Datasheet AD7738 Datasheet (Analog Devices)

8-Channel, High Throughput,

SCLK

DIN

DOUT

CS

RDY

RESET

SERIAL

INTERFACE

CONTROL

LOGIC

AIN4

AIN5

AIN6

AIN7

MUX

AINCOM/P0

AIN0

AIN1

AIN2

AIN3

DGND DV

DD

REFERENCE

DETECT

REFIN– REFIN+

I/O PORT

SYNC/P1

CLOCK

GENERATOR

MCLKINMCLKOUT

MUXOUT ADCIN

AGND AV

DD

24-BIT

- ADC

BUFFER

AD7738

CALIBRATION

CIRCUITRY

a

FEATURES
High Resolution ADC

24 Bits No Missing Codes
0.0015% Nonlinearity

Optimized for Fast Channel Switching

18-Bits p-p Resolution (21 Bits Effective) at 500 Hz
16-Bits p-p Resolution (19 Bits Effective) at 8.5 kHz
15-Bits p-p Resolution (18 Bits Effective) at 15 kHz
On-Chip Per Channel System Calibration

Configurable Inputs

8 Single-Ended or 4 Fully Differential

Input Ranges

+625 mV, +1.25 V, +2.5 V, 625 mV, 1.25 V, 2.5 V

3-Wire Serial Interface

SPI™, QSPI™, MICROWIRE™ and DSP Compatible
Schmitt Trigger on Logic Inputs

Single-Supply Operation

5 V Analog Supply
3 V or 5 V Digital Supply

Package: 28-Lead TSSOP

24-Bit - ADC

AD7738

FUNCTIONAL BLOCK DIAGRAM

APPLICATIONS
PLCs/DCS
Multiplexing Applications
Process Control
Industrial Instrumentation

GENERAL DESCRIPTION

The AD7738 is a high precision, high throughput analog front
end. True 16-bit p-p resolution is achievable with a total con­version time of 117 µs (8.5 kHz channel switching), making it
ideally suitable for high resolution multiplexing applications.

The part can be configured via a simple digital interface, which
allows users to balance the noise performance against data
throughput up to a 15.4 kHz.

The analog front end features eight single-ended or four fully
differential input channels with unipolar or bipolar 625 mV,

1.25 V, and 2.5 V input ranges and accepts a common-mode
input voltage from 200 mV above AGND to AVDD – 300 mV.
The multiplexer output is pinned out externally, allowing the
user to implement programmable gain or signal conditioning
before applying the input to the ADC.

SPI and QSPI are trademarks of Motorola, Inc.
MICROWIRE is a trademark of National Semiconductor Corporation

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.

The differential reference input features “No-Reference” detect
capability. The ADC also supports per channel system calibra­tion options.

The digital serial interface can be configured for 3-wire opera­tion and is compatible with microcontrollers and digital signal
processors. All interface inputs are Schmitt triggered.

The part is specified for operation over the extended industrial
temperature range of –40C to +105C.

Other parts in the AD7738 family are the AD7734 and the
AD7732.

The AD7734 analog front end features four single-ended input
channels with unipolar or true bipolar input ranges to ±10 V
while operating from a single 5 V analog supply. The AD7734
accepts an analog input overvoltage to ±16.5 V while not
degrading the performance of the adjacent channels.

The AD7732 is similar to AD7734, but its analog front end
features two fully differential input channels.

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: [phone]www.analog.com
Fax: [phone] © Analog Devices, Inc., 2002

AD7738–SPECIFICATIONS

(–40ⴗC to +105ⴗC, AVDD = 5 V ⴞ 5%, DVDD = 2.7 V to 3.6 V or 5 V ⴞ 5%,

REFIN(+) = 2.5 V, REFIN(–) = 0 V, AINCOM = 2.5 V, MUXOUT(+) = ADCIN(+), MUXOUT(–) = ADCIN(–), Internal Buffer ON, AIN Range = ⴞ1.25 V,
f

= 6.144 MHz; unless otherwise noted.)

MCLK

Parameter Min Typ Max Unit Test Conditions/Comment

ADC PERFORMANCE—
CHOPPING ENABLED

Conversion Time Rate 372 12190 Hz Configure via Conversion Time Register
No Missing Codes

1

24 Bits FW ≥ 6 (Conversion Time ≥ 165 µs)

See Typical Performance Characteristics
Output Noise See Table I
Resolution See Tables II and III
Integral Nonlinearity (INL) ± 0.0015 % of FSR AIN Range = ± 2.5 V

Offset Error (Unipolar, Bipolar)
Offset Drift vs. Temperature
Gain Error

2

Gain Drift vs. Temperature
Positive Full-Scale Error
Positive Full-Scale Drift vs. Temperature
Bipolar Negative Full-Scale Error

2

1

1

2

1

3

± 0.0015 % of FSR AIN Range = ± 1.25 V
± 10 µV Before Calibration

± 280 nV/°C
± 0.2 % Before Calibration
± 2.5 ppm of FS/ⴗC
± 0.2 % of FSR Before Calibration
± 2.5 ppm of FS/ⴗC

± 0.0030 % of FSR After Calibration

3

Common-Mode Rejection 80 100 dB At DC, AIN = 1 V
Power Supply Rejection 70 80 dB At DC, AIN = 1 V

ADC PERFORMANCE—
CHOPPING DISABLED

Conversion Time Rate 737 15437 Hz Configure via Conversion Time Register
No Missing Codes

1

24 Bits FW ≥ 8 (Conversion Time ≥ 117 µs)

See Typical Perfomance Charateristics
Output Noise See Table IV
Resolution See Tables V and VI
Integral Nonlinearity (INL) ± 0.0015 % of FSR
Offset Error (Unipolar, Bipolar)
Offset Drift vs. Temperature ± 1.5 µV/ⴗC
Gain Error

2

Gain Drift vs. Temperature ± 2.5 ppm of FS/ⴗC
Positive Full-Scale Error
Positive Full-Scale Drift vs. Temperature ± 2.5 ppm of FS/ⴗC
Bipolar Negative Full-Scale Error

