Datasheet AD7718EB, AD7708EB, AD7718BRU, AD7708BRU, AD7708BR Datasheet (Analog Devices)

8-/10-Channel, Low Voltage,

FEATURES 8-/10-Channel, High Resolution ⌺-⌬ ADCs AD7708 Has 16-Bit Resolution AD7718 Has 24-Bit Resolution Factory-Calibrated Single Conversion Cycle Setting Programmable Gain Front End Simultaneous 50 Hz and 60 Hz Rejection

VREF Select

Measurement Capability

Operation Can Be Optimized for

Analog Performance (CHOP = 0) or Channel Throughput (CHOP = 1)

INTERFACE 3-Wire Serial

SPI Schmitt Trigger on SCLK

POWER Specified for Single 3 V and 5 V Operation Normal: 1.28 mA Typ @ 3 V Power-Down: 30 ␮A (32 kHz Crystal Running) On-Chip Functions

Rail-to-Rail Input Buffer and PGA 2-Bit Digital I/O Port

APPLICATIONS Industrial Process Control Instrumentation Pressure Transducers Portable Instrumentation Smart Transmitters

™ Allows Absolute and Ratiometric

, QSPITM, MICROWIRETM, and DSP-Compatible

Low Power, ⌺-⌬ ADCs

AD7708/AD7718

GENERAL DESCRIPTION

The AD7708/AD7718 are complete analog front-ends for low frequency measurement applications. The AD7718 contains a 24-bit Σ-∆ ADC with PGA and can be configured as 4/5 fullydifferential input channels or 8/10 pseudo-differential input channels. Two pins on the device are configurable as analog inputs or reference inputs. The AD7708 is a 16-bit version of the AD7718. Input signal ranges from 20 mV to 2.56 V can be directly converted using these ADCs. Signals can be converted directly from a transducer without the need for signal conditioning.

The device operates from a 32 kHz crystal with an on-board PLL generating the required internal operating frequency. The output data rate from the part is software programmable. The peak-topeak resolution from the part varies with the programmed gain and output data rate.

The part operates from a single 3 V or 5 V supply. When operating from 3 V supplies, the power dissipation for the part is 3.84 mW typ. Both parts are pin-for-pin compatible allowing an upgradable path from 16 to 24 bits without the need for hardware modifications. The AD7708/AD7718 are housed in 28-lead SOIC and TSSOP packages.

FUNCTIONAL BLOCK DIAGRAM

DVD D XTAL1 XTAL2

DGND

AIN1 AIN2 AIN3 AIN4 AIN5 AIN6 AIN7 AIN8

AINCOM

SPI and QSPI are trademarks of Motorola Inc. MICROWIRE is a trademark of National Semiconductor Corp. VREF Select is a trademark of Analog Devices, Inc.

MUX

AV DD

NEG BUF

REFIN2(+)/ AIN9 REFIN1(+) REFIN2(–)/ AIN10 REFIN1(–)

POS BUF

REFIN(+) REFIN(–)

PGA

AD7708/ AD7718

AGND

REV. 0

Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices.

OSC AND

PLL

DOUT DIN SCLK

CS RDY RESET

I/O PORT

SERIAL

INTERFACE

AND

CONTROL

LOGIC

P2 P1

⌺-⌬ ADC*

*AD7708 16-BIT ADC *AD7718 24-BIT ADC

AV DD

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781/329-4700 www.analog.com Fax: 781/326-8703 © Analog Devices, Inc., 2001

AD7708/AD7718

FEATURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

FUNCTIONAL BLOCK DIAGRAM . . . . . . . . . . . . . . . . . 1

GENERAL DESCRIPTION . . . . . . . . . . . . . . . . . . . . . . . . . 1

AD7718 SPECIFICATIONS . . . . . . . . . . . . . . . . . . . . . . . . 3

AD7708 SPECIFICATIONS . . . . . . . . . . . . . . . . . . . . . . . . 6

TIMING CHARACTERISTICS . . . . . . . . . . . . . . . . . . . . . 9

ABSOLUTE MAXIMUM RATINGS . . . . . . . . . . . . . . . . 10

ORDERING GUIDE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

PIN FUNCTION DESCRIPTIONS . . . . . . . . . . . . . . . . . 12

PIN CONFIGURATION . . . . . . . . . . . . . . . . . . . . . . . . . . 13

ADC CIRCUIT INFORMATION . . . . . . . . . . . . . . . . . . . 15

Signal Chain Overview (CHOP Enabled, CHOP = 0) . . . 15

ADC NOISE PERFORMANCE CHOP ENABLED

(CHOP = 0) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Signal Chain Overview (CHOP Disabled CHOP = 1) . . . 19

ADC NOISE PERFORMANCE CHOP DISABLED

(CHOP = 1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

ON-CHIP REGISTERS . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Communications Register . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Status Register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Mode Register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Operating Characteristics when Addressing the

Mode and Control Registers . . . . . . . . . . . . . . . . . . . . . . . 28

ADC Control Register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Filter Register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

I/O Control Register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

ADC Data Result Register . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Unipolar Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Bipolar Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

ADC Offset Calibration Coefficient Registers . . . . . . . . . . . 31

ADC Gain Calibration Coefficient Register . . . . . . . . . . . . . 31

ID Register (ID) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

User Nonprogrammable Test Registers . . . . . . . . . . . . . . . . 31

Configuring the AD7708/AD7718 . . . . . . . . . . . . . . . . . . . . 32

DIGITAL INTERFACE . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

MICROCOMPUTER/MICROPROCESSOR

INTERFACING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

AD7708/AD7718 to 68HC11 Interface . . . . . . . . . . . . . . . . 34

AD7708/AD7718-to-8051 Interface . . . . . . . . . . . . . . . . . . 35

AD7708/AD7718-to-ADSP-2103/ADSP-2105 Interface . . . 36

BASIC CONFIGURATION . . . . . . . . . . . . . . . . . . . . . . . . 36

Analog Input Channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

Single-Ended Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

Chop Mode of Operation (CHOP = 0) . . . . . . . . . . . . . . . . 37

Nonchop Mode of Operation (CHOP = 1) . . . . . . . . . . . . . 38

Programmable Gain Amplifier . . . . . . . . . . . . . . . . . . . . . . . 38

Bipolar/Unipolar Configuration . . . . . . . . . . . . . . . . . . . . . . 38

Data Output Coding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

Oscillator Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Reference Input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

RESET Input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Power-Down Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

Grounding and Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

APPLICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

Data Acquisition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

Programmable Logic Controllers . . . . . . . . . . . . . . . . . . . . . 41

Converting Single-Ended Inputs. . . . . . . . . . . . . . . . . . . . . 42

Combined Ratiometric and Absolute Value

Measurement System . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

Optimizing Throughput while Maximizing 50 Hz

and 60 Hz Rejection in a Multiplexed Data

Acquisition System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

OUTLINE DIMENSIONS . . . . . . . . . . . . . . . . . . . . . . . . . 44

–2–

REV. 0

AD7708/AD7718

AD7718 SPECIFICATIONS

REFIN(+) = 2.5 V; REFIN(–) = AGND; AGND = DGND = 0 V; XTAL1/XTAL2 = 32.768 kHz Crystal Input Buffer Enabled. All specifications T T

unless otherwise noted.)

MAX

Parameter B Grade Unit Test Conditions

AD7718 (CHOP DISABLED)

Output Update Rate 16.06 Hz min CHOP = 1

No Missing Codes Resolution 13 Bits p-p ±20 mV Range, SF = 69

Output Noise and Update Rates See Tables in

Integral Nonlinearity ±10 ppm of FSR max 2 ppm Typical Offset Error

Offset Error Drift vs. Temp Full-Scale Error Gain Drift vs. Temp Negative Full-Scale Error ±0.003 % FSR max

ANALOG INPUTS

Differential Input Full-Scale Voltage ±1.024 × REFIN/GAIN V nom REFIN Refers to Both REFIN1 and

Absolute AIN Voltage Limits AGND + 100 mV V min AIN1–AIN10 and AINCOM with

Absolute AINCOM Voltage Limits AGND – 30 mV V min NEGBUF = 0

Analog Input Current AIN1–AIN10 and AINCOM with NEGBUF = 1

DC Input Current DC Bias Current Drift ±5pA/°C typ

AINCOM Input Current NEGBUF = 0

DC Input Current DC Bias Current Drift ±2 pA/V/°C typ

Normal-Mode Rejection

@ 50 Hz 100 dB min 50 Hz ± 1 Hz, SF Word = 82 @ 60 Hz 100 dB min 60 Hz ± 1 Hz, SF Word = 68

Common-Mode Rejection

@ DC 90 dB min 100 dB typ, Analog Input = 1 V,

@ 50 Hz 100 dB typ 50 Hz ± 1 Hz, SF Word = 82 @ 60 Hz 100 dB typ 60 Hz ± 1 Hz, SF Word = 68

REFERENCE INPUTS (REFIN1 AND REFIN2)

REFIN(+) to REFIN(–) Voltage 2.5 V nom REFIN Refers to Both REFIN1 and REFIN2 REFIN(+) to REFIN(–) Range

REFIN Common-Mode Range AGND – 30 mV V min

Reference DC Input Current 0.5 µA/V typ Reference DC Input Current Drift ± 0.1 nA/V/°C typ Normal-Mode Rejection

@ 50 Hz 100 dB min 50 Hz ± 1 Hz, SF Word = 82 @ 60 Hz 100 dB min 60 Hz ± 1 Hz, SF Word = 68

Common-Mode Rejection Input Range = ±2.56 V

@ DC 100 dB typ Analog Input = 1 V. Input Range = ±2.56 V @ 50 Hz 100 dB typ @ 60 Hz 100 dB typ

(AVDD = 2.7 V to 3.6 V or 4.75 V to 5.25 V, DVDD = 2.7 V to 3.6 V or 4.75 V to 5.25 V,

MIN

1.365 kHz max 24 Bits min

18 Bits p-p ±2.56 V Range, SF = 69

ADC Description

Table VII µV typ Offset Error is in the order of the noise for the

±200 nV/°C typ ±10 µV typ ±0.5 ppm/°C typ

– 100 mV V max NEGBUF = 1

+ 30 mV V max

±1 nA max

±125 nA/V typ ±2.56 V Range

1V min AV

+ 30 mV V max

V max

programmed gain and update rate following a calibration

REFIN2. REFIN = REFIN(+) –REFIN(–) GAIN = 1 to 128

Input Range = ±2.56 V 110 dB typ on ±20 mV Range

REV. 0

–3–

AD7718–SPECIFICATIONS

2.5 V ; REFIN(–) = AGND; AGND = DGND = 0 V; XTAL1/XTAL2 = 32.768 kHz Crystal Input Buffer Enabled. All specifications T

Parameter B Grade Unit Test Conditions

AD7718 (CHOP ENABLED)

Output Update Rate 5.4 Hz min CHOP = 0

No Missing Codes Resolution 13 Bits p-p ±20 mV Range, 20 Hz Update Rate

Output Noise and Update Rates See Tables in

Integral Nonlinearity ±10 ppm of FSR max 2 ppm Typical Offset Error Offset Error Drift vs. Temp Full-Scale Error Gain Drift vs. Temp

ANALOG INPUTS

Differential Input Full-Scale Voltage ±1.024 × REFIN/GAIN V nom REFIN Refers to Both REFIN1 and

Range Matching ±2 µV typ Analog Input = 18 mV Absolute AIN Voltage Limits AGND + 100 mV V min AIN1–AIN10 and AINCOM with

Absolute AINCOM Voltage Limits AGND – 30 mV V min NEGBUF = 0

Analog Input Current AIN1–AIN10 and AINCOM with

DC Input Current DC Input Current Drift ±5 pA/°C typ

AINCOM Input Current NEGBUF = 0

DC Input Current DC Bias Current Drift ±2 pA/V/°C typ

Normal-Mode Rejection

@ 50 Hz 100 dB min 50 Hz ± 1 Hz, SF Word = 82 @ 60 Hz 100 dB min 60 Hz ± 1 Hz, SF Word = 68

Common-Mode Rejection

@ DC 90 dB min 100 dB typ, Analog Input = 1 V,

@ 50 Hz @ 60 Hz

REFERENCE INPUTS (REFIN1 AND REFIN2)

REFIN(+) to REFIN(–) Voltage 2.5 V nom REFIN Refers to Both REFIN1 and

REFIN(+) to REFIN(–) Range

REFIN Common-Mode Range AGND – 30 mV V min

Reference DC Input Current Reference DC Input Current Drift ± 0.01 nA/V/°C typ Normal-Mode Rejection

@ 50 Hz 100 dB min 50 Hz ± 1 Hz, SF Word = 82 @ 60 Hz 100 dB min 60 Hz ± 1 Hz, SF Word = 68

Common-Mode Rejection

@ DC 110 dB typ Analog Input = 1 V @ 50 Hz 110 dB typ 50 Hz ± 1 Hz, 20 Hz Update Rate @ 60 Hz 110 dB typ 60 Hz ± 1 Hz, 20 Hz Update Rate

LOGIC INPUTS

All Inputs Except SCLK and XTAL1

, Input Low Voltage 0.8 V max DVDD = 5 V

INL

, Input Low Voltage 0.4 V max DVDD = 3 V

INL

, Input High Voltage 2.0 V min DVDD = 3 V or 5 V

INH

(AVDD = 2.7 V to 3.6 V or 4.75 V to 5.25 V, DVDD = 2.7 V to 3.6 V or 4.75 V to 5.25 V, REFIN(+) =

to T

MIN

105 Hz max 24 Bits min 20 Hz Update Rate

18 Bits p-p ±2.56 V Range, 20 Hz Update Rate

ADC Description

±3 µV typ 10 nV/°C typ

±10 µV/°C typ ±0.5 ppm/°C typ

REFIN2. REFIN = REFIN(+) REFIN(–) GAIN = 1 to 128

– 100 mV V max NEGBUF = 1

+ 30 mV V max

NEGBUF = 1

±1 nA max

±125 nA/V typ ±2.56 V Range

Input Range = ±2.56 V

110 dB typ on ±20 mV Range 100 dB min 50 Hz ± 1 Hz, 20 Hz Update Rate 100 dB min 60 Hz ± 1 Hz, 20 Hz Update Rate

REFIN2 1V min AV

+ 30 mV V max

V max

±0.5 µA/V typ

Input Range = ±2.56 V

unless otherwise noted.)

MAX

–4–

REV. 0

AD7708/AD7718

Parameter B Grade Unit Test Conditions

LOGIC INPUTS (Continued)

SCLK Only (Schmitt-Triggered Input)

T(+)

T(–)

– V

T(+)

V V V

XTAL1 Only

V V V V

T(–)

T(+)

T(–)

T(+)–VT(–)

INL

INH

INL

INH

, Input Low Voltage 0.8 V max DVDD = 5 V

, Input High Voltage 3.5 V min DVDD = 5 V

, Input Low Voltage 0.4 V max DVDD = 3 V

, Input High Voltage 2.5 V min DVDD = 3 V

Input Currents ±10 µA max Logic Input = DV

Input Capacitance 10 pF typ All Digital Inputs

LOGIC OUTPUTS (Excluding XTAL2)

VOH, Output High Voltage

, Output Low Voltage

, Output High Voltage

, Output Low Voltage

Floating State Leakage Current ±10 µA max Floating State Output Capacitance ±10 pF typ Data Output Coding Binary Unipolar Mode

SYSTEM CALIBRATION

Full-Scale Calibration Limit 1.05 × FS V max Zero-Scale Calibration Limit –1.05 × FS V min Input Span 0.8 × FS V min

START-UP TIME

From Power-On 300 ms typ From Power-Down Mode 1 ms typ Oscillator Enabled

POWER REQUIREMENTS

Power Supply Voltages AV

–AGND 2.7/3.6 V min/max AVDD = 3 V nom

–DGND 2.7/3.6 V min/max DVDD = 3 V nom

(Normal Mode) 0.55 mA max DVDD = 3 V, 0.43 mA typ

(Normal Mode) 1.1 mA max AVDD = 3 V or 5 V, 0.85 mA typ

(Power-Down Mode) 10 µA max DVDD = 3 V, 32.768 kHz Osc. Running

(Power-Down Mode) 1 µA max AVDD = 3 V or 5 V

Power Supply Rejection (PSR) Input Range = ±2.56 V, AIN = 1 V

Chop Disabled 70 dB min 95 dB typ Chop Enabled 100 dB typ

NOTES

Temperature range is –40°C to +85°C.

Not production tested, guaranteed by design and/or characterization data at release.

Following a self-calibration this error will be in the order of the noise for the programmed gain and update selected. A system calibration will completely remove this error.

Recalibration at any temperature will remove these errors.

I/O Port Logic Levels are with respect to AV

Specifications are subject to change without notice.

and AGND.

1.4/2 V min/V max DVDD = 5 V

0.8/1.4 V min/V max DVDD = 5 V

0.3/0.85 V min/V max DVDD = 5 V

0.95/2 V min/V max DVDD = 3 V

0.4/1.1 V min/V max DVDD = 3 V

0.3/0.85 V min/V max DVDD = 3 V

–70 µA max

Logic Input = DGND, Typical –40 µA @ 5 V

and –20 µA at 3 V

– 0.6 V min DVDD = 3 V, I

0.4 V max DVDD = 3 V, I 4V minDV

= 5 V, I

0.4 V max DVDD = 5 V, I

SOURCE

= 100 µA

SINK

SOURCE

= 1.6 mA

SINK

= 100 µA

= 200 µA

Offset Binary Bipolar Mode

2.1 × FS V max

300 ms typ Oscillator Powered Down

and DVDD can be operated independently of each other.