4

± 1mVBefore Calibration

± 0.2 % Before Calibration

2

3

± 0.2 % of FSR Before Calibration

± 0.0030 % of FSR After Calibration

3

Common-Mode Rejection 75 dB At DC, AIN = 1 V
Power Supply Rejection 65 dB At DC, AIN = 1 V

ANALOG INPUTS

Analog Input Voltage Ranges

1, 5

± 2.5 V Range –2.9 ± 2.5 +2.9 V
+2.5 V Range 0 0 to 2.5 2.9 V
± 1.25 V Range –1.45 ± 1.25 +1.45 V
+1.25 V Range 0 0 to 1.25 1.45 V
± 0.625 V Range –725 ±625 +725 mV

+0.625 V Range 0 0 to 625 725 mV
AIN, AINCOM Common-Mode Voltage
AIN, AINCOM Input Current

6

AIN to MUXOUT On Resistance

REFERENCE INPUT

REFIN(+) to REFIN(–) Voltage

1, 7

1

0.2 AV

1

200 Ω

– 0.3 V

DD

200 nA Only One Channel, Chop Disabled

2.475 2.5 2.525 V
NOREF Trigger Voltage 0.5 V NOREF Bit in Channel Status Register
REFIN(+), REFIN(–)

Common-Mode Voltage

Reference Input Current

SYSTEM CALIBRATION

1, 9

1

8

0AV

DD

V

400 µA

Full Scale Calibration Limit +1.05 ⫻ FS V
Zero Scale Calibration Limit –1.05 ⫻ FS V
Input Span 0.8 ⫻ FS 2.1 ⫻ FS V

REV. 0–2–

AD7738

Parameter Min Typ Max Unit Test Conditions/Comment

LOGIC INPUTS

SCLK, DIN, CS, and RESET Inputs

Input Current ± 1 µA
Input Current CS ± 10 µA CS = AV

–40 µA Internal Pull-Up Resistor

Input Capacitance 4 pF

1

V

T+

1

V

T–

V

T+

V

T+

V

T–

V

T+

– V

1

1

– V

1

T–

1

T–

1.4 2 V DVDD = 5 V

0.8 1.4 V DVDD = 5 V

0.3 0.85 V DVDD = 5 V

0.95 2 V DVDD = 3 V

0.4 1.1 V DVDD = 3 V

0.3 0.85 V DVDD = 3 V

MCLK IN Only

Input Current ± 10 µA
Input Capacitance 4 pF

Input Low Voltage 0.8 V DVDD = 5 V

V

INL

Input High Voltage 3.5 V DVDD = 5 V

V

INH

Input Low Voltage 0.4 V DVDD = 3 V

V

INL

V

Input High Voltage 2.5 V DVDD = 3 V

INH

LOGIC OUTPUTS

MCLKOUT

V

OL

V

OH

V

OL

V

OH

10

, DOUT, RDY

Output Low Voltage 0.4 V I

Output High Voltage 4.0 V I

Output Low Voltage 0.4 V I

Output High Voltage DVDD – 0.6 V I

SINK

SOURCE

SINK

SOURCE

Floating State Leakage Current ± 1 µA
Floating State Leakage Capacitance 3 pF

P1 INPUT Levels Referenced to Analog Supplies

Input Current ± 10 µA
V

Input Low Voltage 0.8 V AVDD = 5 V

INL

V

Input High Voltage 3.5 V AVDD = 5 V

INH

P0, P1 OUTPUT

Output Low Voltage 0.4 V

V

OL

0.4 V

0.4 V I

VOH Output High Voltage 4.0 V I

I

SINK

I

SINK

SINK

SOURCE

POWER REQUIREMENTS

– AGND Voltage 4.75 5.25 V

AV

DD

DV

– DGND Voltage 4.75 5.25 V

DD

2.70 3.60 V

Current (Normal Mode) 13.6 16 mA AVDD = 5 V

AV

DD

Current (Internal Buffer Off ) 8.5 mA AVDD = 5 V

AV

DD

Current (Normal Mode)

DV

DD

Current (Normal Mode)

DV

DD

AV

+ DVDD Current (Standby Mode)

DD

Power Dissipation (Normal Mode)
Power Dissipation (Standby Mode)

NOTES

1

Specifications are not production tested, but guaranteed by design and/or characterization data at initial product release.

2

Specifications before calibration. Channel System Calibration reduces these errors to the order of the noise.

3

Applies after the Zero Scale and Full-Scale calibration. The Negative Full Scale error represents the remaining error after removing the offset and gain error.

4

Specifications before calibration. ADC Zero Scale Self-Calibration or Channel Zero Scale System Calibration reduces this error to the order of the noise.

5

The output data span corresponds to the Nominal (Typical) Input Voltage Range. Correct operation of the ADC is guaranteed within the specified min/max.
Outside the Nominal Input Voltage Range, the OVR bit in the Channel Status register is set and the Channel Data register value depends on CLAMP bit in the
Mode register. See the register description and circuit descr iption for more details.

6

If chopping is enabled or when switching between channels, there will be a dynamic current charging the capacitance of the multiplex er, capacitance of the pins,

and any additional capacitance connected to the MUXOUT. See the circuit description for more details.

7

For specified performance. Part is functional with Lower V

8

Dynamic current charging the sigma-delta modulator input switching capacitor.

9

Outside the specified calibration range, calibration is possible but the performance may degrade.

10

These logic output levels apply to the MCLK OUT output when it is loaded with a single CMOS load.

11

With external MCLK, MCLKOUT disabled (CLKDIS bit set in the Mode register).

12

External MCLKIN = 0 V or DVDD, Digital Inputs = 0 V or DVDD, P0 and P1 = 0 V or AVDD.

Specifications are subject to change without notice.