4.75/5.25 V min/max AV

4.75/5.25 V min DV

0.65 mA max DV

2 µA max DV 30 µA max DV 8 µA max DV

= 5 V nom

= 5 V, 0.5 mA typ

= 3 V, Oscillator Powered Down

= 5 V, 32.768 kHz Osc. Running

= 5 V, Oscillator Powered Down

REV. 0

–5–

AD7708/AD7718

AD7708 SPECIFICATIONS

REFIN(+) = 2.5 V; REFIN(–) = AGND; AGND = DGND = 0 V; XTAL1/XTAL2 = 32.768 kHz Crystal Input Buffers Enabled. All specifications T T

unless otherwise noted.)

MAX

Parameter B Grade Unit Test Conditions

AD7708 (CHOP DISABLED)

Output Update Rate 16.06 Hz min CHOP = 1

No Missing Codes Resolution 13 Bits p-p ±20 mV Range, SF Word = 69

Output Noise and Update Rates See Tables in ADC Description Integral Nonlinearity ±15 ppm of FSR max 2ppm Typical Offset Error Offset Error Drift vs. Temp Full-Scale Error Gain Drift vs. Temp Negative Full-Scale Error ±0.003 % FSR typ

ANALOG INPUTS

Differential Input Full-Scale Voltage ±1.024 × REFIN/GAIN V nom REFIN Refers to Both REFIN1 and

Absolute AIN Voltage Limits AGND + 100 mV V min AIN1–AIN10 and AINCOM with

Absolute AINCOM Voltage Limits AGND – 30 mV V min NEGBUF = 0

Analog Input Current AIN1–AIN10 and AINCOM with

DC Input Current DC Bias Current Drift ±5 pA/°C typ

AINCOM Input Current NEGBUF = 0

DC Input Current DC Bias Current Drift ±2 pA/V/°C typ

Normal-Mode Rejection

@ 50 Hz 100 dB min 50 Hz ± 1 Hz, SF Word = 82 @ 60 Hz 100 dB min 60 Hz ± 1 Hz, SF Word = 68

Common-Mode Rejection

@ DC 90 dB min 100 dB typ, Analog Input = 1 V,

@ 50 Hz 100 dB typ 50 Hz ± 1 Hz, SF Word = 82 @ 60 Hz 100 dB typ 60 Hz ± 1 Hz, SF Word = 68

REFERENCE INPUTS (REFIN1 AND REFIN2)

REFIN(+) to REFIN(–) Voltage 2.5 V nom REFIN Refers to Both REFIN1 and

REFIN(+) to REFIN(–) Range

REFIN Common-Mode Range AGND – 30 mV V min

Reference DC Input Current 0.5 µA/V typ Reference DC Input Current Drift ± 0.1 nA/V/°C typ Normal-Mode Rejection

@ 50 Hz 100 dB min 50 Hz ± 1 Hz, SF Word = 82 @ 60 Hz 100 dB min 60 Hz ± 1 Hz, SF Word = 68

Common-Mode Rejection Input Range = ±2.56 V

@ DC 100 dB typ Analog Input = 1 V. Input Range = ± 2.56 V @ 50 Hz 100 dB typ @ 60 Hz 100 dB typ

(AVDD = 2.7 V to 3.6 V or 4.75 V to 5.25 V, DVDD = 2.7 V to 3.6 V or 4.75 V to 5.25 V,

1.365 kHz max 16 Bits min

16 Bits p-p ±2.56 V Range, SF Word = 69

±0.65 LSB typ Following a Self-Calibration ±200 nV/°C typ ±0.75 LSB typ ±0.5 ppm/°C typ

REFIN2. REFIN = REFIN(+) – REFIN(–)

GAIN = 1 to 128

– 100 mV V max NEGBUF = 1

+ 30 mV V max

±1 nA max

±125 nA/V typ ±2.56 V Range

1V min AV

+ 30 mV V max

V max

NEGBUF = 1

Input Range = ±2.56 V

110 dB typ on ±20 mV Range

REFIN2

MIN

–6–

REV. 0

AD7708/AD7718

Parameter B Grade Unit Test Conditions

AD7708 (CHOP ENABLED)

Output Update Rate 5.4 Hz min CHOP = 1

No Missing Codes

Resolution 13 Bits p-p ±20 mV Range, 20 Hz Update Rate

Output Noise and Update Rates See Tables in

Integral Nonlinearity ±15 ppm of FSR max 2 ppm Typical Offset Error Offset Error Drift vs. Temp Full-Scale Error Gain Drift vs. Temp

ANALOG INPUTS

Differential Input Full-Scale Voltage ±1.024 × REFIN/GAIN V nom REFIN Refers to Both REFIN1 and

Range Matching ±2 µV typ Analog Input = 18 mV Absolute AIN Voltage Limits AGND + 100 mV V min AIN1–AIN10 and AINCOM with

Absolute AINCOM Voltage Limits AGND – 30 mV V min NEGBUF = 0

Analog Input Current AIN1–AIN10 and AINCOM with

DC Input Current

DC Input Current Drift ±5 pA/°C typ

AINCOM Input Current NEGBUF = 0

DC Input Current DC Bias Current Drift ±2 pA/V/°C typ

Normal-Mode Rejection

@ 50 Hz 100 dB min 50 Hz ± 1 Hz, SF Word = 82 @ 60 Hz 94 dB min 60 Hz ± 1 Hz, SF Word = 68

Common-Mode Rejection

@ DC 90 dB min 100 dB typ, Analog Input = 1 V,

@ 50 Hz @ 60 Hz

REFERENCE INPUTS (REFIN1 AND REFIN2)

REFIN(+) to REFIN(–) Voltage 2.5 V nom REFIN Refers to Both REFIN1 and

REFIN(+) to REFIN(–) Range

REFIN Common-Mode Range AGND – 30 mV V min

Reference DC Input Current Reference DC Input Current Drift ± 0.01 nA/V/°C typ Normal-Mode Rejection

@ 50 Hz 100 dB min 50 Hz ± 1 Hz, SF Word = 82 @ 60 Hz 100 dB min 60 Hz ± 1 Hz, SF Word = 68

Common-Mode Rejection Input Range = ±2.56 V

@ DC 110 dB typ Analog Input = 1 V @ 50 Hz 110 dB typ 50 Hz ± 1 Hz, 20 Hz Update Rate @ 60 Hz 110 dB typ 60 Hz ± 1 Hz, 20 Hz Update Rate

LOGIC INPUTS

All Inputs Except SCLK and XTAL1

INL

INH

, Input Low Voltage 0.8 V max DVDD = 5 V

, Input High Voltage 2.0 V min DVDD = 3 V or 5 V

105 Hz max 0.732 ms Increments 16 Bits min 20 Hz Update Rate

16 Bits p-p ±2.56 V Range, 20 Hz Update Rate

ADC Description

±3 µV typ Calibration is Accurate to ±0.5 LSB 10 nV/°C typ

±0.75 LSB typ Includes Positive and Negative ERRORS ±0.5 ppm/°C typ

REFIN2. REFIN = REFIN(+) REFIN(–) GAIN = 1 to 128

– 100 mV V max NEGBUF = 1

+ 30 mV V max

NEGBUF = 1

±1 nA max

±125 nA/V typ

Input Range = ±2.56 V

110 dB typ on ±20 mV Range 100 dB min 50 Hz ± 1 Hz, 20 Hz Update Rate 100 dB min 60 Hz ± 1 Hz, 20 Hz Update Rate

REFIN2 1V min AV

+ 30 mV V max

V max

±0.5 µA/V typ

0.4 V max DV

= 3 V

REV. 0

–7–

AD7718–SPECIFICATIONS

AD7708

2.5 V ; REFIN(–) = AGND; AGND = DGND = 0 V; XTAL1/XTAL2 = 32.768 kHz Crystal Input Buffer Enabled. All specifications T

Parameter B Grade Unit Test Conditions

LOGIC INPUTS (Continued)

SCLK Only (Schmitt-Triggered Input)

T(+)

T(–)

T(+)–VT(–)

T(+)

T(–)

T(+)–VT(–)

XTAL1 Only

INL

INH

INL

INH

, Input Low Voltage 0.8 V max DVDD = 5 V

, Input High Voltage 3.5 V min DVDD = 5 V

, Input Low Voltage 0.4 V max DVDD = 3 V

, Input High Voltage 2.5 V min DVDD = 3 V

Input Currents ±10 µA max Logic Input = DV

Input Capacitance 10 pF typ All Digital Inputs

LOGIC OUTPUTS (Excluding XTAL2)

VOH, Output High Voltage

, Output Low Voltage

, Output High Voltage

, Output Low Voltage

Floating State Leakage Current ±10 µA max Floating State Output Capacitance ±10 pF typ Data Output Coding Binary Unipolar Mode

SYSTEM CALIBRATION

Full-Scale Calibration Limit 1.05 × FS V max Zero-Scale Calibration Limit –1.05 × FS V min Input Span 0.8 × FS V min

START-UP TIME

From Power-On 300 ms typ From Power-Down Mode 1 ms typ

POWER REQUIREMENTS

Power Supply Voltages AV

–AGND 2.7/3.6 V min/max AVDD = 3 V nom

–DGND 2.7/3.6 V min/max DVDD = 3 V nom

(Normal Mode) 0.55 mA max DVDD = 3 V, 0.43 mA typ

(Normal Mode) 1.1 mA AVDD = 3 V or 5 V, 0.85 mA typ

(Power-Down Mode) 10 µA max DVDD = 3 V, 32.768 kHz Osc. Running

(Power-Down Mode) 1 µA max AVDD = 3 V or 5 V

Power Supply Rejection (PSR) Input Range = ±2.56 V, AIN = 1 V

Chop Disabled 70 dB min 95 dB typ Chop Enabled 100 dB typ

NOTES

Temperature range is –40°C to +85°C.

Not production tested, guaranteed by design and/or characterization data at release.

Following a self-calibration this error will be in the order of the noise for the programmed gain and update selected. A system calibration will completely remove this error.

Recalibration at any temperature will remove these errors.

I/O Port Logic Levels are with respect to AV

Specifications are subject to change without notice.

and AGND.

(AVDD = 2.7 V to 3.6 V or 4.75 V to 5.25 V, DVDD = 2.7 V to 3.6 V or 4.75 V to 5.25 V, REFIN(+) =

to T

MIN

1.4/2 V min/V max DVDD = 5 V

0.8/1.4 V min/V max DVDD = 5 V

0.3/0.85 V min/V max DVDD = 5 V

0.95/2 V min/V max DVDD = 3 V

0.4/1.1 V min/V max DVDD = 3 V

0.3/0.85 V min/V max DVDD = 3 V

–70 µA max

Logic Input = DGND, Typical –40 µA @ 5 V and –20 µA at 3 V

DVDD – 0.6 V min DVDD = 3 V, I

0.4 V max DVDD = 3 V, I 4V minDV

= 5 V, I

0.4 V max DVDD = 5 V, I

Offset Binary Bipolar Mode

2.1 × FS V max

300 ms typ Oscillator Powered Down

and DVDD can be operated independently of each other.

4.75/5.25 V min/max AV

4.75/5.25 V min DV

0.65 mA DV

2 µA max DV 30 µA max DV 8 µA max DV

= 5 V nom

= 5 V, 0.5 mA typ

= 3 V, Oscillator Powered Down

= 5 V, 32.768 kHz Osc. Running

= 5 V, Oscillator Powered Down

unless otherwise noted.)

MAX

= 100 µA

SOURCE

= 100 µA

SINK

= 200 µA

SOURCE

= 1.6 mA

SINK

–8–

REV. 0

AD7708/AD7718

TIMING CHARACTERISTICS

1, 2

(AVDD = 2.7 V to 3.6 V or AVDD = 5 V ⴞ 5%; DVDD = 2.7 V to 3.6 V or DVDD = 5 V ⴞ 5%; AGND =

DGND = 0 V; XTAL = 32.768 kHz; Input Logic 0 = 0 V, Logic 1 = DVDD unless otherwise noted.

Limit at T

MIN

, T

MAX

Parameter (B Version) Unit Conditions/Comments

32.768 kHz typ Crystal Oscillator Frequency 50 ns min RESET Pulsewidth

Read Operation

0 ns min RDY to CS Setup Time 0 ns min CS Falling Edge to SCLK Active Edge Setup Time 0 ns min SCLK Active Edge to Data Valid Delay

60 ns max DVDD = 4.5 V to 5.5 V

4, 5

80 ns max DV 0 ns min CS Falling Edge to Data Valid Delay

= 2.7 V to 3.6 V

60 ns max DVDD = 4.5 V to 5.5 V 80 ns max DV

100 ns min SCLK High Pulsewidth 100 ns min SCLK Low Pulsewidth 0 ns min CS Rising Edge to SCLK Inactive Edge Hold Time 10 ns min Bus Relinquish Time after SCLK Inactive Edge 80 ns max

100 ns max SCLK Active Edge to RDY High

Write Operation

NOTES

Sample tested during initial release to ensure compliance. All input signals are specified with tr = tf = 5 ns (10% to 90% of DVDD) and timed from a voltage level of 1.6 V.

See Figures 1 and 2.

SCLK active edge is falling edge of SCLK.

These numbers are measured with the load circuit of Figure 1 and defined as the time required for the output to cross the VOL or VOH limits.

This specification only comes into play if CS goes low while SCLK is low. It is required primarily for interfacing to DSP machines.

These numbers are derived from the measured time taken by the data output to change 0.5 V when loaded with the load circuit of Figure 1. The measured number is then extrapolated back to remove effects of charging or discharging the 50 pF capacitor. This means that the times quoted in the timing characteristics are the true bus relinquish times of the part and as such are independent of external bus loading capacitances.

RDY returns high after the first read from the device after an output update. The same data can be read again, if required, while RDY is high, although care should be taken that subsequent reads do not occur close to the next output update.

Specifications subject to change without notice.

0 ns min CS Falling Edge to SCLK Active Edge Setup Time 30 ns min Data Valid to SCLK Edge Setup Time 25 ns min Data Valid to SCLK Edge Hold Time 100 ns min SCLK High Pulsewidth 100 ns min SCLK Low Pulsewidth 0 ns min CS Rising Edge to SCLK Edge Hold Time

= 2.7 V to 3.6 V

3, 7

REV. 0

= 5V

= 3V)

= 5V

= 3V)

TO OUTPUT

50pF

(1.6mA WITH DV

SINK

100␮A WITH DV

1.6V

SOURCE

(200␮A WITH DV 100␮A WITH DV

Figure 1. Load Circuit for Timing Characterization

–9–

AD7708/AD7718

WARNING!

ESD SENSITIVE DEVICE

ABSOLUTE MAXIMUM RATINGS*

(TA = 25°C unless otherwise noted)

AVDD to AGND . . . . . . . . . . . . . . . . . . . . . . . –0.3 V to +7 V

to DGND . . . . . . . . . . . . . . . . . . . . . . . –0.3 V to +7 V

to AGND . . . . . . . . . . . . . . . . . . . . . . . –0.3 V to +7 V

to DGND . . . . . . . . . . . . . . . . . . . . . . . –0.3 V to +7 V

AGND to DGND . . . . . . . . . . . . . . . . . . –0.05 V to +0.05 V

to DVDD . . . . . . . . . . . . . . . . . . . . . . . . . –5 V to +5 V

Analog Input Voltage to AGND . . . . –0.3 V to AV

Reference Input Voltage to AGND . . –0.3 V to AV

+0.3 V

Total AIN/REFIN Current (Indefinite) . . . . . . . . . . . . 30 mA

Digital Input Voltage to DGND . . . . –0.3 V to DV

Digital Output Voltage to DGND . . . –0.3 V to DV

+0.3 V

Operating Temperature Range . . . . . . . . . . . –40°C to +85°C

Storage Temperature Range . . . . . . . . . . . . –65°C to +150°C

ORDERING GUIDE

Temperature Package Package

Model Range Description Option

AD7708BR –40°C to +85°C SOIC R-28 AD7708BRU –40°C to +85°C TSSOP RU-28 EVAL-AD7708EB Evaluation Board AD7718BR –40°C to +85°C SOIC R-28 AD7718BRU –40°C to +85°C TSSOP RU-28 EVAL-AD7718EB Evaluation Board

Junction Temperature . . . . . . . . . . . . . . . . . . . . . . . . . 150°C

SOIC Package

Thermal Impedance . . . . . . . . . . . . . . . . . . . 71.4°C/W

Thermal Impedance . . . . . . . . . . . . . . . . . . . . . 23°C/W

TSSOP Package

Thermal Impedance . . . . . . . . . . . . . . . . . . . 97.9°C/W

Thermal Impedance . . . . . . . . . . . . . . . . . . . . . 14°C/W

Lead Temperature, Soldering

Vapor Phase (60 sec) . . . . . . . . . . . . . . . . . . . . . . . . 215°C

Infrared (15 sec) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220°C

*Stresses above those listed under Absolute Maximum Ratings may cause perma-

nent damage to the device. This is a stress rating only; functional operation of the device at these or any other conditions above those listed in the operational sections of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.

CAUTION

ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on the human body and test equipment and can discharge without detection. Although the AD7708/AD7718 features proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high-energy electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance degradation or loss of functionality.

–10–

REV. 0

SCLK

RDY

SCLK

DIN

AD7708/AD7718

MSB

Figure 2. Write Cycle Timing Diagram

LSB

DOUT

MSB

LSB

Figure 3. Read Cycle Timing Diagram

REV. 0

–11–

AD7708/AD7718

PIN FUNCTION DESCRIPTIONS

Pin No Mnemonic Function

1 AIN7 Analog Input Channel 7. Programmable-gain analog input that can be used as a pseudo-

differential input when used with AINCOM, or as the positive input of a fully-differential input pair when used with AIN8. (See ADC Control Register section.)