REV. 0

11

11

12

11

12

REF

2.7 3 mA DVDD = 5 V

1.0 1.5 mA DVDD = 3 V
80 µAAV
85 100 mW
500 µWAV

–3–

DD

= 800 µA, DVDD = 5 V

= 200 µA, DVDD = 5 V

= 100 µA, DVDD = 3 V

= 100 µA, DVDD = 3 V

= 8 mA, T
= 5 mA, T

= 2.5 mA, T

= 70°C, AVDD = 5 V

MAX

= 85°C, AVDD = 5 V

MAX

= 105°C, AVDD = 5 V

MAX

= 200 µA, AVDD = 5 V

= DVDD = 5 V

DD

= DVDD = 5 V

DD

AD7738

TIMING SPECIFICATIONS

1, 2, 3

(AVDD = 5 V  5%; DVDD = 2.7 V to 3.6 V or 5 V  5%; Input Logic 0 = 0 V, Logic 1 = DVDD unless otherwise noted.)

Parameter Min Typ Max Unit Test Conditions/Comment

MASTER CLOCK RANGE 1 6.144 MHz

t

1

t

2

50 ns SYNC Pulsewidth
500 ns RESET Pulsewidth

READ OPERATION

t

4

4

t

5

4, 5

t

5A

t

6

t

7

t

8

6

t

9

0nsCS Falling Edge to SCLK Falling Edge Setup Time

SCLK Falling Edge to Data Valid Delay
060nsDV
080nsDV

of 4.75 V to 5.25 V

DD

of 2.7 V to 3.3 V

DD

CS Falling Edge to Data Valid Delay
060nsDV
080nsDV

of 4.75 V to 5.25 V

DD

of 2.7 V to 3.3 V

DD

50 ns SCLK High Pulsewidth
50 ns SCLK Low Pulsewidth
0nsCS Rising Edge after SCLK Rising Edge Hold Time

10 80 ns Bus Relinquish Time after SCLK Rising Edge

WRITE OPERATION

t

11

t

12

t

13

t

14

t

15

t

16

NOTES

1

Sample tested during initial release to ensure compliance.

2

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.

3

See Figures 1 and 2.

4

These numbers are measured with the load circuit of Figure 3 and defined as the time required for the output to cross the VOL or VOH limits.

5

This specification is relevant only if CS goes low while SCLK is low.

6

These numbers are derived from the measured time taken by the data output to change 0.5 V when loaded with the circuit of Figure 3.
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.

0nsCS Falling Edge to SCLK Falling Edge Setup
30 ns Data Valid to SCLK Rising Edge Setup Time
25 ns Data Valid after SCLK Rising Edge Hold Time
50 ns SCLK High Pulsewidth
50 ns SCLK Low Pulsewidth
0nsCS Rising Edge after SCLK Rising Edge Hold Time

Specifications are subject to change without notice.

REV. 0–4–

CS

t

4

SCLK

t

5

t

5A

DOUT MSB

Figure 1. Read Cycle Timing Diagram

CS

t

11

SCLK

t

12

DIN

MSB

AD7738

LSB

LSB

t

8

t

9

t

16

t

6

t

7

t

14

t

15

t

13

Figure 2. Write Cycle Timing Diagram

I

OUTPUT

PIN

(800A AT DV

SINK

100A AT DV

TO

50pF

I

( 200A AT DVDD = 5V

SOURCE

100A AT DV

1.6V

DD

DD

= 5V
= 3V)

DD

= 3V)

Figure 3. Load Circuit for Access Time and Bus Relinquish Time

REV. 0

–5–

AD7738

SCLK

DIN

DOUT

CS

RDY

RESET

SERIAL

INTERFACE

CONTROL

LOGIC

AIN4

AIN5

AIN6

AIN7

MUX

AINCOM/P0

AIN0

AIN1

AIN2

AIN3

DGND DV

DD

REFERENCE

DETECT

REFIN– REFIN+

I/O PORT

SYNC/P1

CLOCK

GENERATOR

MCLKINMCLKOUT

MUXOUT ADCIN

AGND AV

DD

24-BIT

- ADC

BUFFER

AD7738

CALIBRATION

CIRCUITRY

DV

DD

AV

DD

ABSOLUTE MAXIMUM RATINGS*

(TA = 25C unless otherwise noted.)

AVDD to AGND, DVDD to DGND . . . . . . . . . –0.3 V to +7 V

AGND to DGND . . . . . . . . . . . . . . . . . . . . . –0.3 V to +0.3 V

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

AV

DD

AIN, AINCOM to AGND . . . . . . . . . –0.3 V to AV

REFIN(+), REFIN(–) to AGND . . . . . –0.3 V to AV

MUXOUT(+) to AGND . . . . . . . . . . . –0.3 V to AV

MUXOUT(–) to AGND . . . . . . . . . . . –0.3 V to AV

ADCIN(+), ADCIN(–) to AGND . . . . –0.3 V to AV

P1 Voltage to AGND . . . . . . . . . . . . . . –0.3 V to AV

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

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

+ 0.3 V

DD

+ 0.3 V

DD

+ 0.3 V

DD

+ 0.3 V

DD

+ 0.3 V

DD

+ 0.3 V

DD

+ 0.3 V

DD

+ 0.3 V

DD

Operating Temperature Range . . . . . . . . . . –40C to +105C

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

Junction Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . 150C

TSSOP Package, Power Dissipation . . . . . . . . . . . . . 660 mW

Thermal Impedance . . . . . . . . . . . . . . . . . . . . . . 97.9C/W

␪

JA

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 indicated in the operational
section of this specification is not implied. Exposure to absolute maximum rating
conditions for extended periods may affect device reliability.

ORDERING GUIDE

Temperature Package Package

Model Range Description Options

AD7738BRU –40C to +105CTSSOP 28 RU-28

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
AD7738 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.

Figure 4. Block Diagram

REV. 0–6–

PIN CONFIGURATION

AD7738

SCLK

MCLKIN

MCLKOUT

CS

RESET

AV

AINCOM/P0

SYNC/P1

AIN7

AIN6

AIN5

AIN4

MUXOUT(+)

MUXOUT(–)

DD

1

2

3

4

5

6

AD7738

7

TOP VIEW

(Not to Scale)

8

9

10

11

12

13

14

28

DGND

27

DV

26

DIN

25

DOUT

24

RDY

23

AGND

22

REFIN(–)

21

REFIN(+)

20

AIN0

19

AIN1

18

AIN2

17

AIN3

16

ADCIN(+)

15

ADCIN(–)

DD

PIN FUNCTION DESCRIPTION

Pin No. Mnemonic Description

1 SCLK Serial Clock. Schmitt-Triggered Logic Input. An external serial clock is applied to this input to transfer

serial data to or from the AD7738.

2 MCLKIN Master Clock Signal for the ADC. This can be provided in the form of a crystal/resonator or external

clock. A crystal/resonator can be tied across the MCLKIN and MCLKOUT pins. Alternatively, the MCLKIN
pin can be driven with a CMOS compatible clock and MCLKOUT left unconnected.

3MCLKOUT When the master clock for the device is a crystal/resonator, the crystal/resonator is connected between

MCLKIN and MCLKOUT. If an external clock is applied to the MCLKIN, MCLKOUT provides an
inverted clock signal or can be switched off to lower the device power consumption. MCLKOUT is
capable of driving one CMOS load.

4 CS Chip Select. Active low Schmitt triggered logic input with an internal pull-up resistor. With this input

hardwired low, the AD7738 can operate in its 3-wire interface mode using SCLK, DIN, and DOUT. CS
can be used to select the device in systems with more than one device on the serial bus. It can also be used as
an 8-bit frame synchronization signal.

5 RESET Schmitt-Triggered Logic Input. Active low input that resets the control logic, interface logic, digital filter,

analog modulator, and all on-chip registers of the part to power-on status. Effectively, everything on the
part except the clock oscillator is reset when the RESET pin is exercised.

6AV

DD

Analog Positive Supply Voltage. 5 V to AGND nominal.

7 AINCOM/P0 Analog Inputs Common Terminal/Digital Output. The pin is determined by the P0 Dir bit; the digital

value can be written as the P0 bit in the I/O Port register. The digital voltage is referenced to analog
supplies. When configured as an input (P0 Dir bit set to 1), the single-ended Analog Inputs 0 to 7 can be
referenced to this pin’s voltage level.

8 SYNC/P1 SYNC/Digital Input/Digital Output. The pin direction is determined by the P1 Dir bit; the digital value

can be read/written as the P1 bit in the I/O Port register. When the SYNC Enable bit in the I/O Port
register is set to 1, the SYNC/P1 pin can be used to synchronize the AD7738 modulator and digital filter
with other devices in the system. The digital voltage is referenced to the analog supplies. When configured
as an input, the pin should be tied high or low.

9–12, AIN0–AIN7 Analog Inputs
17–20

13 MUXOUT(+) Analog Multiplexer Positive Output

14 MUXOUT(–) Analog Multiplexer Negative Output

REV. 0

–7–

AD7738

PIN FUNCTION DESCRIPTION (continued)

Pin No. Mnemonic Pin Description

15 ADCIN(–) ADC Negative Input. In normal circuit configuration, this pin should be connected to the MUXOUT– pin.

16 ADCIN(+) ADC Positive Input. In normal circuit configuration, this pin should be connected to the MUXOUT+ pin.

21 REFIN(+) Positive Terminal of the Differential Reference Input. REFIN+ voltage potential can lie any where between

AV

and AGND. In normal circuit configuration, this pin should be connected to a 2.5 V reference voltage.

DD

22 REFIN(–) Negative Terminal of the Differential Reference Input. REFIN– voltage potential can lie any where between

AV

and AGND. In normal circuit configuration, this pin should be connected to a 0 V reference voltage.

DD

23 AGND Ground Reference Point for Analog Circuitry
24 RDY Logic Output. Used as a status output in both conversion mode and calibration mode. In conversion

mode, a falling edge on this output indicates that either any channel or all channels have unread data
available—according to the RDY function bit in the I/O Port register. In calibration mode, a falling edge
on this output indicates that calibration is complete. See more details in Digital Interface Description
section later in this data sheet.

25 DOUT Serial Data Output with serial data being read from the output shift register on the part. This output

shift register can contain information from any AD7738 register depending on the address bits of the
Communications register.

26 DIN Serial Data Input (Schmitt triggered) with serial data being written to the input shift register on the part.

Data from this input shift register is transferred to any AD7738 register depending on the address bits of
the Communications register.

27 DV

DD

28 DGND Ground Reference Point for Digital Circuitry

Digital Supply Voltage, 3 V or 5 V Nominal

REV. 0–8–

AD7738

OUTPUT NOISE AND RESOLUTION SPECIFICATION

The AD7738 can be operated with chopping enabled or disabled, allowing the ADC to be programmed either to optimize the
throughput rate and channel switching time or to optimize offset drift performance. Noise tables for these two primary modes of
operation are outlined below for a selection of output rates and settling times.

CHOPPING ENABLED

The first mode, in which the AD7738 is configured with chopping enabled (CHOP = 1), provides very low noise numbers with lower
output rates. Tables I to III show the –3 dB frequencies and typical performance versus channel conversion time or equivalent output
data rate, respectively. Table I shows the typical output rms noise. Table II shows the typical effective resolution based on the rms
noise. Table III shows the typical output peak-to-peak resolution, representing values for which there will be no code flicker within a
six-sigma limit. The peak-to-peak resolutions are not calculated based on rms noise, but on peak-to-peak noise.

These typical numbers are generated from 4096 data samples acquired in Continuous Conversion mode with an analog input voltage
set to 0 V and MCLK = 6.144 MHz. The Conversion Time is selected via the Channel Conversion Time register.