2 AIN8 Analog Input Channel 8. Programmable-gain analog input that can be used as a pseudo-

differential input when used with AINCOM, or as the negative input of a fully-differential input pair when used with AIN7. (See ADC Control Register section.)

3AV

4 AGND Analog Ground 5 REFIN1(–) Negative Reference Input. This reference input can lie anywhere between AGND and AV 6 REFIN1(+) Positive reference input. REFIN(+) can lie anywhere between AVDD and AGND. The nominal

7 AIN1 Analog Input Channel 1. Programmable-gain analog input that can be used as a pseudo-

8 AIN2 Analog Input Channel 2. Programmable-gain analog input that can be used as a pseudo-

9 AIN3 Analog Input Channel 3. Programmable-gain analog input that can be used as a pseudo-

10 AIN4 Analog Input Channel 4. Programmable-gain analog input that can be used as a pseudo-

11 AIN5 Analog Input Channel 5. Programmable-gain analog input that can be used as a pseudo-

12 AINCOM All analog inputs are referenced to this input when configured in pseudo-differential input mode. 13 REFIN2(+)/AIN9 Positive reference input/analog input. This input can be configured as a reference input with the

14 REFIN2(–)/AIN10 Negative reference input/analog input. This pin can be configured as a reference or analog input.

15 AIN6 Analog Input Channel 6. Programmable-gain analog input that can be used as a pseudo-

16 P2 P2 can act as a general-purpose Input/Output bit referenced between AV

17 AGND It is recommended that this pin be tied directly to AGND. 18 P1 P1 can act as a general-purpose Input/Output bit referenced between AV

19 RESET Digital input used to reset the ADC to its power-on-reset status. This pin has a weak pull-up

20 SCLK Serial clock input for data transfers to and from the ADC. The SCLK has a Schmitt-trigger

Analog Supply Voltage

– 1 V.

reference voltage [REFIN(+)–REFIN(–)] is 2.5 V but the part is functional with a reference range from 1 V to AV

differential input when used with AINCOM, or as the positive input of a fully-differential input pair when used with AIN2. (See ADC Control Register Section.)

differential input when used with AINCOM, or as the negative input of a fully-differential input pair when used with AIN1. (See ADC Control Register section.)

differential input when used with AINCOM, or as the positive input of a fully-differential input pair when used with AIN4. (See ADC Control Register section.)

differential input when used with AINCOM, or as the negative input of a fully-differential input pair when used with AIN3. (See ADC Control Register section.)

differential input when used with AINCOM, or as the positive input of a fully-differential input pair when used with AIN6. (See ADC Control Register section ADCCON.)

same characteristics as REFIN1(+) or as an additional analog input. When configured as an analog input this pin provides a programmable-gain analog input that can be used as a pseudodifferential input when used with AINCOM, or as the positive input of a fully-differential input pair when used with AIN10. (See ADC Control Register section.)

When configured as a reference input it provides the negative reference input for REFIN2. When configured as an analog input it provides a programmable-gain analog input that can be used as a pseudo-differential input when used with AINCOM, or as the negative input of a fullydifferential input pair when used with AIN9. (See ADC Control Register section.)

differential input when used with AINCOM, or as the negative input of a fully-differential input pair when used with AIN5. (See ADC Control Register section.)

and AGND. There

is a weak pull-up to AV

internally to DV

internally on this pin.

and AGND. There

input making an opto-isolated interface more robust. The serial clock can be continuous with all data transmitted in a continuous train of pulses. Alternatively, it can be a noncontinuous clock with the information being transmitted to or from the AD7708/AD7718 in smaller batches of data.

–12–

REV. 0

AD7708/AD7718

Pin No Mnemonic Function

21 CS Chip Select Input. This is an active low logic input used to select the AD7708/AD7718. CS can

be used to select the AD7708/AD7718 in systems with more than one device on the serial bus or as a frame synchronization signal in communicating with the device. CS can be hardwired low, allowing the AD7708/AD7718 to be operated in 3-wire mode with SCLK, DIN, and DOUT used to interface with the device.

22 RDY RDY is a logic low status output from the AD7708/AD7718. RDY is low when valid data exists

in the data register for the selected channel. This output returns high on completion of a read operation from the data register. If data is not read, RDY will return high prior to the next update indicating to the user that a read operation should not be initiated. The RDY pin also returns low following the completion of a calibration cycle. RDY does not return high after a calibration until the mode bits are written to enabling a new conversion or calibration.

23 DOUT Serial data output with serial data being read from the output shift register of the ADC. The output

shift register can contain data from any of the on-chip data, calibration or control registers.

24 DIN Serial Data Input with serial data being written to the input shift register on the AD7708/AD7718

Data in this shift register is transferred to the calibration or control registers within the ADC

depending on the selection bits of the Communications register. 25 DGND Ground Reference Point for the Digital Circuitry. 26 DV

27 XTAL2 Output from the 32 kHz Crystal Oscillator or Resonator Inverter. 28 XTAL1 Input to the 32 kHz Crystal Oscillator or Resonator Inverter.

Digital Supply Voltage, 3 V or 5 V Nominal.

PIN CONFIGURATION

AIN7

AIN8

AGND

AIN1

AIN2

AIN3

AIN4

AIN5

AD7708/

AD7718

TOP VIEW

(Not to Scale)

REFIN1(–)

REFIN1(+)

AINCOM

REFIN2(+)/AIN9

REFIN2(–)/AIN10

XTAL1

XTAL2

DGND

DIN

DOUT

RDY

SCLK

RESET

AGND

AIN6

REV. 0

–13–

AD7708/AD7718

–Typical Performance Characteristics

8389600

8389400

8389200

8389000

8388800

8388600

CODE READ

8388400

8388200

8388000

AVDD = DVDD = 5V INPUT RANGE = ⴞ20mV

RMS NOISE = 0.58␮V rms

0 1000100

200 300

REFIN1(+)–REFIN1(–) = 2.5V UPDATE RATE = 19.79Hz

400 500 600 700 800 900

READING NUMBER

TA = 25ⴗC V

= 2.5V

REF

TPC 1. AD7718 Typical Noise Plot on ±20 mV Input Range with 19.79 Hz Update Rate

8388721

8388499

8388547

8388615

8388579

8388687

8388657

8388779

8388754

8388841

8388805

8388906

8388874

8388985

8388941

8389033

8389110

8388382

8388039

8388449

TPC 2. AD7718 Noise Distribution Histogram

CHOP = 0

NO MISSING CODES – Min

040302010 50 10090807060

UPDATE RATE – Hz

110

TPC 4. AD7718 No-Missing Codes Performance

32772

32771

32770

32769

32768

CODE READ

32767

32766

32765

32764

1000 200 400300

AVDD = DVDD = 5V INPUT RANGE = ⴞ20mV

UPDATE RATE = 19.79Hz

500

READING NUMBER

600 700 800 900 1000

REF

= 2.5V

= 25 C

TPC 5. AD7708 Typical Noise Plot on ±20 mV Input Range

3.0

= 2.5V

= 25ⴗC

ⴞ2.56V RANGE

REF –

ⴞ20mV RANGE

2.5

2.0

AVDD = DVDD = 5V V

REF

1.5

INPUT RANGE = ⴞ2.56V UPDATE RATE = 19.79Hz T

RMS NOISE – ␮V

1.0

0.5

1.0 3.02.52.01.5 3.5 5.04.54.0

TPC 3. RMS Noise vs. Reference Input (AD7718 andAD7708)

–14–

700

600

500

400

300

OCCURRENCE

200

100

3276732766 32768 3277032769

CODE

TPC 6. AD7708 Noise Histogram

32771

REV. 0

AD7708/AD7718

ADC CIRCUIT INFORMATION

The AD7708/AD7718 incorporates a 10-channel multiplexer with a sigma-delta ADC, on-chip programmable gain amplifier and digital filtering intended for the measurement of wide dynamic range, low frequency signals such as those in weigh-scale, strain-gauge, pressure transducer, or temperature measurement applications. The AD7708 offers 16-bit resolution while the AD7718 offers 24-bit resolution. The AD7718 is a pin-for-pin compatible version of the AD7708. The AD7718 offers a direct upgradable path from a 16-bit to a 24-bit system without requiring any hardware changes and only minimal software changes.

These parts can be configured as four/five fully-differential input channels or as eight/ten pseudo-differential input channels referenced to AINCOM. The channel is buffered and can be programmed for one of eight input ranges from ±20 mV to ±2.56 V. Buffering the input channel means that the part can handle significant source impedances on the analog input and that R, C filtering (for noise rejection or RFI reduction) can be placed on the analog inputs if required. These input channels are intended to convert signals directly from sensors without the need for external signal conditioning.

The ADC employs a sigma-delta conversion technique to realize up to 24 bits of no missing codes performance. The sigma-delta modulator converts the sampled input signal into a digital pulse train whose duty cycle contains the digital information. A Sinc

programmable low-pass filter is then employed to decimate the modulator output data stream to give a valid data conversion result at programmable output rates. The signal chain has two modes of operation, CHOP enabled and CHOP disabled. The CHOP bit in the mode register enables and disables the chopping scheme.

Signal Chain Overview (CHOP Enabled, CHOP = 0)

With CHOP = 0, chopping is enabled, this is the default and gives optimum performance in terms of drift performance. With chopping enabled, the available output rates vary from 5.35 Hz (186.77 ms) to 105.03 Hz (9.52 ms). A block diagram of the ADC input channel with chop enabled is shown in Figure 4.

The sampling frequency of the modulator loop is many times higher than the bandwidth of the input signal. The integrator in the modulator shapes the quantization noise (which results from the analog-to-digital conversion) so that the noise is pushed toward one-half of the modulator frequency. The output of the sigma-delta modulator feeds directly into the digital filter. The digital filter then band-limits the response to a frequency significantly lower than one-half of the modulator frequency. In this manner, the 1-bit output of the comparator is translated into a band limited, low noise output from the AD7708/AD7718 ADC. The AD7708/AD7718 filter is a low-pass, Sinc

or (sinx/x)

filter whose primary function is to remove the quantization noise introduced at the modulator. The cutoff frequency and decimated output data rate of the filter are programmable via the SF word loaded to the filter register. The complete signal chain is chopped resulting in excellent dc offset and offset drift specifications and is extremely beneficial in applications where drift, noise rejection, and optimum EMI rejection are important factors. With chopping, the ADC repeatedly reverses its inputs. The decimated digital output words from the Sinc

filters, therefore, have a positive offset and negative offset term included. As a result, a final summing stage is included so that each output word from the filter is summed and averaged with the previous filter output to produce a new valid output result to be written to the ADC data register. The programming of the Sinc

decimation factor is restricted to an 8-bit register SF, the actual decimation factor is the register value times 8. The decimated output rate from the Sinc

filter (and the ADC conversion rate)

will therefore be

=×

ADC MOD

318

where

in the ADC conversion rate.

ADC

SF is the decimal equivalent of the word loaded to the filter

is the modulator sampling rate of 32.768 kHz.

MOD

The chop rate of the channel is half the output data rate:

CHOP

×12

ADC

As shown in the block diagram, the Sinc3 filter outputs alternately contain +V

and –VOS, where VOS is the respective

channel offset. This offset is removed by performing a running average of two. This average by two means that the settling time to any change in programming of the ADC will be twice the normal conversion time, while an asynchronous step change on the analog input will not be fully reflected until the third subsequent output.

SETTLE

==×

ADC

The allowable range for SF is 13 to 255 with a default of 69 (45H). The corresponding conversion rates, conversion times, and settling times are shown in Table I. Note that the conversion time increases by 0.732 ms for each increment in SF.

REV. 0

ANALOG

INPUT

CHOP

MUX

BUF

PGA

MOD

⌺-⌬

MOD0

CHOP

XOR

(

8 ⴛ SF

⌺

(

SINC 3 FILTER

Figure 4. ADC Channel Block Diagram with CHOP Enabled

–15–

3 ⴛ

(8 ⴛ SF )

ADC

AIN + V AIN – V

⌺

DIGITAL OUTPUT

AD7708/AD7718

Table I. ADC Conversion and Settling Times for Various SF Words with CHOP = 0

SF Data Update Rate Settling Time Word f

(Hz) t

ADC

SETTLE

(ms)

13 105.3 19.04 23 59.36 33.69 27 50.56 39.55 45 30.3 65.9 69 (Default) 19.79 101.07 91 15 133.1 182 7.5 266.6 255 5.35 373.54

The overall frequency response is the product of a sinc sinc response. There are sinc 3 × f of f

and there are sinc notches at odd integer multiples

ADC

/2. The 3 dB frequency for all values of SF obeys the

ADC

notches at integer multiples of

and a

following equation:

f (3 dB) = 0.24 × f

ADC

Normal-mode rejection is the major function of the digital filter on the AD7708/AD7718. The normal mode 50 ±1 Hz rejection with an SF word of 82 is typically –100 dB. The 60 ± 1 Hz rejection with SF = 68 is typically –100 dB. Simultaneous 50 Hz and 60 Hz rejection of better than 60 dB is achieved with an SF of 69. Choosing an SF word of 69 places notches at both 50 Hz and 60 Hz. Figures 5 to 9 show the filter rejection for a selection of SF words.

The frequency response of the filter H (f) is as follows:



 



sin ( / )

SF f f

×××

sin ( / )

MOD



××









sin ( / )

××

sin ( / )

OUT

 



where

= 32,768 Hz,

MOD

SF = value programmed into SF Register, f

OUT

= f

/(SF × 8 × 3).

MOD

The following plots show the filter frequency response for a variety of update rates from 5 Hz to 105 Hz.

–20

–40

–60

–80

–100

–120

–140

ATTENUATION – dB

–160

–180

–200

100

SF = 13 OUTPUT DATA RATE = 105Hz INPUT BANDWIDTH = 25.2Hz FIRST NOTCH = 52.5Hz 50Hz REJECTION = –23.6dB, 50Hzⴞ1Hz REJECTION = –20.5dB 60Hz REJECTION = –14.6dB, 60Hzⴞ1Hz REJECTION = –13.6dB

150 200 250 300 350 400

FREQUENCY – Hz

450 500 550 600

Figure 5. Filter Profile with SF = 13

–20

–40

–60

–80

–100

ATTENUATION – dB

–120

–140

–160

0 10010

SF = 82 OUTPUT DATA RATE = 16.65Hz INPUT BANDWIDTH = 4Hz 50Hz REJECTION = –171dB, 50Hzⴞ1Hz REJECTION = –100dB 60Hz REJECTION = –58dB, 60Hzⴞ1Hz REJECTION = –53dB

20 30 40 50 60 70 80 90

FREQUENCY – Hz

Figure 6. Filter Profile with SF = 82

650

700

–16–

REV. 0

AD7708/AD7718

–20

–40

–60

–80

–100

ATTENUATION – dB

–120

–140

–160

0 10010

SF = 255 OUTPUT DATA RATE = 5.35Hz INPUT BANDWIDTH = 1.28Hz 50Hz REJECTION = –93dB, 50Hzⴞ1Hz REJECTION = –93dB 60Hz REJECTION = –74dB, 60Hzⴞ1Hz REJECTION = –68dB

20 30 40 50 60 70 80 90

FREQUENCY – Hz

Figure 7. Filter Profile with SF = 255

–20

–40

–60

–80

–100

ATTENUATION – dB

–120

–140

–160

0 100

SF = 69 OUTPUT DATA RATE = 19.8Hz INPUT BANDWIDTH = 4.74Hz FIRST NOTCH = 9.9Hz 50Hz REJECTION = –66dB, 50Hzⴞ1Hz REJECTION = –60dB 60Hz REJECTION = –117dB, 60Hzⴞ1Hz REJECTION = –94dB

20 30 40 50 60 70 80 90

FREQUENCY – Hz

Figure 8. Filter Profile with Default SF = 69 Giving Filter Notches at Both 50 Hz and 60 Hz

–20

–40

–60

–80

–100

ATTENUATION – dB

–120

–140

–160

0 10010

SF = 68 OUTPUT DATA RATE = 20.07Hz INPUT BANDWIDTH = 4.82Hz 50Hz REJECTION = –74dB, 50Hzⴞ1Hz REJECTION = –54.6dB 60Hz REJECTION = –147dB, 60Hzⴞ1Hz REJECTION = –101dB

20 30 40 50 60 70 80 90

FREQUENCY – Hz

Figure 9. Filter Profile with SF = 68

ADC NOISE PERFORMANCE CHOP ENABLED (CHOP = 0)

Tables II to V show the output rms noise and output peak-topeak resolution in bits (rounded to the nearest 0.5 LSB) for a selection of output update rates. The numbers are typical and generated at a differential input voltage of 0 V with AV DV

= 5 V and using a 2.5 V reference. The output update

rate is selected via the SF7–SF0 bits in the Filter Register. It is important to note that the peak-to-peak resolution figures represent the resolution for which there will be no code flicker within a six-sigma limit. The output noise comes from two sources. The first is the electrical noise in the semiconductor devices (device noise) used in the implementation of the modulator. Secondly, when the analog input is converted into the digital domain, quantization noise is added. The device noise is at a low level and is independent of frequency. The quantization noise starts at an even lower level but rises rapidly with increasing frequency to become the dominant noise source. The numbers in the tables are given for the bipolar input ranges. For the unipolar ranges the rms noise numbers will be the same as the bipolar range, but the peak-to-peak resolution is now based on half the signal range which effectively means losing one bit of resolution.