Table I. Typical Output RMS Noise in V vs. Conversion Time and Input Range with
Chopping Enabled

Conversion Conversion Output –3 dB
Time Time Data Rate Frequency

FW Register (s) (Hz) (Hz) 2.5 V, +2.5 V 1.25 V, +1.25 V, 625 mV, +625 mV

127 FFh 2686 372 194 1.8 1.1
46 AEh 999 1001 521 3.0 1.8
17 91h 395 2534 1317 5.1 3.0
8 88h 207 4826 2510 8.1 4.5
4 84h 124 8074 4198 9.3 5.3
2 82h 82 12166 6326 17.0 10.6

Table II. Typical Effective Resolution in Bits vs. Conversion Time and Input Range with
Chopping Enabled

Conversion Conversion Output –3 dB
Time Time Data Rate Frequency

FW Register (s) (Hz) (Hz) 2.5 V +2.5 V 1.25 V +1.25 V 625 mV +625 mV

127 FFh 2686 372 194 21.4 20.4 21.1 20.1 20.1 19.1
46 AEh 999 1001 521 20.6 19.6 20.4 19.4 19.4 18.4
17 91h 395 2534 1317 19.9 18.9 19.6 18.6 18.6 17.6
8 88h 207 4826 2510 19.2 18.2 19.0 18.0 18.0 17.0
4 84h 124 8074 4198 19.0 18.0 18.8 17.8 17.8 16.8
2 82h 82 12166 6326 18.1 17.1 17.8 16.8 16.8 15.8

Table III. Typical Peak-to-Peak Resolution in Bits vs. Conversion Time and Input Range with
Chopping Enabled

Input Range

Input Range

Conversion Conversion Output –3 dB
Time Time Data Rate Frequency

FW Register (s) (Hz) (Hz) 2.5 V +2.5 V 1.25 V +1.25 V 625 mV +625 mV

127 FFh 2686 372 194 18.4 17.4 18.2 17.2 17.2 16.2
46 AEh 999 1001 521 17.8 16.8 17.5 16.5 16.5 15.5
17 91h 395 2534 1317 16.8 15.8 16.7 15.7 15.7 14.7
8 88h 207 4826 2510 16.5 15.5 16.2 15.2 15.2 14.2
4 84h 124 8074 4198 16.0 15.0 16.0 15.0 15.0 14.0
2 82h 82 12166 6326 15.0 14.0 15.0 14.0 14.0 13.0

REV. 0

–9–

Input Range

AD7738

CHOPPING DISABLED

The second mode, in which the AD7738 is configured with chopping disabled (CHOP = 0), provides faster conversion time while still
maintaining high resolution. Tables IV to VI show the –3 dB frequencies and typical performance versus channel conversion time or
equivalent output data rate, respectively. Table IV shows the typical output rms noise. Table V shows the typical effective resolution
based on the rms noise. Table VI shows the typical output peak-to-peak resolution, representing values for which there will be no code
flicker within a six-sigma limit. The peak-to-peak resolutions are not calculated based on rms noise, but on peak-to-peak noise.

These typical numbers are generated from 4096 data samples acquired in Continuous Conversion mode with an analog input voltage
set to 0 V and MCLK = 6.144 MHz. The Conversion Time is selected via the Channel Conversion Time register.

Table IV. Typical Output RMS Noise in V vs. Conversion Time and Input Range with
Chopping Disabled

Conversion Conversion Output –3 dB
Time Time Data Rate Frequency

FW Register (s) (Hz) (Hz) 2.5 V, +2.5 V 1.25 V, +1.25 V, 625 mV, +625 mV

127 7Fh 1357 737 671 2.7 1.5
92 5Ch 992 1008 917 3.0 1.8
35 23h 398 2511 2285 5.1 3.0
16 10h200 4991 2510 7.5 4.5
99h127 7847 7141 10.2 5.9
88h 117 8545 7776 11.4 6.5
33h6515398 14013 15.5 10.3

Table V. Typical RMS Resolution in Bits vs. Conversion Time and Input Range with
Chopping Disabled

Conversion Conversion Output –3 dB
Time Time Data Rate Frequency

FW Register (s) (Hz) (Hz) 2.5 V +2.5 V 1.25 V +1.25 V 625 mV +625 mV

127 7Fh 1357 737 671 20.8 19.8 20.6 19.6 19.6 18.6
92 5Ch 992 1008 917 20.6 19.6 20.4 19.4 19.4 18.4
35 23h 398 2511 2285 19.9 18.9 19.6 18.6 18.6 17.6
16 10h200 4991 2510 19.3 18.3 19.0 18.0 18.0 17.0
99h127 7847 7141 18.9 17.9 18.7 17.7 17.7 16.7
88h 117 8545 7776 18.7 17.7 18.5 17.5 17.5 16.5
33h6515398 14013 18.0 16.7 17.8 17.1 17.1 16.1

Table VI. Typical Peak-to-Peak Resolution in Bits vs. Conversion Time and Input Range with
Chopping Disabled

Input Range

Input Range

Conversion Conversion Output –3 dB
Time Time Data Rate Frequency

FW Register (s) (Hz) (Hz) 2.5 V +2.5 V 1.25 V +1.25 V 625 mV +625 mV

127 7Fh 1357 737 671 17.9 16.9 17.8 16.8 16.8 15.8
92 5Ch 992 1008 917 17.8 16.8 17.4 16.4 16.4 15.4
35 23h 398 2511 2285 17.0 16.0 16.8 15.8 15.8 14.8
16 10h200 4991 2510 16.3 15.3 16.2 15.2 15.2 14.2
99h127 7847 7141 16.1 15.1 15.9 14.9 14.9 13.9
88h 117 8545 7776 16.0 15.0 15.7 14.7 14.7 13.7
33h6515398 14013 15.0 14.0 14.8 13.8 13.8 12.8

Input Range

REV. 0–10–

Typical Performance Characteristics–AD7738

25

24

23

22

21

20

19

NO MISSING CODES

18

17

16

1102

CHOP = 1

3456789

FILTER WORD

TPC 1. No Missing Codes Performance, Chopping Enabled

25

24

23

22

21

20

19

NO MISSING CODES

18

17

16

1102

CHOP = 0

3456789

FILTER WORD

TPC 2. No Missing Codes Performance, Chopping Disabled

0

–20

–40

–60

–80

–100

GAIN – dB

–120

–140

–160

–180

0 1400200

THD = 115dB

400 600 800 1000 1200

INPUT FREQUENCY

TPC 3. Typical FFT Plot; Input Sinewave 183 Hz,

1.2 V Peak, Range

µ

s, Chopping Enabled

394

140

EFFECTIVE RES. 19.9 BITS
P-P RES. 17.0 BITS

120

100

80

60

40

NUMBER OF CODES

20

0

–80 80–60

±

1.25 V, Conversion Time

–40 –20 0 20 40

VA LU E

60

TPC 4. Typical Histogram; Analog Inputs Shorted;

±

Range

2.5 V, Conversion Time 394 µs;

Chopping Enabled

REV. 0

–11–

AD7738

Table VII. Register Summary

Addr Dir Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0

Register hex Default Value

Communications 00 W 0 R/W 6-Bit Register Address

I/O Port 01 R/W P0 P1 P0 DIR P1 DIR RDY FN 0 0 SYNC

P0 Pin P1 Pin 1 1 0 0 0 0

Revision 02 R Chip Revision Code Chip Generic Code

xxxx00 01

Test 03 R/W 24 Bits Manufacturing Test Register

ADC Status 04 R RDY7 RDY6 RDY5 RDY4 RDY3 RDY2 RDY1 RDY0

000000 00

Checksum 05 R/W 16-Bit Checksum Register

ADC ZS Calibration 06 R/W 24-Bit ADC Zero-Scale Calibration Register

800000h

ADC FS 07 R/W 24-Bit ADC Full-Scale Register

800000h

Channel Data

1

Channel ZS Calibration

Channel FS Calibration

Channel Status

Channel Setup

1

1

Channel Conv. Time

2

Mode

NOTES

1

The three LSBs of the register address, i.e., Bit 2, Bit 1, and Bit 0 in the Communication register, specify the channel number of the register being accessed.

2

There is only one Mode register, although the Mode register can be accessed in one of eight address locations The address used to write the Mode register specifies

the ADC channel on which the mode will be applied. Address 38h only must be used for reading from the Mode register.

08-0F R 16-/24-Bit Data Registers

8000h

1

10–17 R/W 24-Bit Channel Zero-Scale Calibration Registers

800000h

1

18–1F R/W 24-Bits Channel Full-Scale Calibration Registers

200000h

20–27 R CH2 CH1 CH0 0/P0 RDY/P1 NOREF SIGN OVR

Channel Number 0 0 0 0 0

28–2F R/W BUF OFF COM1 COM0 Stat. Opt. ENABLE RNG2 RNG1 RNG0

000000 00

1

30–37 R/W CHOP FW (7-Bit Filter Word)

1 11h

38–3F R/W MD2 MD1 MD0 CLKDIS DUMP Cont. RD 24/16 Bits CLAMP

000000 00

Table VIII. Operational Mode Summary

MD2 MD1 MD0 Mode

000 Idle Mode
001 Continuous Conversion Mode
010 Single Conversion Mode
011 Power-Down (Standby) Mode
100 ADC Zero-Scale Self Calibration
101 For Future Use
110 Channel Zero-Scale System Calibration
111 Channel Full-Scale System Calibration

Table IX. Input Range Summary

RNG2 RNG1 RNG0 Nominal Input Voltage Range

100± 2.5 V
1010 V to +2.5 V
000± 1.25 V
0010 V to +1.25 V
010± 0.625 V
0110 V to +0.625 V

REV. 0–12–

AD7738

REGISTER DESCRIPTION

The AD7738 is configurable through a series of registers. Some of them configure and control general AD7738 features, others are
specific to each channel. The register data widths vary from 8 bits to 24 bits. All registers are accessed through the Communi
register, i.e., any communication to the AD7738 must start with a write to the Communication register, specifying which register
will be subsequently read or written.

Communications Register

8 Bits, Write-Only Register, Address 00h

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 subsequent operation will be a read or write and to which register this operation will be
directly placed.
after the subsequent read or write operation to the selected register is complete. If the interface sequence is lost, the part can be reset
by writing at least 32 serial clock cycles with DIN high and CS low (Note that all of the parts including modulator, filter, interface
and all registers are reset in this case). Remember to keep DIN low while reading 32 or more bits either in Continuous Read mode or
with the DUMP bit and “24/16” bit in the Mode register set.

Bit Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0

Mnemonic 0 R/W 6-Bit Register Address

Bit Mnemonic Description

70 This bit must be zero for proper operation.
6R/W A zero in this bit indicates that the next operation will be a write to a specified register.

5–0 Address Address specifying to which register the read or write operation will be directed.

The digital interface defaults to expect write operation to the Communication register after power on, after reset, or

A one in this bit indicates that the next operation will be a read from a specified register.

For channel specific registers the three LSBs, i.e., Bit 2, Bit 1, and Bit 0, specify the channel number.
When the subsequent operation writes to the Mode register, then the three LSBs specify the channel
selected for operation determined by the Mode register value. See Table X.
(The analog input’s configuration depends on the COM1, COM0 bits in the Channel Setup register.)

cation

Table X.

Bit 2 Bit 1 Bit 0 Channel Single Input Differential Input

0000 AIN0 AIN0–AIN1
0011 AIN1 AIN2–AIN3
0102 AIN2 AIN4–AIN5
0113 AIN3 AIN6–AIN7
1004 AIN4 AIN0–AIN1
1015 AIN5 AIN2–AIN3
1106 AIN6 AIN4–AIN5
1117 AIN7 AIN6–AIN7

REV. 0

–13–

AD7738

I/O Port Register

8 Bits, Read/Write Register, Address 01h, Default Value 30h + Digital Input Value  40h

The bits in this register are used to configure and access the digital I/O pin on the AD7738.

Bit Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0

Mnemonic P0 P1 P0 DIR P1 DIR RDY FN 0 0 SYNC

Default P0 Pin P1 Pin 1 1 0 0 0 0

Bit Mnemonic Description

7P0 When the AINCOM/P0 pin is configured as a digital output, the P0 bit determines the pin’s output level.
6P1When the P1 pin is configured as an output, the P1 bit determines the pin’s output level. When the P1

pin is configured as an input, the P1 bit reflects the current input level on the pin.

5 P0 DIRWhen set to 1, the AINCOM/P0 pin is configured as an analog input. When set to 0, the AINCOM/P0

pin is configured as a digital output.

4 P1 DIRThis bit determines whether P1 pin is configured as an input or an output. When set to 1, the P1 pin will

be a digital input; when reset to 0, the pin will be a digital output.