REV. 0

–17–

AD7708/AD7718

Table II. Typical Output RMS Noise vs. Input Range and Update Rate for AD7718 with Chop Enabled (CHOP = 0); Output RMS Noise in ␮V

SF Data Update Word Rate (Hz) ⴞ20 mV ⴞ40 mV ⴞ80 mV ⴞ160 mV ⴞ320 mV ⴞ640 mV ⴞ1.28 V ⴞ2.56 V

13 105.3 1.50 1.50 1.60 1.75 3.50 4.50 6.70 11.75 23 59.36 1.0 1.02 1.06 1.15 1.22 1.77 3.0 5.08 27 50.56 0.95 0.95 0.98 1.00 1.10 1.66 5.0 69 19.79 0.60 0.65 0.65 0.65 0.65 0.95 1.40 2.30 255 5.35 0.35 0.35 0.37 0.37 0.37 0.51 0.82 1.25

Table III. Peak-to-Peak Resolution vs. Input Range and Update Rate for AD7718 with Chop Enabled (CHOP = 0); Peak-to-Peak Resolution in Bits

SF Data Update Word Rate (Hz) ⴞ20 mV ⴞ40 mV ⴞ80 mV ⴞ160 mV ⴞ320 mV ⴞ640 mV ⴞ1.28 V ⴞ2.56 V

13 105.3 12 13 14 15 15 15.5 16 16 23 59.36 12.5 13.5 14.5 15 16 17 17 17 27 50.56 12.5 13.5 14.5 15.5 16.5 17 17 17 69 19.79 13 14 15 16 17 17.5 18 18.5 255 5.35 14 15 16 17 18 18.5 18.8 19.2

Table IV. Typical Output RMS Noise vs. Input Range and Update Rate for AD7708 with Chop Enabled ( CHOP = 0); Output RMS Noise in ␮V

SF Data Update Word Rate (Hz) ⴞ20 mV ⴞ40 mV ⴞ80 mV ⴞ160 mV ⴞ320 mV ⴞ640 mV ⴞ1.28 V ⴞ2.56 V

Input Range

Table V. Peak-to-Peak Resolution vs. Input Range and Update Rate for AD7708 with Chop Enabled (CHOP = 0); Peak-to-Peak Resolution in Bits

SF Data Update Word Rate (Hz) ⴞ20 mV ⴞ40 mV ⴞ80 mV ⴞ160 mV ⴞ320 mV ⴞ640 mV ⴞ1.28 V ⴞ2.56 V

13 105.3 12 13 14 15 15 15.5 16 16 23 59.35 12.5 13.5 14.5 15 16 16 16 16 27 50.56 12.5 13.5 14.5 15.5 16 16 16 16 69 19.79 13 14 15 16 16 16 16 16 255 5.35 14 15 16 16 16 16 16 16

Input Range

–18–

REV. 0

AD7708/AD7718

SIGNAL CHAIN OVERVIEW CHOP DISABLED (CHOP = 1)

With CHOP =1 chopping is disabled. With chopping disabled the available output rates vary from 16.06 Hz (62.26 ms) to

1365.33 Hz (0.73 ms). The range of applicable SF words is from 3 to 255. When switching between channels with chop disabled, the channel throughput is increased by a factor of two over the case where chop is enabled. When used in multiplexed applications operation with chop disabled will offer the best throughput time when cycling through all channels. The drawback with chop disabled is that the drift performance is degraded and calibration is required following a gain change or significant temperature change. A block diagram of the ADC input channel with chop disabled is shown in Figure 10. The signal chain includes a mux, buffer, PGA, sigma-delta modulator, and digital filter. The modulator bit stream is applied to

filter. The programming of the Sinc3 decimation

a Sinc factor is restricted to an 8-bit register SF, the actual decimation factor is the register value times 8. The decimated output rate from the Sinc

filter (and the ADC conversion rate) will there-

fore be:

ADC

MOD

×8

where

is the ADC conversion rate,

ADC

SF is the decimal equivalent of the word loaded to the filter

is the modulator sampling rate of 32.768 kHz.

MOD

The settling time to a step input is governed by the digital filter. A synchronized step change will require a settling time of three times the programmed update rate, a channel change can be treated as a synchronized step change. An unsynchronized step change will require four outputs to reflect the new analog input at its output.

SETTLE

==×

ADC

The allowable range for SF is 3 to 255 with a default of 69 (45H). The corresponding conversion rates, conversion times, and settling times are shown in Table VI. Note that the conversion time increases by 0.245 ms for each increment in SF.

Table VI. ADC Conversion and Settling Times for Various SF Words with CHOP = 1

SF Data Update Rate Settling Time Word f

(Hz) t

ADC

SETTLE

(ms)

03 1365.33 2.20 68 60.2 49.8 69 (Default) 59.36 50.54 75 54.6 54.93 82 49.95 60 151 27.13 110.6 255 16.06 186.76

The frequency response of the digital filter H (f) is as follows:

 



sin( / )

SF f f

×× ×

sin( / )

MOD

 



where

= 32,768 Hz,

MOD

SF = value programmed into SF SFR.

The following shows plots of the filter frequency response using different SF words for output data rates

There are sinc

notches at integer multiples of the update rate.

of 16 Hz to 1.36 kHz.

The 3 dB frequency for all values of SF obeys the following equation:

f (3 dB) = 0.262 × f

ADC

The following plots show frequency response of the AD7708/ AD7718 digital filter for various filter words. The AD7708/ AD7718 are targeted at multiplexed applications. One of the key requirements in these applications is to optimize the SF word to obtain the maximum filter rejection at 50 Hz and 60 Hz while minimizing the channel throughput rate. Figure 12 shows the AD7708/AD7718 optimized throughput while maximizing 50 Hz and 60 Hz rejection. This is achieved with an SF word of

75. In Figure 13, by using a higher SF word of 151, 50 Hz and 60 Hz rejection can be maximized at 60 dB with a channel throughput rate of 110 ms. An SF word of 255 gives maximum rejection at both 50 Hz and 60 Hz but the channel throughput rate is restricted to 186 ms as shown in Figure 14.

REV. 0

ANALOG

INPUT

MUX

BUF

PGA

MOD

⌺-⌬

MOD0

SINC3 FILTER

Figure 10. ADC Channel Block Diagram with CHOP Disabled

–19–

ADC

DIGITAL OUTPUT

AD7708/AD7718

–20

–40

–60

–80

–100

–120

–140

ATTENUATION – dB

–160

–180

–200

0 10010

SF = 68 OUTPUT DATA RATE = 60.2Hz SETTLING TIME = 49.8ms INPUT BANDWIDTH = 15.5Hz 50Hz REJECTION = –43dB, 50Hzⴞ1Hz REJECTION = –40dB 60Hz REJECTION = –147dB, 60Hzⴞ1Hz REJECTION = –101dB

20 30 40 50 60 70 80 90

FREQUENCY – Hz

Figure 11. Frequency Response Operating with the SF Word of 68

–20

–40

–60

–80

–100

–120

–140

ATTENUATION – dB

–160

–180

–200

0 10010

SF = 75 OUTPUT DATA RATE = 54.6Hz SETTLING TIME = 55ms INPUT BANDWIDTH = 14.3Hz 50Hz REJECTION = –62.5dB, 50Hzⴞ1Hz REJECTION = –57dB 60Hz REJECTION = –63dB, 60Hzⴞ1Hz REJECTION = –60dB

20 30 40 50 60 70 80 90

FREQUENCY – Hz

Figure 12. Optimizing Filter Response for Throughput while Maximizing the Simultaneous 50 Hz and 60 Hz Rejection

–20

–40

–60

–80

–100

–120

–140

ATTENUATION – dB

–160

–180

–200

0 10010

SF = 151 OUTPUT DATA RATE = 27.12Hz SETTLING TIME = 110ms INPUT BANDWIDTH = 27.12Hz 50Hz REJECTION = –65.4dB, 50Hzⴞ1Hz REJECTION = –60dB 60Hz REJECTION = –63dB, 60Hzⴞ1Hz REJECTION = –60dB

20 30 40 50 60 70 80 90

FREQUENCY – Hz

Figure 13. Optimizing Filter Response for Maximum Simultaneous 50 Hz and 60 Hz Rejection

–20

–40

–60

–80

–100

–120

–140

ATTENUATION – dB

–160

–180

–200

0 10010

SF = 255 OUTPUT DATA RATE = 16.06Hz SETTLING TIME = 186ms INPUT BANDWIDTH = 4.21Hz 50Hz REJECTION = –87dB, 50Hzⴞ1Hz REJECTION = –77dB 60Hz REJECTION = –72dB, 60Hzⴞ1Hz REJECTION = –68dB

20 30 40 50 60 70 80 90

FREQUENCY – Hz

Figure 14. Frequency with Maximum SF Word = 255

ADC NOISE PERFORMANCE CHOP DISABLED (CHOP = 1)

Tables VII to X show the output rms noise and output peak-topeak resolution in bits (rounded to the nearest 0.5 LSB) for some typical output update rates. The numbers are typical and generated at a differential input voltage of 0 V. The output update rate is selected via the SF7–SF0 bits in the Filter Register. It is important to note that the peak-to-peak resolution figures represent the resolution for which there will be no code flicker within a six-sigma limit. The output noise comes from two sources. The first is the electrical noise in the semiconductor devices (device noise) used in the implementation of the modulator. Secondly,

–20–

when the analog input is converted into the digital domain, quantization noise is added. The device noise is at a low level and is independent of frequency. The quantization noise starts at an even lower level but rises rapidly with increasing frequency to become the dominant noise source. The numbers in the tables are given for the bipolar input ranges. For the unipolar ranges the rms noise numbers will be the same as the bipolar range, but the peak-to-peak resolution is now based on half the signal range which effectively means losing 1 bit of resolution.

REV. 0

AD7708/AD7718

Table VII. Typical Output RMS Noise vs. Input Range and Update Rate for AD7718 with Chop Disabled (CHOP = 1); Output RMS Noise in ␮V

SF Data Update Word Rate (Hz) ⴞ20 mV ⴞ40 mV ⴞ80 mV ⴞ160 mV ⴞ320 mV ⴞ640 mV ⴞ1.28 V ⴞ2.56 V

03 1365.33 30.31 29.02 58.33 112.7 282.44 361.72 616.89 1660 13 315.08 2.47 2.49 2.37 3.87 7.18 12.61 16.65 32.45 66 62.06 0.743 0.852 0.9183 0.8788 0.8795 1.29 1.99 3.59 69 59.38 0.961 0.971 0.949 0.922 0.923 1.32 2.03 3.73 81 50.57 0.894 0.872 0.872 0.806 0.793 1.34 2.18 2.96 255 16.06 0.475 0.468 0.434 0.485 0.458 0.688 1.18 1.78

Table VIII. Peak-to-Peak Resolution vs. Input Range and Update Rate for AD7718 with Chop Disabled (CHOP = 1); Peak-to-Peak Resolution in Bits

SF Data Update Word Rate (Hz) ⴞ20 mV ⴞ40 mV ⴞ80 mV ⴞ160 mV ⴞ320 mV ⴞ640 mV ⴞ1.28 V ⴞ2.56 V

03 1365.33 8999 9 9 9 9 13 315.08 11 12 14 14 14 14 15 15 66 62.06 13 14 15 16 17 17 18 18 69 59.36 13 14 15 16 17 17 18 18 81 50.57 13 14 15 16 17 17 18 18 255 16.06 14 15 16 17 18 18 19 19

Input Range

Table IX. Typical Output RMS Noise vs. Input Range and Update Rate for AD7708 with Chop Disabled (CHOP = 1); Output RMS Noise in ␮V

Input Range

SF Data Update Word Rate (Hz) ⴞ20 mV ⴞ40 mV ⴞ80 mV ⴞ160 mV ⴞ320 mV ⴞ640 mV ⴞ1.28 V ⴞ2.56 V

Table X. Peak-to-Peak Resolution vs. Input Range and Update Rate for AD7708 with Chop Disabled (CHOP = 1); Peak-to-Peak Resolution in Bits

SF Data Update Word Rate (Hz) ⴞ20 mV ⴞ40 mV ⴞ80 mV ⴞ160 mV ⴞ320 mV ⴞ640 mV ⴞ1.28 V ⴞ2.56 V

03 1365.33 8999 9 9 9 9 13 315.08 11 12 14 14 14 14 15 15 66 62.06 13 14 15 16 16 16 16 16 69 59.36 13 14 15 16 16 16 16 16 81 50.57 13 14 15 16 16 16 16 16 255 16.06 14 15 16 16 16 16 16 16

Input Range

REV. 0

–21–

AD7708/AD7718

ON-CHIP REGISTERS

The AD7708 and AD7718 are controlled and configured via a number of on-chip registers which are shown in Figure 15. The first of these registers is the communications register which is used to control all operations on these converters. All communications with these parts must start with a write to the communications register to specify the next operation to be performed. After a power-on or RESET, the device defaults to waiting for a write to the communications register. The STATUS register contains information pertaining to the operating conditions of the converter. The STATUS register is a read only register. The MODE register is used to configure the conversion mode, calibration, chop enable/disable, reference select, channel configuration and buffered or unbuffered operation on the AINCOM analog input. The MODE register is a read/write register. The ADC Control register is a read/write register used to select the active channel and program its input range and bipolar/unipolar operation. The I/O control register is a read/ write register used to configure the operation of the 2-pin I/O

DIN

COMMUNICATIONS REGISTER

WEN R/W 00 A1A2A3 A0

port. The filter register is a read/write register used to program the data update rate of the converter. The ADC Data register is a read only register that contains the result of a data conversion on the selected channel. The ADC offset registers are read/write registers that contain the offset calibration data. There are five offset registers, one for each of the fully differential input channels. When configured for pseudo-differential input mode the channels share offset registers. The ADC gain registers are read/write registers that contain the gain calibration data. There are five ADC gain registers, one for each of the fully differential input channels. When configured for pseudo differential input mode the channels share gain registers. The ADC contains Test registers for factory use only, the user is advised not to alter the operating conditions of these registers. The ID register is a read only register and is used for silicon identification purposes. The following sections contains more in-depth detail on all of these registers. In the following descriptions, SET implies a Logic 1 state and CLEARED implies a Logic 0 state unless otherwise stated.

DOUT

STATUS REGISTER

DIN

MODE REGISTER

DIN

ADC CONTROL REGISTER

DIN

I/O CONTROL REGISTER

DIN

FILTER REGISTER

ADC DATA REGISTER

DIN

ADC OFFSET REGISTER

DIN

ADC GAIN REGISTER

DIN

TEST REGISTER

ID REGISTER

Figure 15. On-Chip Registers

SELECT

DECODER

–22–

REV. 0

AD7708/AD7718

Table XI. Registers—Quick Reference Guide

Power-On/Reset

Communications Write Only 8 Bits Not Applicable All operations to other registers are initiated through

CR7 CR6 CR5 CR4 CR3 CR2 CR1 CR0

WEN R/W 0(0) 0(0) A3(0) A2(0) A1(0) A0(0)

Status Register Read Only 8 Bits 00 Hex Provides status information on conversions, calibra-

BSMBSL

YDR0 LAC0RRE00 KCOL

Mode Register Read/Write 8 Bits 00 Hex Controls functions such as mode of operation, chan-

MSB LSB

CHOP NEGBUF REFSEL CHCON OSCPD MD2 MD1 MD0

Control Register Read/Write 8 Bits 07 Hex This register is used to select the active channel

BSMBSL

3HC2HC1HC0HC/U B 2NR1NR0NR

the Communications Register. This controls whether subsequent operations are read or write operations and also selects the register for that subsequent operation.

tions and error conditions.

nel configuration, oscillator operation in power-down.

ADC (ADCCON)

input, configure the operating input range, and select unipolar or bipolar operation.

I/O (IOCON)

I/O Control Register Read/Write 8 Bits 00 Hex This register is used to control and configure the

I/O port.

BSMBSL

00 RID2PRID1P00TAD2PTAD1P

Filter Register Read/Write 8 Bits 45 Hex This register determines the amount of averaging

BSMBSL

7FS6FS5FS4FS3FS2FS1FS0FS

performed by the sinc filter and consequently determines the data update rate of the AD7708/AD7718. The filter register determines the update rate for operation with CHOP enabled and CHOP disabled.

AD7718 ADC (DATA)

Data Register Read Only 24 Bits 000000 Hex Provides the most up-to-date conversion result for

the selected channel on the AD7718.

AD7708 (DATA)

Data Register Read Only 16 Bits 0000 Hex Provides the most up-to-date conversion result for

the selected channel on the AD7708.

REV. 0

–23–

AD7708/AD7718

Table XI. Registers—Quick Reference Guide (continued)

Power-On/Reset Register Name Type Size Default Value Function

AD7718

Offset Register Read/Write 24 Bits 800 000 Hex Contains a 24-bit word which is the offset calibration

coefficient for the part. The contents of this register are used to provide offset correction on the output from the digital filter. There are five Offset Registers on the part and these are associated with input channels as outlined in the ADCCON register.

AD7718

Gain Register Read/Write 24 Bits 5XXXX5 Hex Contains a 24-bit word which is the gain calibration

coefficient for the part. The contents of this register are used to provide gain correction on the output from the digital filter. There are five Gain Registers on the part and these are associated with input channels as outlined in the ADCCON register.

AD7708

Offset Register Read/Write 16 Bits 8000 Hex Contains a 16-bit word which is the offset calibration

AD7708

Gain Register Read/Write 16 Bits 5XXX Hex Contains a 16-bit word which is the gain calibration

AD7708

ID Register Read 8 Bits 5X Hex Contains an 8-bit byte which is the identifier for

the part.

AD7718

ID Register Read 8 Bits 4X Hex Contains an 8-bit byte which is the identifier for

the part.

Test Registers Read/Write 16 Bits 0000 Hex Controls the test modes of the part that are used

when testing the part. The user is advised not to

change the contents of these registers.