3 RDY FN This bit is used to control the function of the RDY pin on the AD7738. When this bit is reset to 0 the RDY

pin goes low when any channel has unread data. When this bit is set to 1, the RDY pin will only go low if all
enabled channels have unread data.

2, 1 0 These bits must be zero for proper operation.
0 SYNC This bit enables the SYNC pin function. By default, this bit is 0 and SYNC/P1 can be used as a

digital I/O pin. When the SYNC EN bit is set to 1, the SYNC pin can be used to synchronize the
AD7738 modulator and digital filter with other devices in the system.

Revision Register

8 Bits, Read-Only Register, Address 02h, Default Value 01h + Chip Revision  10h

Bit Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0

Mnemonic Chip Revision Code Chip Generic Code

Default x x x x 0 0 0 1

Bit Mnemonic Description

7–4 Chip Revision Code 4-Bit Factory Chip Revision Code
3–0 Chip Generic Code On the AD7738, these bits will read back as 01h.

Test Register

24 Bits, Read/Write Register, Address 03h

This register is used for testing the part in the manufacturing process. The user must not change the default configuration of this register.

ADC Status Register

8 Bits, Read-Only Register, Address 04h, Default Value 00h

In conversion modes, the register bits reflect the individual channel status. When a conversion is complete, the corresponding Channel
Data register is updated and the corresponding RDY bit is set to 1. When the Channel Data register is read, the corresponding bit is
reset to 0. The bit is also reset to 0 when no read operation has taken place and the result of the next conversion is being updated to
the Channel Data register. Writing to the Mode register resets all the bits to 0.

In calibration modes, all the register bits are reset to 0 while a calibration is in progress and all the bits are set to 1 when the
calibration is complete.

The RDY pin output is related to the content of ADC Status register as defined by the RDY Function bit in the I/O Port register.

Bit Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0

Mnemonic RDY7 RDY6 RDY5 RDY4 RDY3 RDY2 RDY1 RDY0

Default 0 0 0 0 0 0 0 0

The RDY0 bit corresponds to Channel 0, RDY1 bit to Channel 1, and so on.

REV. 0–14–

AD7738

Checksum Register

16 Bits, Read/Write Register, Address 05h

This register is described in the “AD7732/34/38 Checksum Register” Technical Note.

ADC Zero Scale Calibration Register

24 Bits, Read/Write Register, Address 06h, Default Value 800000h

The register holds the ADC Zero-Scale Calibration coefficient. The value in this register is used in conjunction with the value in the
ADC Full-Scale Calibration register and corresponding Channel Zero-Scale and Channel Full-Scale Calibration registers to scale
digitally all channels’ conversion results. The value in this register is updated automatically following the execution of an ADC Zero­Scale ADC Self-Calibration. Writing to this register is possible in the Idle Mode only. See the calibration description for more details.

ADC Full-Scale Register

24 Bits, Read/Write Register, Address 07h, Default Value 800000h

The register holds the ADC Full-Scale coefficient. The user is advised not to change the default configuration of this register.

Channel Data Registers

16/24 Bits, Read-Only Registers, Address 08h–0Fh, Default Width 16 Bits, Default Value 8000h

These registers contain the most up-to-date conversion results corresponding to each analog input channel. The 16- or 24-bit data
width can be configured by setting the “16/24” bit in the Mode register. The relevant RDY bit in the Channel Status register goes
high when the result is updated. The RDY bit will return low once the Data register reading has begun. The RDY pin can be configured
to indicate when any channel has unread data or waits until all enabled channels have unread data. If any Channel Data Register read
operation is in progress when the new result is updated, then no update of the Data register occurs. This is to avoid getting corrupted
data. Reading the Status registers can be associated with reading the Data registers in the Dump mode. Reading the Status registers is
always associated with reading the Data registers in the Continuous Read mode. See the digital interface description for more details.

Channel Zero-Scale Calibration Registers

24 Bits, Read/Write Registers, Address 10h–17h, Default Value 800000h

These registers hold the particular channel Zero-Scale Calibration coefficients. The value in these registers is used in conjunction
with the value in the corresponding Channel Full-Scale Calibration register, the ADC Zero-Scale Calibration register, and ADC
Full-Scale Calibration register to scale digitally the particular channel conversion results. The value in this register is updated auto­matically following the execution of a Channel Zero-Scale System Calibration.

The format of the Channel Zero-Scale Calibration register is a sign bit and 22 bits unsigned value.

Writing this register is possible in the Idle Mode only. See the calibration description for more details.

Channel Full-Scale Calibration Registers

24 Bits, Read/Write Registers, Address 18h–1Fh, Default Value 200000h

These registers hold the particular channel Full-Scale Calibration coefficients. The value in these registers is used in conjunction with
the value in the corresponding Channel Zero-Scale Calibration register, the ADC Zero-Scale Calibration register, and ADC Full
Scale Calibration register to scale digitally the particular channel conversion results. The value in this register is updated automati­cally following the execution of a Channel Full-Scale System Calibration. Writing this register is possible in the Idle mode only. See
the calibration description for more details.

REV. 0

–15–

AD7738

Channel Status Registers

8 Bits, Read-Only Register, Address 20h–27h, Default Value 20h  Channel Number

These registers contain individual channel status information and some general AD7738 status information. Reading the Status
registers can be associated with reading the Data registers in the Dump mode. Reading the Status registers is always associated with
reading the Data registers in the Continuous Read mode. See the Digital Interface Description section for more details.

Bit Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0

Mnemonic CH2 CH1 CH0 0/P0 RDY/P1 NOREF SIGN OVR

Default Channel Number 0 0 0 0 0

Bit Mnemonic Description

7–5 CH2–CH0 These bits reflect the channel number. This can be used for current channel identification and easier

operation in the Dump mode and Continuous Read mode.

40/P0 When the Status Option bit in the corresponding Channel Setup register is reset to 0, this bit is read

as a zero. When the Status Option bit in set to 1, this bit reflects the state of the P0 output pin.