–24–

REV. 0

AD7708/AD7718

Communications Register (A3, A2, A1, A0 = 0, 0, 0, 0)

The Communications Register is an 8-bit write-only register. All communications to the part must start with a write operation to the Communications Register. The data written to the Communications Register determines whether the next operation is a read or write operation, the type of read operation, and on which register this operation takes place. For read or write operations, once the subsequent read or write operation to the selected register is complete, the interface returns to where it expects a write operation to the Communications Register. This is the default state of the interface and, on power-up or after a RESET, the AD7708/AD7718 is in this default state waiting for a write operation to the Communications Register. In situations where the interface sequence is lost, a write operation of at least 32 serial clock cycles with DIN high returns the AD7708/AD7718 to this default state by resetting the part. Table XII outlines the bit designations for the Communications Register. CR0 through CR7 indicate the bit location, CR denoting the bits are in the Communications Register. CR7 denotes the first bit of the data stream.

7RC6RC5RC4RC3RC2RC1RC0RC

NEW )0(

Bit Bit Location Mnemonic Description

CR7 WEN Write Enable Bit. A 0 must be written to this bit so the write operation to the Communications Register

CR6 R/W A zero in this bit location indicates that the next operation will be a write to a specified register. A one in

CR5 0 A zero must be written to this bit position to ensure correct operation of the AD7708/AD7718.

CR4 0 A zero must be written to this bit position to ensure correct operation of the AD7708/AD7718.

CR3–CR0 A3–A0 Register Address Bits. These address bits are used to select which of the AD7708/AD7718’s registers are

/R W )0()0(0)0(0)0(3A)0(2A)0(1A)0(0A

Table XII. Communications Register Bit Designations

actually takes place. If a 1 is written to this bit, the part will not clock on to subsequent bits in the regis ter. It will stay at this bit location until a 0 is written to this bit. Once a 0 is written to the WEN bit, the next seven bits will be loaded to the Communications Register.

this position indicates that the next operation will be a read from the designated register.

being accessed during this serial interface communication. A3 is the MSB of the three selection bits.

Table XIII. Register Selection Table

A3 A2 A1 A0 Register

0000Communications Register during a Write Operation 0000Status Register during a Read Operation 0001Mode Register 0010ADC Control Register 0011Filter Register 0100ADC Data Register 0101ADC Offset Register 0110ADC Gain Register 0111I/O Control Register 1000Undefined 1001Undefined 1010Undefined 1011Undefined 1100Test 1 Register 1101Test 2 Register 1110Undefined 1111ID Register

REV. 0

–25–

AD7708/AD7718

Status Register (A3, A2, A1, A0 = 0, 0, 0, 0; Power-On-Reset = 00Hex)

The ADC Status Register is an 8-bit read-only register. To access the ADC Status Register, the user must write to the Communications Register selecting the next operation to be a read and load Bits A3-A0 with 0, 0, 0,0. Table XIV outlines the bit designations for the Status Register. SR0 through SR7 indicate the bit location, SR denoting the bits are in the Status Register. SR7 denotes the first bit of the data stream. The number in brackets indicates the power-on/reset default status of that bit.

RS7 6RS5RS4RS3RS2RS1RS0RS

YDR)0()0(0)0(LAC)0(0)0(RRE)0(0)0(0)0(KCOL

Table XIV. Status Register Bit Designations

Bit Bit Location Mnemonic Description

SR7 RDY Ready Bit for the ADC

Set when data is transferred to the ADC data registers or on completion of calibration cycle. The RDY bit is cleared automatically a period of time before the data register is updated with a new conversion result or after the ADC data register has been read. This bit is also cleared by a write to the mode bits to indicate a conversion or calibration. The RDY pin is the complement of the RDY bit.

SR6 0 Bit is automatically cleared. Reserved for future use

SR5 CAL Calibration Status Bit

Set to indicate completion of calibration. It is set at the same time that the RDY is set high. Cleared by a write to the mode bits to start another ADC conversion or calibration.

SR4 0 This bit is automatically cleared. Reserved for future use

SR3 ERR ADC Error Bit

Set to indicate that the result written to the ADC data register has been clamped to all zeros or all ones. After a calibration this bit also flags error conditions that caused the calibration registers not to be written. Error sources include Overrange. Cleared by a write to the mode bits to initiate a conversion or calibration.

SR2 0 This bit is automatically cleared. Reserved for future use

SR1 0 This bit is automatically cleared. Reserved for future use

SR0 LOCK PLL Lock Status Bit.

Set if the PLL has locked onto the 32.768 kHz crystal oscillator clock. If the user is worried about exact sampling frequencies etc., the LOCK bit should be interrogated and the result discarded if the LOCK bit is zero.

–26–

REV. 0

AD7708/AD7718

Mode Register (A3, A2, A1, A0 = 0, 0, 0, 1; Power-On-Reset = 00Hex)

The Mode Register is an 8-bit register from which data can be read or to which data can be written. This register configures the operating modes of the AD7708/AD7718. Table XV outlines the bit designations for the Mode Register. MR7 through MR0 indicate the bit location, MR denoting the bits are in the Mode Register. MR7 denotes the first bit of the data stream. The number in brackets indicates the power-on/reset default status of that bit.

7RM6RM5RM4RM3RM2RM1RM0RM

POHC )0(

)0(FUBGEN)0(LESFER)0(NOCHC)0(DPCSO)0(2DM)0(1DM)0(0DM

Table XV. Mode Register Bit Designations

Bit Bit Location Mnemonic Description

MR7 CHOP If this bit is cleared, chopping is enabled. When this bit is set chopping is disabled. The default is for

chop enabled.

MR6 NEGBUF This bit controls the operation of the input buffer on the AINCOM input when a channel is config-

ured for pseudo-differential mode of operation. If cleared, the analog negative input (AINCOM) is unbuffered allowing it to be tied to AGND in single-ended input configuration. If this bit is set the analog negative input (AINCOM) is buffered, placing a restriction on its common-mode input range.

MR5 REFSEL If this bit is cleared, the reference selected is REFIN1(+) and REFIN1(–) for the active channel. If

this bit is set, the reference selected is REFIN2(+) and REFIN2(–) for the active channel. The contents of the CHCON bit overrides the REFSEL bit. If the ADC is configured in five fully-differential or 10 pseudo-differential input channel mode, the REFSEL bit setting is irrelevant as only one reference input is available. V

Select implemented using the REFSEL bit enables the user to

REF

perform both absolute and ratiometric measurements.

MR4 CHCON When cleared the device is configured as an 8-input channel converter, configured as eight pseudo-

differential input channels with respect to AINCOM or four differential input arrangements with two reference input selection options. When set the device is configured as a 10 pseudodifferential input or a five differential input channel arrangement with a single reference input option.

MR3 OSCPD Oscillator Power-Down Bit.

If this bit is set, placing the AD7708/AD7718 in standby mode will stop the crystal oscillator reducing the power drawn by these parts to a minimum. The oscillator will require 300 ms to begin oscillating when the ADC is taken out of standby mode. If this bit is cleared, the oscillator is not shut off when the ADC is put into standby mode and will not require the 300 ms start-up time when the ADC is taken out of standby.

MR2–MR0 MD2–MD0 ADC Mode Bits.

These bits select the operational mode of the ADC as follows:

MD2 MD1 MD0

0 0 0 Power-Down Mode (Power-On Default) 0 0 1 Idle Mode

In Idle Mode the ADC filter and modulator are held in a reset state although the modulator clocks are still provided.

0 1 0 Single Conversion Mode

In Single Conversion Mode, a single conversion is performed on the enabled channels. On completion of the conversion the ADC data registers are updated, the relevant flags in the STATUS register are written, and idle mode is reentered with the MD2–MD0 being written accordingly to 001.

0 1 1 Continuous Conversion

In continuous conversion mode, the ADC data registers are regularly updated at the selected update rate (see Filter register).

1 0 0 Internal Zero-Scale Calibration

Internal short automatically connected to the enabled channel(s)

1 0 1 Internal Full-Scale Calibration

External V

is connected automatically to the ADC input for this calibration.

REF

1 1 0 System Zero-Scale Calibration

User should connect system zero-scale input to the channel input pins as selected by CH3–CH0 bits in the control registers.

1 1 1 System Full-Scale Calibration

User should connect system full-scale input to the channel input pins as selected by CH3–CH0 bits in the control registers.

REV. 0

–27–

AD7708/AD7718

Operating Characteristics when Addressing the Mode and Control Registers

1. Any change to the MD bits will immediately reset the ADCs. A write to the MD2–MD0 bits with no change is also treated as a reset.

2. Once the MODE has been written with a calibration mode, the RDY bit (STATUS) is immediately reset and the calibration commences. On completion the appropriate calibration registers are written, the bit in STATUS register is updated and the MD2–MD0 bits are reset to 001 to indicate the ADC is back in idle mode.

3. Calibrations are performed with the maximum allowable SF value with chop enabled. SF register is reset to user configuration after calibration with chop enabled. Calibrations are performed with the selected value of SF when chop is disabled.

ADC Control Register (ADCCON): (A3, A2, A1, A0 = 0, 0, 1, 0; Power-On-Reset = 07 Hex)

The ADC Control Register is an 8-bit register from which data can be read or to which data can be written. This register is used to configure the ADC for range, channel selection, and unipolar or bipolar coding. Table XVI outlines the bit designations for the ADC control register ADCCON7 through ADCCON0 indicate the bit location, ADCCON denoting the bits are in the ADC Control Register. ADCCON7 denotes the first bit of the data stream. The number in brackets indicates the power-on/reset default status of that bit.

ADCCON7 ADCCON6 ADCCON5 ADCCON4 ADCCON3 ADCCON2 ADCCON1 ADCCON0

CH3 (0) CH2 (0) CH1 (0) CH0 (0) U/B (0) RN2 (1) RN1 (1) RN0 (1)

Table XVI. ADC Control Register (ADCCON) Bit Designations

Bit Bit Location Mnemonic Description

ADCCON7 CH3 ADC Channel Selection Bits. Written by the user to select either pseudo-differential or fullyADCCON6 CH2 differential input pairs used by the ADC as follows: ADCCON5 CH1 ADCCON4 CH0

8-Channel Configuration 10-Channel Configuration

(CHCON = 0) (CHCON = 1)

Positive Negative Cal Register Positive Negative Cal Register

CH3 CH2 CH1 CH0 Input Input Pair Input Input Pair

0 0 0 0 AIN1 AINCOM 1 AIN1 AINCOM 1 0 0 0 1 AIN2 AINCOM 2 AIN2 AINCOM 2 0 0 1 0 AIN3 AINCOM 3 AIN3 AINCOM 3 0 0 1 1 AIN4 AINCOM 4 AIN4 AINCOM 4 0 1 0 0 AIN5 AINCOM 1 AIN5 AINCOM 5 0 1 0 1 AIN6 AINCOM 2 AIN6 AINCOM 1 0 1 1 0 AIN7 AINCOM 3 AIN7 AINCOM 2 0 1 1 1 AIN8 AINCOM 4 AIN8 AINCOM 3 1 0 0 0 AIN1 AIN2 1 AIN1 AIN2 1 1 0 0 1 AIN3 AIN4 2 AIN3 AIN4 2 1 0 1 0 AIN5 AIN6 3 AIN5 AIN6 3 1 0 1 1 AIN7 AIN8 4 AIN7 AIN8 4 1 1 0 0 AIN2 AIN2 1 AIN9 AIN10 5 1 1 0 1 AINCOM AINCOM 1 AINCOM AINCOM 1 1 1 1 0 REFIN(+) REFIN(–) 1 AIN9 AINCOM 4 1 1 1 1 OPEN OPEN 1 AIN10 AINCOM 5

ADCCON3 U/B Unipolar/Bipolar Bit.

Set by user to enable unipolar coding i.e., zero differential input will result in 000000hex output and a full-scale differential input will result in FFFFFF Hex output when operated in 24-bit mode. Cleared by user to enable bipolar coding, Negative full-scale differential input will result in an output code of 000000 Hex, zero differential input will result in an output code of 800000 Hex and a positive full-scale differential input will result in an output code of FFFFFF Hex.

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AD7708/AD7718

Table XVI. ADC Control Register (ADCCON) Bit Designations (continued)

AD0C2 RN2 ADC Range Bits AD0C1 RN1 Written by the user to select the ADC input range as follows AD0C0 RN0 RN2 RN1 RN0 Selected ADC Input Range (VREF = 2.5 V)

000±20 mV 001±40 mV 010±80 mV 011±160 mV 100±320 mV 101±640 mV 110±1.28 V 111±2.56 V

Filter Register (A3, A2, A1, A0 = 0, 0, 1, 1; Power-On Reset = 45Hex)

The Filter Register is an 8-bit register from which data can be read or to which data can be written. This register determines the amount of averaging performed by the sinc filter. Table XVII outlines the bit designations for the Filter Register. FR7 through FR0 indicate the bit location, FR denoting the bits are in the Filter Register. FR7 denotes the first bit of the data stream. The number in brackets indicates the power-on/reset default status of that bit. The number in this register is used to set the decimation factor and thus the output update rate for the ADCs. The filter register cannot be written to by the user the ADC is active. The update rate is used for the ADCs is calculated as follows:

f f CHOP Enabled CHOP

ADC MOD

=× =

ADC MOD

where

= ADC Output Update Rate,

ADC

= Modulator Clock Frequency = 32.768 kHz,

MOD

SF = Decimal Value Written to SF Register.

()

f CHOP Disabled CHOP

×=

()

Table XVII. Filter Register Bit Designations

7RF6RF5RF4RF3RF2RF1RF0RF

)0(7FS6FS)1(5FS)0(4FS)0(3FS)0()1(2FS)0(1FS)1(0FS

The allowable range for SF is 13 decimal to 255 decimal with chop enabled, and the allowable SF range when chop is disabled is 03 decimal to 255 decimal. Examples of SF values and corresponding conversion rate (f

) and time (t

ADC

) are shown in Table XVIII.

ADC

It should be noted that optimum performance is obtained when operating with chop enabled. When chopping is enabled (CHOP = 0), the filter register is loaded with FF HEX during a calibration cycle. With chop disabled (CHOP =1), the value in the filter register is used during calibration.

Table XVIII. Update Rate vs. SF Word

CHOP Enabled CHOP Disabled

SF (Dec) SF (Hex) f

(Hz) t

ADC

(ms) f

ADC

(Hz) t

ADC

(ms)

03 03 N/A N/A 1365.33 0.732 13 0D 105.3 9.52 315 3.17 69 45 19.79 50.34 59.36 16.85 255 FF 5.35 186.77 16.06 62.26

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–29–

AD7708/AD7718

I/O Control Register (IOCON): (A3, A2, A1, A0 = 0, 1, 1, 1; Power-On-Reset = 00Hex)

The IOCON Register is an 8-bit register from which data can be read or to which data can be written. This register is used to control and configure the I/O port. Table XIX outlines the bit designations for this register. IOCON7 through IOCON0 indicate the bit location, IOCON denoting the bits are in the I/O Control Register. IOCON7 denotes the first bit of the data stream. The number in brackets indicates the power-on/reset default status of that bit. A write to the IOCON register has immediate effect and does not reset the ADCs.

7NOCOI6NOCOI5NOCOI4NOCOI3NOCOI2NOCOI1NOCOI0NOCOI

)0(0)0()0(RID2PRID1P)0(0)0()0(0)0(TAD2P)0(TAD1P

Table XIX. IOCON (I/O Control Register) Bit Designations

Bit Bit Location Mnemonic Description

IOCON7 0 This bit should always be cleared. Reserved for future use.

IOCON6 0 This bit should always be cleared. Reserved for future use.

IOCON5 P2DIR P2, I/O Direction Control Bit.

Set by user to enable P2 as an output. Cleared by user to enable P2 as an input. There are weak pull-ups internally when enabled

as an input.

IOCON4 P1DIR P1, I/O Direction Control Bit.

Set by user to enable P1 as an output. Cleared by user to enable P1 as an input. There are weak pull-ups internally when enabled

as an input.

IOCON3 0 This bit should always be cleared. Reserved for future use.

IOCON2 0 This bit should always be cleared. Reserved for future use.

IOCON1 P2DAT Digital I/O Port (P1) Data Bit.

The readback value of this bit indicates the status of the pin regardless of whether this pin is configured as an input or an output. The value written to this data bit will appear at the output port when the I/O pin is enabled as an output.

IOCON0 P1DAT Digital I/O port (P1) Data Bit.

The readback value of this bit indicates the status of the pin, regardless of whether this pin is configured as an input or an output. The value written to this data bit will appear at the output port when the I/O pin is enabled as an output.

ADC Data Result Register (DATA): (A3, A2, A1, A0 = 0, 1, 0, 0; Power-On-Reset = 000000Hex)

The conversion result for the selected ADC channel is stored in the ADC data register (DATA). This register is 16 bits wide on the AD7708 and 24 bits wide on the AD7718. This is a read only register. On completion of a read from this register the RDY bit in the status register is cleared. These ADCs can be operated in either unipolar or bipolar mode of operation.

Unipolar Mode

In unipolar mode of operation the output coding is straight binary. With an analog input voltage of 0 V the output code is 0000Hex for the AD7708 and 000000Hex for the AD7718. With an analog input voltage of 1.024 V

/Gain the output code is FFFFHex

REF

for the AD7708 and FFFFFF Hex for the AD7718. The output code for any analog input voltage can be represented as follows:

Code = (AIN × GAIN × 2

)/(1.024 × V

REF

)

where

AIN is the analog input voltage and N = 16 for the AD7708 and N = 24 for the AD7718.

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AD7708/AD7718

Bipolar Mode

With an analog input voltage of (–1.024 V With an analog input voltage of 0 V, the output code is 8000Hex for the AD7708 and 800000Hex for the AD7718. With an analog input voltage of (+1.024 V

/GAIN), the output code is FFFF Hex for the AD7708 and FFFFFF Hex for the AD7718. Note the

REF

analog inputs are pseudo bipolar inputs and the analog input voltage must remain within the common-mode input range at all times. The output code for any analog input voltage can be represented as follows:

where

AIN is the analog input voltage, N = 16 for the AD7708, and N = 24 for the AD7718.

ADC Offset Calibration Coefficient Registers (OF0): (A3, A2, A1, A0 = 0, 1, 0, 1; Power-On-Reset = 8000(00)Hex)

The offset calibration registers are 16-bit registers on the AD7708 and 24-bit registers on the AD7718. These registers hold the offset calibration coefficient for the ADC. The power-on-reset value of the internal zero-scale calibration coefficient registers is 8000(00). There are five offset registers available, one for each of the fully differential input channels. Calibration register pairs are shared when operating in pseudo-differential input mode. However, these bytes will be automatically overwritten if an internal or system zero-scale calibration is initiated by the user via MD2–MD0 bits in the MODE register. The channel bits, in association with the communication register address for the OF0 register, allow access to this register. This register is a read/write register. The calibration register can only be written to if the ADC is inactive (MD bits in the mode register = 000 or 001). Reading of the calibration register does not clear the RDY bit.

ADC Gain Calibration Coefficient Register (GNO): (A3, A2, A1, A0 = 0, 1, 1, 0; Power-On-Reset = 5XXX(X5) Hex)

The gain calibration registers are 16-bit registers on the AD7708 and 24-bit registers on the AD7718. These registers are configured at power-on with factory-calculated internal full-scale calibration coefficients. There are five full-scale registers available, one for each of the fully differential input channels. Calibration register pairs are shared when operating in pseudo-differential input mode. Every device will have different default coefficients. However, these bytes will be automatically overwritten if an internal or system full-scale calibration is initiated by the user via MD2–MD0 bits in the MODE register. The channel bits, in association with the communication register address, allow access to the data contained in the GN0 register. This is a read/write register. The calibration registers can only be written to if the ADC is inactive (MD bits in the mode register = 000 or 001). Reading of the calibration registers does not clear the RDY bit. A calibration (self or system) is required when operating with chop mode disabled.

ID Register (ID): (A3, A2, A1, A0 = 1, 1, 1, 1; Power-On-Reset = 4X Hex (AD7718) and 5X Hex (AD7708)

This register is a read only 8-bit register. The contents are used to determine the die revision of the silicon. Table XX indicates the bit locations for the AD7708.

/GAIN), the output code is 0000 Hex using the AD7708 and 000000H using the AD7718.

REF

Code = 2

N–1

× [(AIN × GAIN/1.024 × V

REF

) + 1]

Table XX. ID Register Bit Designation

ID7 ID6 ID5 ID4 ID3 ID2 ID1 ID0

0 1 0 0/1 X X X X

User Nonprogrammable Test Registers

The AD7708 and AD7718 contain two test registers. The bits in these test registers control the test modes of these ADCs which are used for the testing of the device. The user is advised not to change the contents of these registers.

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–31–

AD7708/AD7718

Configuring the AD7708/AD7718

All user-accessible registers on the AD7708 and AD7718 are accessed via the serial interface. Communication with any of these registers is initiated by first writing to the Communications Register. Figures 16, 17, and 18 show flow diagrams for initializing the ADC, a sequence for calibrating the ADC channels, and a routine that cycles through and reads all channels. Figure 16 shows a flowchart detailing necessary programming steps required to initialize the ADC. The following are the general programming steps required:

1. Configure and initialize the microcontroller or microprocessor serial port.

2. Initialize the AD7708/AD7718 by configuring the following registers:

a. IOCON to configure the digital I/O port.

b. FILTER to configure the update rate for each channel.

c. ADCCON to select the active input channel, select

the analog input range, and select unipolar or bipolar operation.

d. MODE to configure the operating mode. The mode

register selects chop or nonchop operation, buffered/ unbuffered operation of the AINCOM input, 8-/10channel mode of operation and reference select along with the selection of conversion, calibration or idle modes of operation.

All operations consist of a write to the communications register to specify the next operation as a write to a specified register. Data is then written to the specified register. When each sequence is complete, the ADC defaults to waiting for another write to the communications register to specify the next operation.

CALIBRATION ROUTINE

WRITE TO ADCCON SELECTING CHANNEL AND CONFIGURING INPUT RANGE AND UNIPOLAR/BIPOLAR MODE

WRITE TO MODE REGISTER SELECTING ZERO-SCALE CALIBRATION

MD BITS

= 001?

WRITE TO MODE REGISTER SELECTING FULL-SCALE CALIBRATION

YES

MD BITS

= 001?

YES

CAL ANOTHER CHANNEL

START

POWER-ON/RESET FOR AD7708/AD7718

CONFIGURE AND INITIALIZE ␮ C/␮P SERIAL PORT

WRITE TO COMMUNICATIONS REGISTER SELECTING NEXT OPERATION TO BE A WRITE TO THE IOCON REGISTER AND CONFIGURE THIS REGISTER

WRITE TO COMMUNICATIONS REGISTER SETTING UP NEXT OPERATION TO BE A WRITE TO THE FILTER REGISTER. SET SF WORD FOR REQUIRED UPDATE RATE

WRITE TO COMMUNICATIONS REGISTER SETTING UP NEXT OPERATION TO BE A WRITE TO THE ADC CONTROL REGISTER. CONFIGURE ADCCON

WRITE TO COMMUNICATIONS REGISTER SETTING UP NEXT OPERATION TO BE A WRITE TO THE MODE REGISTER. CALIBRATE AND SELECT MODE OF OPERATION

END

Figure 17. Calibrating the AD7708/AD7718

Figure 17 shows a flowchart detailing necessary programming steps required when calibrating the AD7708/AD7718. The AD7708/AD7718 have dedicated calibration register pairs for each of the fully-differential input channels. Having a dedicated register pair per channel allows each channel to be calibrated as part of the initialization and the ADC picks up the relevant coefficients for each channel during normal operation. When operating is pseudo-differential mode channels share calibration register pairs. Channels that share coefficients should be configured with the same operating conditions to avoid having to calibrate each time a channel is switched, especially with chop mode disabled. The AD7708/AD7718 are factory-calibrated with chop mode enabled and, therefore, if the ADC is operated at the same conditions as the factory-calibration field calibrations will not be required. Extremely low offset error and offset and gain drift errors are a by product of the chopping scheme. When operating with chop mode disabled, the user can achieve faster throughput times. An offset calibration is required with chop disabled when a gain or temperature change occurs. The following are the general programming steps required when calibrating a channel on the AD7708/AD7718.

Figure 16. Initializing AD7708/AD7718

–32–

REV. 0

AD7708/AD7718

POLL RDY PIN

WRITE TO ADCCON SELECTING CHANNEL N AND ITS OPERATING RANGE

END

WRITE TO COMMUNICATIONS REGISTER SETTING UP NEXT OPERATION TO BE A READ OF DATA REGISTER AND THEN READ DATA RESULT

WRITE TO MODE REGISTER SELECTING CONTINUOUS CONVERSION MODE

CYCLE THROUGH AND READ ALL CHANNELS

RDY

LOW?

YES

ALL

CHANNELS

READ

YES

INCREMENT CHANNEL ADDRESS (N = N+1)

1. Write to the ADCCON register to select the channel to be calibrated, its input range, and operation in unipolar or bipolar mode.

2. Write to the mode register selecting chop or nonchop mode of operation, select the reference, buffered/unbuffered operation on the AINCOM, and select zero-scale offset calibration. Zero-scale calibration can be either self-calibration, where the ADC determines the zero point internal to the ADC, or a system calibration where the user must supply the zero-scale voltage to the input for the duration of the calibration.

3. The calibration is initiated following the write to the mode register. The user then needs to determine when the calibration is complete. This can be performed in two ways, by polling the RDY pin or flag or by monitoring the MD2, MD1, MD0 bits in the mode register. These bits are reset to 0, 0, 1 when the calibration is complete. The flowchart uses polling of the mode bits in the mode register to determine when the calibration is complete.

4. The next step is to perform the full-scale calibration. Full-scale calibration can be a self-calibration or system calibration. Using system calibration the user must supply the full-scale signal to the analog inputs for the duration of the calibration. Again the MD2, MD1, MD0 bits in the mode register are monitored to determine when the calibration is complete.

Figure 18 shows a flowchart detailing the necessary programming steps required to cycle through and read data results from all channels in a multiplexed application. This flowchart assumes that all channels have been previously calibrated. The following are the general programming steps required when reading all channels in a multiplexed application.

1. The AD7708/AD7718 is put into continuous conversion mode. In this mode the part continually converts on the specified channel and the RDY line indicates when valid data is available to be read from the data register.

2. The ADCCON register is written to select the channel for conversion, its input range and operation is unipolar/ bipolar mode.

3. In this flowchart hardware polling of the RDY line is per- formed to determine when it is valid to read data from the converter. When RDY is low, valid data is available in the data register. The RDY line is set high on a channel change and will not go low until a new valid data word is available. Alternatively, the RDY bit in the status register can be polled in software to determine when to read data from the converter.

4. When the data is read, increment the channel address pointer

to select the next channel, poll the RDY pin, or RDY bit in the status register, and again read the data. Continue until all channels have been read.

Figure 18. Multichannel Read Operation

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–33–

AD7708/AD7718

DIGITAL INTERFACE

As previously outlined, the AD7708/AD7718’s programmable functions are controlled using a set of on-chip registers. Data is written to these registers via the part’s serial interface and read access to the on-chip registers is also provided by this interface. All communications to the part must start with a write operation to the Communications Register. After power-on or RESET, the device expects a write to its Communications Register. The data written to this register determines whether the next operation to the part is a read or a write operation and also determines to which register this read or write operation occurs. Therefore, write access to any of the other registers on the part starts with a write operation to the Communications Register followed by a write to the selected register. A read operation from any other register on the part (including the output data register) starts with a write operation to the Communications Register followed by a read operation from the selected register.

The AD7708/AD7718s serial interface consists of five signals, CS, SCLK, DIN, DOUT and RDY. The DIN line is used for transferring data into the on-chip registers while the DOUT line is used for accessing data from the on-chip registers. SCLK is the serial clock input for the device and all data transfers (either on DIN or DOUT) take place with respect to this SCLK signal. The RDY line is used as a status signal to indicate when data is ready to be read from the devices’s data register. RDY goes low when a new data word is available in the output register. It is reset high when a read operation from the data register is complete. It also goes high prior to the updating of the output register to indicate when not to read from the device to ensure that a data read is not attempted while the register is being updated. CS is used to select the device. It can be used to decode these devices in systems where a number of parts are connected to the serial bus.

Figures 2 and 3 show timing diagrams for interfacing to the AD7708/AD7718 with CS used to decode the part. Figure 3 is for a read operation from the AD7708/AD7718 output shift register while Figure 2 shows a write operation to the input shift register. It is possible to read the same data twice from the output register even though the RDY line returns high after the first read operation. Care must be taken, however, to ensure that the read operations have been completed before the next output update is about to take place.

The serial interface can operate in three-wire mode by tying the CS input low. In this case, the SCLK, DIN and DOUT lines are used to communicate with the device and the status of the RDY bit can be obtained by interrogating the STATUS Register. This scheme is suitable for interfacing to microcontrollers. If CS is required as a decoding signal, it can be generated from a port bit. For microcontroller interfaces, it is recommended that the SCLK idles high between data transfers.

The AD7708/AD7718 can also be operated with CS used as a frame synchronization signal. This scheme is suitable for DSP interfaces. In this case, the first bit (MSB) is effectively clocked out by CS since CS would normally occur after the falling edge of SCLK in DSPs. The SCLK can continue to run between data transfers provided the timing numbers are obeyed.

The serial interface can be reset by exercising the RESET input on the part. It can also be reset by writing a series of 1s on the DIN input. If a Logic 1 is written to the AD7708/AD7718 DIN line for at least 32 serial clock cycles, the serial interface is reset. This ensures that in three-wire systems, if the interface is lost

either via a software error or by some glitch in the system, it can be reset back to a known state. This state returns the interface to where the ADC is expecting a write operation to its Communications Register. This operation resets the contents of all registers to their power-on-reset values.

Some microprocessor or microcontroller serial interfaces have a single serial data line. In this case, it is possible to connect the ADC’s DOUT and DIN lines together and connect them to the single data line of the processor. A 10 kΩ pull-up resistor should be used on this single data line. In this case, if the interface is lost, because the read and write operations share the same line, the procedure to reset it back to a known state is somewhat different than previously described. It requires a read operation of 24 serial clocks followed by a write operation where a Logic 1 is written for at least 32 serial clock cycles to ensure that the serial interface is back into a known state.

MICROCOMPUTER/MICROPROCESSOR INTERFACING

The flexible serial interface allows for easy interface to most microcomputers and microprocessors. The flowcharts of Figures 16, 17, and 18 outline the sequence that should be followed when interfacing a microcontroller or microprocessor to the AD7708/AD7718. Figures 19, 20, and 21 show some typical interface circuits.

The serial interface on the AD7708/AD7718 is capable of operating from just three wires and is compatible with SPI interface protocols. The three-wire operation makes the part ideal for isolated systems where minimizing the number of interface lines minimizes the number of opto-isolators required in the system. The serial clock input is a Schmitt-triggered input to accommodate slow edges from optocouplers. The rise and fall times of other digital inputs to the AD7708/AD7718 should be no slower than 1 µs.

Most of the registers on the AD7708/AD7718 are 8-bit registers, which facilitates easy interfacing to the 8-bit serial ports of microcontrollers. The Data Register on the AD7718 is 24 bits wide, the ADC data register on the AD7708 is 16 bits wide, and the offset and gain registers are 16-bit registers on the AD7708 and 24-bit registers on the AD7718; however, data transfers to these registers can consist of multiple 8-bit transfers to the serial port of the microcontroller. DSP processors and microprocessors generally transfer 16 bits of data in a serial data operation. Some of these processors, such as the ADSP-2105, have the facility to program the amount of cycles in a serial transfer. This allows the user to tailor the number of bits in any transfer to match the register length of the required register in the AD7708/ AD7718.

Even though some of the registers on the AD7708/AD7718 are only eight bits in length, communicating with two of these registers in successive write operations can be handled as a single 16-bit data transfer if required. For example, if the Filter Register is to be updated, the processor must first write to the Communications Register (saying that the next operation is a write to the Filter Register) and then write eight bits to the Filter Register. If required, this can all be done in a single 16-bit transfer because once the eight serial clocks of the write operation to the Communications Register have been completed, the part immediately sets itself up for a write operation to the Filter Register.

–34–

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AD7708/AD7718

AD7708/AD7718 to 68HC11 Interface

Figure 19 shows an interface between the AD7708/AD7718 and the 68HC11 microcontroller. The diagram shows the minimum (3-wire) interface with CS on the AD7708/AD7718 hardwired low. In this scheme, the RDY bit of the Status Register is monitored to determine when the Data Register is updated. An alternative scheme, which increases the number of interface lines to four, is to monitor the RDY output line from the AD7708/AD7718. The monitoring of the RDY line can be done in two ways. First, RDY can be connected to one of the 68HC11’s port bits (such as PC0), which is configured as an input. This port bit is then polled to determine the status of RDY. The second scheme is to use an interrupt driven system, in which case the RDY output is connected to the IRQ input of the 68HC11. For interfaces that require control of the CS input on the AD7708/AD7718, one of the port bits of the 68HC11 (such as PC1), which is configured as an output, can be used to drive the CS input.

68HC11

SCK

MISO

MOSI

AD7708/

AD7718

RESET

SCLK

DOUT

DIN

AD7708/AD7718-to-8051 Interface

An interface circuit between the AD7708/AD7718 and the 8XC51 microcontroller is shown in Figure 20. The diagram shows the minimum number of interface connections with CS on the AD7708/AD7718 hardwired low. In the case of the 8XC51 interface the minimum number of interconnects is just two. In this scheme, the RDY bit of the Status Register is monitored to determine when the Data Register is updated. The alternative scheme, which increases the number of interface lines to three, is to monitor the RDY output line from the AD7708/AD7718. The monitoring of the RDY line can be done in two ways. First, RDY can be connected to one of the 8XC51’s port bits (such as P1.0) which is configured as an input. This port bit is then polled to determine the status of RDY. The second scheme is to use an interrupt-driven system, in which case the RDY output is connected to the INT1 input of the 8XC51. For interfaces that require control of the CS input on the AD7708/AD7718, one of the port bits of the 8XC51 (such as P1.1), which is configured as an output, can be used to drive the CS input. The 8XC51 is configured in its Mode 0 serial interface mode. Its serial interface contains a single data line. As a result, the DOUT and DIN pins of the AD7708/AD7718 should be connected together with a 10 kΩ pull-up resistor. The serial clock on the 8XC51 idles high between data transfers. The 8XC51 outputs the LSB first in a write operation, while the AD7708/AD7718 expects the MSB first so the data to be transmitted has to be rearranged before being written to the output serial register. Similarly, the AD7708/AD7718 outputs the MSB first during a read operation while the 8XC51 expects the LSB first. Therefore, the data read into the serial buffer needs to be rearranged before the correct data word from the AD7708/ AD7718 is available in the accumulator.

Figure 19. AD7708/AD7718-to-68HC11 Interface

The 68HC11 is configured in the master mode with its CPOL bit set to a Logic 1 and its CPHA bit set to a Logic 1. When the 68HC11 is configured like this, its SCLK line idles high between data transfers. The AD7708/AD7718 is not capable of full duplex operation. If the AD7708/AD7718 is configured for a write operation, no data appears on the DOUT lines even when the SCLK input is active. Similarly, if the AD7708/AD7718 is configured for a read operation, data presented to the part on the DIN line is ignored even when SCLK is active.

8XC51

P3.0

P3.1

10k⍀

AD7708/

AD7718

RESET

DOUT

DIN

SCLK

Figure 20. AD7708/AD7718-to-8XC51 Interface

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–35–

AD7708/AD7718

AD7708/AD7718-to-ADSP-2103/ADSP-2105 Interface

Figure 21 shows an interface between the AD7708/AD7718 and the ADSP-2103/ADSP-2105 DSP processor. In the interface shown, the RDY bit of the Status Register is again monitored to determine when the Data Register is updated. The alternative scheme is to use an interrupt-driven system, in which case the RDY output is connected to the IRQ2 input of the ADSP-2103/ ADSP-2105. The serial interface of the ADSP-2103/ADSP-2105 is set up for alternate framing mode. The RFS and TFS pins of the ADSP-2103/ADSP-2105 are configured as active low outputs and the ADSP-2103/ADSP-2105 serial clock line, SCLK, is also configured as an output. The CS for the AD7708/AD7718 is active when either the RFS or TFS outputs from the ADSP-2103/ ADSP-2105 are active. The serial clock rate on the ADSP2103/ADSP-2105 should be limited to 3 MHz to ensure correct operation with the AD7708/AD7718.

ADSP-2103/

ADSP-2105

SCLK

RFS

TFS

AD7708/

AD7718

RESET

DOUT

DIN

SCLK

Figure 21. AD7708/AD7718-to-ADSP-2103/ADSP-2105 Interface

BASIC CONFIGURATION

The basic connection diagram for the AD7708/AD7718 in 10channel mode is shown in Figure 22. This shows both the AV

and DVDD pins of the converters being driven from the analog 5 V supply. Some applications will have AV from separate supplies. AV

and DVDD can be operated inde-

and DVDD driven

pendently of each other, allowing the device to be operated with 5 V analog supply and 3 V digital supply or vice versa. The parts can be operated in 8- or 10-channel configurations. In 8-channel mode the user has two reference input options. This allows the user to operate some channels in ratiometric mode and others in absolute measurement mode. In 10-channel mode only one reference option is available. An AD780/REF195, precision 2.5 V reference, provides the reference source for the part. A quartz crystal or ceramic resonator provides the 32.768 kHz master clock source for the part. In some cases, it will be necessary to connect capacitors on the crystal or resonator to ensure that it does not oscillate at overtones of its fundamental operating frequency. The values of capacitors will vary, depending on the manufacturer’s specifications.

ANALOG 5V

SUPPLY

ANALOG 5V

SUPPLY

10␮F

OUT

10␮F

0.1␮F

AIN1

AIN2

AIN3

AIN4

AIN5

AIN6

AIN7

AIN8

AIN9

AIN10

AINCOM

REF1IN(+)

REF1IN(–)

0.1␮F

AD7708/

AD7718

MCLKIN

MCLKOUT

0.1␮F

RESET

DOUT

DIN

SCLK

CHIP SELECT

RECEIVE (READ)

SERIAL DATA (WRITE)

SERIAL CLOCK

32kHz CRYSTAL

–36–

AD780/ REF195

GND

AGND DGND

Figure 22. Basic Configuration for 10-Channel Mode

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AD7708/AD7718

Analog Input Channels

The input multiplexer on AD7708/AD7718 can be configured as either an 8- or 10-input channel device. This configuration is selected using the CHCON bit in the MODE register. With CHCON = 0 (Figure 23), the user has eight input channels; these can be configured as eight pseudo-differential input channels with respect to AINCOM or four fully-differential input channels.

In this configuration the user can select REFIN1 or REFIN 2 as the reference for the selected channel using the REFSEL bit in the mode register.

REFIN1(+)

REFIN1(–)

AIN1

AIN2

AIN3

AIN4

AIN5

AIN6

AIN7

AIN8

AINCOM

REFIN2(+)

REFIN2(–)

REFIN1(+)

REFIN1(–)

AIN1

AIN2

AIN3

AIN4

AIN5

AIN6

AIN7

AIN8

AINCOM

REFIN2(+)

REFIN2(–)

AD7708/

AD7718

Figure 23. Analog Input and Reference Options with CHCON = 0

With CHCON = 1 (Figure 24), the user has 10 input channels that can be configured as 10 pseudo-differential input channels with respect to AINCOM or as five fully-differential input channels. The contents of the CHCON bit overrides the REFSEL bit. If the ADC is configured in five fully-differential or 10 pseudodifferential input channel mode, the REFSEL bit setting is irrelevant as only REFIN1 is available. Channel selection Bits CH3, CH2, CHI, and CH0 in the ADCCON register select the input channel.

The input multiplexer switches the selected input channel to the on-chip buffer amplifier and sigma-delta converter. When the analog input channel is switched, the settling time of the part must elapse before a new valid word is available from the ADC. If any two inputs are configured as a differential input pair, this input is buffered and the common-mode and absolute input voltage is restricted to a range between AGND + 100 mV and AV

– 100 mV. Care must be taken in setting up the common-mode voltage and input voltage range to ensure that these limits are not exceeded, otherwise there will be a degradation in linearity and noise performance.

REFIN1(+)

REFIN1(–)

AIN1

AIN2

AIN3

AIN4

AIN5

AIN6

AIN7

AIN8

AIN9

AIN10

AINCOM

REFIN1(+)

REFIN1(–)

AIN1

AIN2

AIN3

AIN4

AIN5

AIN6

AIN7

AIN8

AIN9

AIN10

AINCOM

AD7708/

AD7718

Figure 24. Analog Input and Reference Options with CHCON = 1

Single-Ended Operation

The NEGBUF bit in the mode register is used to control the operation of the input buffer on the AINCOM pin when configured for pseudo-differential mode of operation. If cleared, the analog negative input (AINCOM) is unbuffered. It should be noted that the unbuffered input path on the AINCOM provides a dynamic load to the driving source. Therefore, resistor/capacitor combinations on this input pin can cause dc gain errors depending on the output impedance of the source that is driving the AINCOM input. AINCOM is tied to AGND for single-ended operation. This enables all pseudo-differential inputs to act as single-ended analog inputs. All analog inputs still operate in buffered mode and their common-mode and absolute input voltage is restricted to a range between AGND + 100 mV and

– 100 mV.

Chop Mode of Operation (CHOP = 0)

The signal chain on the AD7708/AD7718 can be operated with chopping enabled or disabled. Chopping is enabled or disabled using the CHOP bit in the mode register. The default mode of operation is for chop enabled (CHOP = 0). Optimum performance in terms of minimizing offset error and offset and gain drift performance is achieved when chopping is enabled. The digital filter decimation rate, and consequently the output data rate, is programmable via the SF word loaded to the filter register. Output data rates vary from 5.35 Hz (186.77 ms) to 105.03 Hz

(9.52 ms). The output data rate

ADC

The overall frequency response from the digital filter with chopping enabled is the product of a sinc

are sinc

notches at integer multiples of 3 × f

sinc notches at odd integer multiples of f

MOD

and a sinc response. There

ADC

×24

and there are

ADC

/2. Normal mode

rejection is the major function of the digital filter on the AD7708/ AD7718. The normal mode 50 ± 1 Hz rejection with an SF word of 82 is typically –100 dB. The 60 ± 1 Hz rejection with SF = 68 is typically –100 dB. Simultaneous 50 Hz and 60 Hz rejection of better than 60 dB is achieved with an SF of 69 and gives a data update rate of 19.8 Hz and a channel settling time of 101 ms. The AD7708/AD7718 are factory-calibrated so field calibration will only be required if the ADC is operated at temperatures that differ substantially from the factory-calibration conditions.

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–37–

AD7708/AD7718

Nonchop Mode of Operation (CHOP = 1)

Chopping is enabled and disabled using the CHOP bit in the mode register. Chopping is disabled by loading a 1 to the chop bit in the mode register. With chopping disabled the available output rates vary from 16.06 Hz (62.26 ms) to 1365.33 Hz (0.73 ms). The range of applicable SF words is from 3 to 255. When the chopping is disabled the channel output data rate is increased by a factor of 3 compared to the situation when chopping is enabled and using the same SF word. When used in multiplexed applications, operation with chop disabled will offer the best throughput time when cycling through all channels. The drawback with chop disabled is that the drift performance is degraded and calibration is required following a gain change or significant temperature change. The output update and filter decimation rate is again controlled by the SF word loaded to the filter register. The digital filter frequency response places sinc

notches at integer multiples of the update rate. The output

update rate

ADC

MOD

. The AD7708/AD7718 are targeted

×8

at multiplexed applications and therefore operating with chop disabled optimizes channel throughput time. One of the key requirements in these applications is the selection of an SF word so as to obtain the maximum filter rejection at 50 Hz and 60 Hz while minimizing the channel throughput rate. This is achieved with an SF word of 75 giving 57 dB rejection at 50 Hz, and 60 dB rejection at 60 Hz while offering a channel throughput time of 55 ms. Using a higher SF word of 151, 50 Hz and 60 Hz rejection can be maximized at 60 dB with a channel throughput rate of 110 ms. An SF word of 255 gives maximum rejection at both 50 Hz and 60 Hz but the channel throughput rate is restricted to 186 ms. Table XXI shows a quick comparison of normal mode 50 Hz and 60 Hz rejection, settling time, and update rate for a selection of SF words with chop both enabled and disabled.

Programmable Gain Amplifier

The output from the buffer is applied to the input of the programmable gain amplifier (PGA). The PGA gain range is programmed via the range bits in the ADCCON register. The PGA has eight ranges. With an external 2.5 V reference applied, and a PGA setting of 128, the unipolar analog input range is 0 mV to 20 mV, while the bipolar analog input range is ±20 mV. With a PGA setting of 1, the unipolar and bipolar input ranges are 2.56 V. When operating with chop mode enabled (CHOP = 0), the ADC range-matching specification of 2 µV (typ) across all ranges means that calibration need only be carried out on a single range and does not have to be repeated when the PGA range is changed. This is a significant advantage when compared with similar ADCs available on the market. Typical matching across ranges is shown in Figure 25. Here, the ADC is configured in fully-differential, bipolar mode with an external 2.5 V reference, while an analog input voltage of just greater than 19 mV is forced on its analog inputs. The ADC continuously converts the dc voltage at an update rate of 5.35 Hz, i.e., SF = FFhex, 800 conversion results in total are gathered. The first 100 results are gathered with the ADC operating with a PGA setting of 128.

19.372

19.371

19.370

19.369

19.368

19.367

ADC INPUT VOLTAGE – mV

19.366

19.365

19.364

ADC RANGE

1000 200 400300

20mVⴞ40mVⴞ80mV

ⴞ

SAMPLE COUNT

160mVⴞ320mVⴞ640mV

ⴞ

600 700 800

500

1.28Vⴞ2.56V

ⴞ

Figure 25. ADC Range Matching

The PGA setting is then switched to 64 and 100 more results are gathered, and so on until the last 100 samples are gathered with a PGA setting of 1. From Figure 25, the variation in the sample mean through each range, i.e., the range matching, is seen to be of the order of 2 µV. When operating with chop mode disabled (CHOP = 1), new calibration data is needed (but not necessarily a new calibration) to remove offset error when switching channels.

Bipolar/Unipolar Configuration

The analog inputs on the AD7708/AD7718 can accept either unipolar or bipolar input voltage ranges. Bipolar input ranges does not imply that the part can handle negative voltages with respect to system AGND. Signals in pseudo-differential mode are referenced to AINCOM, while in fully differential mode they are referenced to the negative input of the differential input. For example, if AINCOM is 2.5 V and the AD7708/AD7718 AIN1 analog input is configured for an analog input range of 0 mV to +20 mV, the input voltage range on the AIN1 input is 2.5 V to

2.52 V. If AINCOM is 2.5 V and the AD7708/AD7718 is configured for an analog input range of ±1.28 V, the analog input range on the AIN1 input is 1.22 V to 3.78 V (i.e., 2.5 V ±

1.28 V). Bipolar or unipolar options are chosen by programming U/B bit in the ADCCON register. Programming for either unipolar or bipolar operation does not change any of the input signal conditioning; it simply changes the data output coding and the points on the transfer function where calibrations occur.

Data Output Coding

When the AD7718 is configured for unipolar operation, the output coding is natural (straight) binary with a zero differential input voltage resulting in a code of 000 . . . 000, a midscale voltage resulting in a code of 100 . . . 000, and a full-scale input voltage resulting in a code of 111 . . . 111. The output code for any analog input voltage can be represented as follows:

Code = (AIN × GAIN × 2

)/(1.024 × V

REF

)

where

AIN is the analog input voltage, GAIN is the PGA gain, i.e., 1 on the 2.5 V range and 128 on

the 20 mV range.

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AD7708/AD7718

The output code for any analog input voltage on the AD7708 can be represented as follows:

Code = (AIN × GAIN × 2

)/(1.024 × V

REF

)

where

AIN is the analog input voltage, GAIN is the PGA gain, i.e., 1 on the 2.5 V range and 128 on

the 20 mV range.

When an ADC is configured for bipolar operation, the coding is offset binary with a negative full-scale voltage resulting in a code of 000 . . . 000, a zero differential voltage resulting in a code of 100 . . . 000, and a positive full-scale voltage resulting in a code of 111 . . . 111. The output code from the AD7718 for any analog input voltage can be represented as follows:

Code = 2

× [(AIN × GAIN/(1.024 × V

REF

)) + 1]

where

AIN is the analog input voltage, GAIN is the PGA gain, i.e., 1 on the ±2.5 V range and 128 on

the ±20 mV range.

The output code from the AD7708 for any analog input voltage can be represented as follows:

Code = 2

× [(AIN × GAIN/(1.024 × V

REF

)) + 1]

where

AIN is the analog input voltage, GAIN is the PGA gain, i.e., 1 on the ±2.5 V range and 128 on

the ±20 mV range.

Oscillator Circuit

The AD7708/AD7718 is intended for use with a 32.768 kHz watch crystal or ceramic resonator. A PLL internally locks onto a multiple of this frequency to provide a stable 4.194304 MHz clock for the ADC. The modulator sample rate is the same as the oscillator frequency.

The start-up time associated with 32 kHz crystals is typically 300 ms. The OSPD bit in the mode register can be used to prevent the oscillator from powering down when the AD7708/ AD7718 is placed in power-down mode. This avoids having to wait 300 ms after exiting power-down to start a conversion at the expense of raising the power-down current.

Reference Input

The AD7708/AD7718 has a fully differential reference input capability. When the AD7708/AD7718 is configured in 8-channel mode (CHCON = 0) the user has the option of selecting one of two reference options. This allows the user to configure some channels, for example, for ratiometric operation while others can be configured for absolute value measurements. The REFSEL bit in the mode register allows selection of the required reference. If the REFSEL bit is cleared, the reference selected is REFIN1(+) –REFIN1(–) for the active channel. If this bit is set, the reference selected is REFIN2(+) – REFIN2(–) for the active channel. When the AD7708/AD7718 is configured in 10-channel mode (CHCON = 1) the user has only one reference option (REFIN1). The contents of the CHCON bit overrides the REFSEL bit. If

the ADC is configured in five fully-differential or 10 pseudodifferential input channel mode, the REFSEL bit setting is irrelevant as only one reference input is available.

The common-mode range for these differential inputs is from AGND to AV

. The reference inputs are unbuffered and

therefore excessive R-C source impedances will introduce gain errors. The nominal reference voltage for specified operation, VREF, (REFIN1(+)–REFIN1(–) or REFIN2(+)–REFIN2(–)), is 2.5 V, but the AD7708/AD7718 is functional with reference voltages from 1 V to AV

. In applications where the excitation

(voltage or current) for the transducer on the analog input also drives the reference voltage for the part, the effect of the low frequency noise in the excitation source will be removed as the application is ratiometric. If the AD7708/AD7718 is used in a nonratiometric application, a low noise reference should be used. Recommended reference voltage sources for the AD7708/ AD7718 include the AD780, REF43, and REF192. It should also be noted that the reference inputs provide a high impedance, dynamic load. Because the input impedance of each reference input is dynamic, resistor/capacitor combinations on these inputs can cause dc gain errors, depending on the output impedance of the source that is driving the reference inputs. Reference voltage sources, like those recommended above (e.g., AD780) will typically have low output impedances and are therefore tolerant of having decoupling capacitors on the REFIN(+) without introducing gain errors in the system. Deriving the reference input voltage across an external resistor will mean that the reference input sees a significant external source impedance. External decoupling on the REFIN(+) and REFIN(–) pins would not be recommended in this type configuration.

RESET Input

The RESET input on the AD7708/AD7718 resets all the logic, the digital filter and the analog modulator while all on-chip registers are reset to their default state. RDY is driven high and the AD7708/AD7718 ignores all communications to any of its registers while the RESET input is low. When the RESET input returns high the AD7708/AD7718 operates with its default setup conditions and it is necessary to set up all registers and carry out a system calibration if required after a RESET command.

Power-Down Mode

Loading 0, 0, 0 to the MD2, MD1, MD0 bits in the ADC mode register places the ADC in device power-down mode. Device power-down mode is the default condition for the AD7708/ AD7718 on power-up. The ADC retains the contents of all its on-chip registers (including the data register) while in powerdown. The device power-down mode does not affect the digital interface, but does affect the status of the RDY pin. Writing the AD7708/AD7718 into power-down will reset the RDY line high. Placing the part in power-down mode reduces the total current

+ DIDD) to 31 µA max when the part is operated at 5 V

(AI

and the oscillator allowed to run during power-down mode. With the oscillator shut down the total I

is typically 9 µA.

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–39–

AD7708/AD7718

Calibration

The AD7708/AD7718 provides four calibration modes that can be programmed via the mode bits in the mode register. One of the major benefits of the AD7708/AD7718 is that it is factorycalibrated with chopping enabled as part of the final test process with the generated coefficients stored within the ADC. At poweron, the factory gain calibration coefficients are automatically loaded to the gain calibration registers on the AD7708/AD7718. This gives excellent offset and drift performance and it is envisaged that in the majority of applications the user will not need to perform any field calibrations. gain calibration coefficients (generated at 25°C ambient) are automatically present at power-on, an internal full-scale calibration will only be required if the part is being operated at temperatures significantly different from 25°C.

When chopping is disabled (CHOP =1) the AD7708/AD7718 requires an offset calibration or new calibration coefficients on range changing or when significant temperature changes occur as the signal chain is no longer chopped and offset and drift errors are no longer removed as part of the conversion process.

The factory-calibration values for any one channel will be overwritten if any one of the four calibration options is initiated. The AD7708/AD7718 offers “internal” or “system” calibration facilities. For full calibration to occur, the calibration logic must record the modulator output for two different input conditions. These are “zero-scale” and “full-scale” points. These points are derived by performing a conversion on the different input voltages provided to the input of the modulator during calibration. The result of the “zero-scale” calibration conversion is stored in the Offset Calibration Registers for the appropriate channel. The result of the “full-scale” calibration conversion is stored in the Gain Calibration Registers. With these readings, the calibration logic can calculate the offset and the gain slope for the input-to-output transfer function of the converter. During an “internal” zero-scale or full-scale calibration, the respective “zero” input and “full-scale” input are automatically connected to the ADC input pins internally to the device. A “system” calibration, however, expects the system zero-scale and system full-scale voltages to be applied to the external ADC pins before the calibration mode is initiated. In this way external ADC errors are taken into account and minimized as a result of system calibration. It should also be noted that to optimize calibration accuracy, all AD7708/AD7718 ADC calibrations are carried out automatically at the slowest update rate with chop enabled. When chop mode is disabled calibrations are carried out at the update rate defined by the SF word in the filter register.

Internally in the AD7708/AD7718, the coefficients are normalized before being used to scale the words coming out of the digital filter. The offset calibration coefficient is subtracted from the result prior to the multiplication by the gain coefficient. With chopping disabled AD7708/AD7718 ADC specifications will only apply after a zero-scale calibration at the operating point of interest. From an operational point of view, a calibration should be treated like another ADC conversion. A zero-scale calibration (if required) should always be carried out before a full-scale

Also, because factory

calibration. System software should monitor the RDY bit in the STATUS register to determine end of calibration via a polling sequence or interrupt driven routine.

Grounding and Layout

Since the analog inputs and reference inputs are differential, most of the voltages in the analog modulator are common-mode voltages. The excellent common-mode rejection of the part will remove common-mode noise on these inputs. The analog and digital supplies to the AD7708/AD7718 are independent and separately pinned out to minimize coupling between the analog and digital sections of the device. The AD7708/AD7718 can be operated with 5 V analog and 3 V digital supplies or vice versa. The digital filter will provide rejection of broadband noise on the power supplies, except at integer multiples of the modulator sampling frequency. The digital filter also removes noise from the analog and reference inputs provided these noise sources do not saturate the analog modulator. As a result, the AD7708/ AD7718 is more immune to noise interference than a conventional high-resolution converter. However, because the resolution of the AD7708/AD7718 is so high and the noise converter so low, care must be taken with regard to grounding and layout.

The printed circuit board that houses the ADC should be designed so the analog and digital sections are separated and confined to certain areas of the board. This facilitates the use of ground planes that can be easily separated. A minimum etch technique is generally best for ground planes as it gives the best shielding.

Although the AD7708/AD7718 has separate pins for analog and digital ground, the AGND and DGND pins are tied together internally via the substrate. Therefore, the user must not tie these two pins to separate ground planes unless the ground planes are connected together near the AD7708/AD7718.

In systems where the AGND and DGND are connected somewhere else in the system, i.e., the systems power supply, they should not be connected again at the AD7708/AD7718 or a ground loop will result. In these situations it is recommended that ground pins of the AD7708/AD7718 be tied to the AGND plane.

In any layout it is implicit that the user keep in mind the flow of currents in the system, ensuring that the paths for all currents are as close as possible to the paths the currents took to reach their destinations. Avoid forcing digital currents to flow through the AGND.

Avoid running digital lines under the device as these will couple noise onto the die. The analog ground plane should be allowed to run under the AD7708/AD7718 to prevent noise coupling. The power supply lines to the AD7708/AD7718 should use as wide a trace as possible to provide low impedance paths and reduce the effects of glitches on the power supply line. Fast switching signals like clocks should be shielded with digital ground to avoid radiating noise to other sections of the board and clock signals should never be run near the analog inputs. Avoid crossover of digital and analog signals. Traces on opposite sides of the board should run at right angles to each other. This will reduce the effects of feedthrough through the board. A microstrip technique is by far the best, but is not always possible with a double-sided board. In this technique, the component side of the board is dedicated to ground planes while signals are placed on the solder side.

levels from the

–40–

REV. 0

AD7708/AD7718

AD780

AIN6

AIN5

AIN4

AIN3

AIN2

AIN1

GND

OUT

AGND

DGND

REF1IN(–)

MCLKIN

MCLKOUT

32kHz

AD7708/

AD7718

AIN8

AIN7

AIN9

AINCOM

AIN10

MICRO-

CONTROLLER

SCLK

DIN

DOUT

RDY

RESET

REFIN1(+)

Good decoupling is important when using high resolution ADCs. All analog supplies should be decoupled with 10 µF tantalum in parallel with 0.1 µF capacitors to AGND. To achieve the best from these decoupling components, they have to be placed as close as possible to the device, ideally right up against the device. All logic chips should be decoupled with 0.1 µF ceramic capacitors to DGND. In systems where a common supply voltage is used to drive both the AV AD7718, it is recommended that the system’s AV

and DVDD of the AD7708/

supply is

used. This supply should have the recommended analog supply decoupling capacitors between the AV

pin of the AD7708/

AD7718 and AGND and the recommended digital supply decoupling capacitor between the DV

pin of the AD7708/

AD7718 and DGND.

APPLICATIONS

The AD7708/AD7718 provides a low cost, high resolution analog-to-digital function. The AD7708 offers 16-bit resolution while the AD7718 offers 24-bit resolution. The AD7708 and AD7718 are pin and function compatible. The AD7718 allows a direct upgradable path from a 16-bit to a 24-bit system with

The buffer on the negative analog input can be bypassed allowing the AD7708/AD7718 be operated as eight or ten single-ended input channels. The PGA allows the user to connect transducers directly to the input of the AD7708/AD7718. The programmable gain front end on the AD7708/AD7718 allows the part to handle unipolar analog input ranges from 0 mV to +20 mV to 0 V to +2.5 V and bipolar inputs of ±20 mV to ±2.5 V. Because the part operates from a single supply these bipolar ranges are with respect to a biased-up differential input.

Data Acquisition

The AD7708/AD7718, with its different configuration options (five fully-differential input or 10 pseudo-differential input channels with one reference input or four fully-differential input or eight pseudo-differential input channels with two reference inputs), is suited to low bandwidth, high resolution data acquisition systems. In addition, the 3-wire digital interface allows this data acquisition front end to be isolated with just three optoisolators. The entire system can be operated from a single 3 V or 5 V supply, provided that the input signals to the AD7708/ AD7718’s analog inputs are all of positive polarity.

minimal software and no hardware changes. Because the analogto-digital function is provided by a sigma-delta architecture, it makes the part more immune to noisy environments, thus making the part ideal for use in sensor measurement and in industrial and process control applications. There are two modes of operation associated with the AD7708/AD7718, chop enabled (CHOP = 0) or chop disabled (CHOP = 1). With chop enabled the signal chain is chopped and the device is factory-calibrated at final test in this mode. Field calibration can be avoided due to the extremely low offset and gain drifts exhibited by the converter in this mode. While operating in this mode gives optimum performance in terms of offset error and offset and gain drift performance, it offers limited throughput when cycling through all channels. With chopping disabled, the signal chain is not chopped and therefore the user needs to ensure that the ADC is calibrated on range changes and if there is a significant temperature change as the gain and offset drift performance is degraded.

The key advantage in using the AD7708/AD7718 with chopping disabled is in channel cycling applications where system throughput is of prime importance. The max conversion rate with chop disabled is 1.36 kHz compared with 105 Hz with chop enabled.

The AD7708/AD7718 also provides a programmable gain amplifier, a digital filter, and system calibration options. Thus, it provides far more system level functionality than off-the-shelf integrating ADCs without the disadvantage of having to supply a high quality integrating capacitor. In addition, using the AD7708/AD7718 in a system allows the system designer to achieve a much higher level of resolution because noise performance of the AD7708/AD7718 is significantly better than that of integrating ADCs.

The on-chip PGA allows the AD7708/AD7718 to handle an analog input voltage range as low as 10 mV full scale with V = 1.25 V. The AD7708/AD7718 can be operated in 8-channel mode with two reference input options or 10-channel mode with one reference input. Eight-channel mode allows both ratiometric or absolute measurements to be performed on any channel using the two reference input options. The differential analog inputs of the part allow this analog input range to have an absolute value anywhere between AGND + 100 mV and AV

REV. 0

– 100 mV.

REF

Figure 26. Data Acquisition Using the AD7708/AD7718

Programmable Logic Controllers

The AD7708/AD7718 is also suited to programmable logic controller applications. In such applications, the ADC is required to handle signals from a variety of different transducers. The AD7708/AD7718’s programmable gain front end allows the part to either handle low level signals directly from a transducer or full-scale signals that have already been conditioned. The faster throughput rate and settling time of the part when operated with chopping disabled makes this the optimum mode of operation in PLC applications as an important feature in these applications is loop response time. The configuration of the AD7708/AD7718 in PLC applications is similar to that outlined for a data acquisition system and is shown in Figure 26. In this application the AD7708/AD7718 is configured in 10-channel mode, (CHCON = 1) and can be operated as 10 pseudo-differential inputs with respect to AINCOM or as five fully-differential input channels.

–41–

AD7708/AD7718

Converting Single-Ended Inputs

The AD7708/AD7718 generally operates in buffered mode. This places a restriction of AGND + 100 mV to AVDD – 100 mV on the absolute and common-mode voltages that can be applied to any input on the AD7708/AD7718.

Some applications may require the measurement of analog inputs with respect to AGND. To enable the AD7708/AD7718 to be used in these single-ended applications, the buffer on the AINCOM can be bypassed. The NEGBUF bit in the mode register controls the operation of the input buffer on the AINCOM input when a channel is configured for pseudo-differential mode of operation. If cleared, the analog negative input (AINCOM) is unbuffered, allowing it to be tied to AGND in single-ended input configuration. If this bit is set, the analog negative input (AINCOM) is buffered, placing a restriction on its commonmode input range. When AINCOM is unbuffered, signals with a common-mode range from AGND – 30 mV to AVDD + 30 mV can be accommodated on this input allowing the end user to connect the AINCOM input to AGND and perform singleended measurements with respect to this input. This unbuffered input path on the AINCOM provides a dynamic load to the driving source. Therefore, resistor/capacitor combinations on this input pin can cause dc gain errors, depending on the output impedance of the source that is driving the AINCOM input. All analog inputs still operate in buffered mode and their common-mode and absolute input voltage is restricted to a range between AGND + 100 mV and AV

– 100 mV.

Combined Ratiometric and Absolute Value Measurement System

The AD7708/AD7718 when operated with CHCON = 0 can be configured for operation as four fully-differential analog inputs or eight pseudo-differential analog inputs with two fully-differential reference inputs. Having the ability to use either REFIN1 or REFIN2 with any channel during the conversion process allows the end user to make both absolute and ratiometric measurements as shown in Figure 27. In this example a fully-differential analog input (AIN1–AIN2) is being converted from a bridge transducer in a ratiometric manner using REFIN1 as the reference input for this channel. AIN3 is configured as a pseudo-differential input channel using REFIN2 to perform an absolute measurement on the potentiometer. The REFSEL bit in the mode register is used to select which reference is used with the active channel during the conversion process. When the AD7708/AD7718 is configured with CHCON = 1, only one reference (REFIN1) is available. The contents of the CHCON bit override the REFSEL bit. If the ADC is configured in five fully-differential or 10 pseudo-differential input channel mode, the REFSEL bit setting is irrelevant as only REFIN1 is available.

REFIN1(+)

AIN1

AIN2 REFIN1(–)

AD7718

EXCITATION VOLTAGE = 4V

WIPER

REFIN2(+)

AIN3

AINCOM

REFIN2(–)

GND

Figure 27. Absolute and Ratiometric Measurement System Using AD7718

–42–

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AD7708/AD7718

Optimizing Throughput while Maximizing 50 Hz and 60 Hz Rejection in a Multiplexed Data Acquisition System

The AD7708/AD7718 can be optimized for one of two modes of operation. Operating the AD7708/AD7718 with chopping enabled (CHOP = 0) optimizes the AD7708/AD7718 for analog performance over channel throughput. Output data rates vary from 5.35 Hz (186.77 ms) to 105.03 Hz (9.52 ms). Optimum performance, in terms of minimizing offset error and offset and gain drift performance, is achieved as a result of chopping the signal chain.

With chopping disabled the available output rates vary from

16.06 Hz (62.26 ms) to 1365.33 Hz (0.73 ms). The range of

applicable SF words is from 3 to 255. When the chopping is disabled the channel output data rate is increased by a factor of 3 compared to the situation when chopping is enabled is disabled. When used in multiplexed applications, operation

cycling through all channels. The drawback with chopping disabled is that the drift performance is degraded and calibration is required following gain and temperature changes. One of the key requirements in these applications is to optimize the SF word to obtain the maximum filter rejection at 50 Hz and 60 Hz while minimizing the channel throughput rate. This is achieved with an SF word of 75 giving 57 dB rejection at 50 Hz and 60 dB rejection at 60 Hz while offering a channel throughput time of 55 ms.

Using a higher SF word of 151, 50 Hz and 60 Hz rejection can be maximized at 60 dB with a channel throughput rate of 110 ms. An SF word of 255 gives maximum rejection at both 50 Hz and 60 Hz, but the channel throughput rate is restricted to 186 ms. Table XXI gives a quick comparison of normal mode 50 Hz and 60 Hz rejection, settling time, and update rate for a selection of SF words with both chopping enabled and disabled.

with chop disabled will offer the best throughput time when

Table XXI. Normal Mode 50 Hz and 60 Hz Rejection vs. Settling Time and Update Rate for a Selection of SF Words

CHOP Disabled CHOP Enabled

50 Hz ⴞ 1 Hz 60 Hz ⴞ 1 Hz 50 Hz ⴞ 1 Hz 60 Hz ⴞ 1 Hz

SF Rejection Rejection Rejection Rejection Word f

ADC

(Hz) t

(ms) (dB) (dB) f

SETTLE

ADC

(Hz) t

(ms) (dB) (dB)

SETTLE

03 1365.33 2.20 0.05 0.08 N/A N/A N/A N/A 13 315.08 9.52 1 1.5 105.03 19.04 21 13.6 69 59.36 50.54 42 94 19.79 101 60 94 75 54.6 54.93 57 60 18.2 110 62 66 82 49.95 60 100 16.65 180 100 53 151 27.13 110.6 60 60 9.04 221 72 63 255 16.06 186.76 77 68 5.35 373.5 93 68

REV. 0

–43–

AD7708/AD7718

OUTLINE DIMENSIONS

Dimensions shown in inches and (mm).

28-Lead Plastic SOIC

(R-28)

0.7125 (18.10)

0.6969 (17.70)

28 15

PIN 1

0.0500

0.0118 (0.30)

0.0040 (0.10)

28 15

PIN 1

0.006 (0.15)

0.002 (0.05)

(1.27)

BSC

0.0192 (0.49)

0.0138 (0.35)

0.386 (9.80)

0.378 (9.60)

0.2992 (7.60)

0.2914 (7.40)

0.1043 (2.65)

0.0926 (2.35)

SEATING PLANE

0.4193 (10.65)

0.3937 (10.00)

0.0125 (0.32)

0.0091 (0.23)

28-Lead Plastic TSSOP

(RU-28)

0.177 (4.50)

0.169 (4.30)

0.256 (6.50)

141

0.0433 (1.10)

0.246 (6.25)

MAX

0.0291 (0.74)

0.0098 (0.25)

8ⴗ 0ⴗ

C01831–1–7/01(0)

ⴛ 45ⴗ

0.0500 (1.27)

0.0157 (0.40)

SEATING

PLANE

0.0256 (0.65) BSC

0.0118 (0.30)

0.0075 (0.19)

–44–

0.0079 (0.20)

0.0035 (0.090)

8ⴗ 0ⴗ

0.028 (0.70)

0.020 (0.50)

PRINTED IN U.S.A.

REV. 0

Datasheet AD7718EB, AD7708EB, AD7718BRU, AD7708BRU, AD7708BR Datasheet (Analog Devices)

Specifications and Main Features

Frequently Asked Questions

User Manual

FUNCTIONAL BLOCK DIAGRAM

TABLE OF CONTENTS

TIMING CHARACTERISTICS

ORDERING GUIDE

CAUTION

–Typical Performance Characteristics

ADC CIRCUIT INFORMATION

ADC NOISE PERFORMANCE CHOP ENABLED (CHOP = 0)

ADC NOISE PERFORMANCE CHOP DISABLED (CHOP = 1)

ON-CHIP REGISTERS

AD7718 ADC (DATA)

AD7708 (DATA)

AD7718

AD7718

AD7708

DIGITAL INTERFACE

MICROCOMPUTER/MICROPROCESSOR INTERFACING

AD7708/AD7718-to-ADSP-2103/ADSP-2105 Interface

APPLICATIONS