3 RDY/P1 When the Status Option bit in the corresponding Channel Setup register is reset to 0, this bit reflects the

selected channel RDY bit in the ADC Status register. When the Status Option bit is set to 1, this
bit reflects the state of the P1 pin whether it is configured as an input or output.

2 NOREF This bit indicates the reference input status. If the voltage between the REFIN+ and REFIN– pins is less

than the NOREF trigger voltage, then the NOREF bit goes to a 1.

1 SIGN The voltage polarity at the analog input. Will be 0 for a positive voltage; will be 1 for a negative voltage.

0 OVR This bit reflects either overrange or underrange on an analog input. The bit is set to 1 when the analog

input voltage goes over or under the Nominal Voltage Range. See the Analog Inputs Extended Voltage
Range section.

REV. 0–16–

AD7738

Channel Setup Registers

8 Bits, Read/Write Register, Address 28h–2Fh, Default Value 00h

These registers are used to configure the selected channel, its input voltage range, and set up the corresponding Channel Status
register.

Bit Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0

Mnemonic BUF OFF COM1 COM0 Stat. Opt. ENABLE RNG2 RNG1 RNG0

Default 0 0 0 0 0 0 0 0

Bit Mnemonic Description

7 BUF OFF Buffer Off. If reset to 0, then internal buffer is enabled. Only operation with internal buffer enabled

6, 5 COM1, COM0 Analog Input Configuration. See Table XI.

4 Stat. Opt. Status Option. When this bit is set to 1, the P1 bit in the Status Channel register will reflect the state

3 ENABLE Channel Enable. Set this bit to 1 to enable the channel in the Continuous Conversion mode. A single

2–0 RNG2–0 The Channel Input Voltage Range. See Table XII.

is recommended.

of the P1 pin. When this bit is reset to 0, the P1 bit in the Status Channel register bit will reflect
the channel corresponding RDY bit in the ADC Status register.

conversion will take place regardless of this bit value.

Table XI.

COM1 COM0 COM1 COM0

Channel 0 0 1 1

0 AIN0–AINCOM AIN0–AIN1
1 AIN1–AINCOM AIN2–AIN3
2 AIN2–AINCOM AIN4–AIN5
3 AIN3–AINCOM AIN6–AIN7
4 AIN4–AINCOM AIN0–AIN1
5 AIN5–AINCOM AIN2–AIN3
6 AIN6–AINCOM AIN4–AIN5
7 AIN7–AINCOM AIN6–AIN7

Channel Conversion Time Registers

8 Bits, Read/Write Register, Address 30h–37h, Default Value 91h

The Conversion Time registers enable or disable chopping and configure the digital filter for a particular channel.

This register value affects the conversion time, frequency response, and noise performance of the ADC.

Bit Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0

Mnemonic CHOP FW (7-Bit Filter Word)

Default 1 11h

Bit Mnemonic Description

7 CHOP Chop Enable Bit. Set to 1 to apply chopping mode for a particular channel.

6–0 FW CHOP = 1, Single Conversion or Continuous Conversion with one channel enabled.

Conversion Time (µs) = (FW  128 + 248)/MCLK Frequency (MHz), the FW in range of 2 to 127.

RNG2 RNG1 RNG0 Voltage Range

100± 2.5 V
1010 V to +2.5 V
000± 1.25 V
0010 V to +1.25 V
010± 0.625 V
0110 V to +0.625 V

Table XII.

Nominal Input

REV. 0

CHOP = 1, Continuous Conversion with two or more channels enabled.
Conversion Time (µs) = (FW  128 + 249)/MCLK Frequency (MHz), the FW in range of 2 to 127.

CHOP = 0, Single Conversion or Continuous Conversion with one channel enabled.
Conversion Time (µs) = (FW  64 + 206)/MCLK Frequency (MHz), the FW in range of 3 to 127.

CHOP = 0, Single Conversion or Continuous Conversion with two or more channels enabled.
Conversion Time (µs) = (FW  64 + 207)/MCLK Frequency (MHz), the FW in range of 3 to 127.

–17–

AD7738

Mode Register

8 Bits Read/Write Register, Address 38h–3Fh, Default Value 00h

The Mode register configures the part and determines the part’s operating mode. Writing to the Mode register will clear the ADC
Status register, set the RDY pin to logic high level, exit all current operations, and start the mode specified by the Mode bits.

The AD7738 contains only one Mode register. The three LSBs of the address used for writing to the Mode register specify the channel
selected for operation determined by the MD2 to MD0 bits. The address 38h only must be used for reading from the Mode register.

Bit Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0

Mnemonic MD2 MD1 MD0 CLKDIS DUMP CONT RD 24/16 BIT CLAMP

Default 0 0 0 0 0 0 0 0

Bit Mnemonic Description

7–5 MD2–MD0 Mode Bits. These three bits determine the AD7738 operation mode. Writing a new value to the Mode

bits will exit the part from the mode in which it has been operating and place it in the new requested
mode immediately. The function of the Mode bits is described in more detail below.

4 CLKDIS Master Clock Output Disable. When this bit is set to 1 the master clock is disabled from appearing

at the MCLKOUT pin and the MCLKOUT pin is in a high impedance state. This allows turning off the
MCLKOUT as a power saving feature. When using an external clock on MCLKIN, the AD7738
continues to have internal clocks and will convert normally regardless of CLKDIS bit state. When using
a crystal oscillator or ceramic resonator across the MCLKIN and MCLKOUT pins, the AD7738 clock is
stopped and no conversions can take place when the CLKDIS bit is active. The AD7738 digital interface
can still be accessed using the SCLK pin.

3 DUMP DUMP Mode. When this bit is reset to 0, the Channel Status register and Channel Data register will

be addressed and read separately. When the DUMP bit is set to 1, the Channel Status register will be followed
immediately by a read of the Channel Data register regardless of whether the Status or Data register
has been addressed through the Communication register. The Continuous Read mode will always be a
“Dump Mode” reading of the Channel Status and Data register regardless of the Dump Bit value. See the
Digital Interface Description section for more details.

2 CONT RD When this bit is set to 1, the AD7738 will operate in the Continuous Read mode. See the Digital

Interface Description section for more details.

1 24/16 BIT The Channel Data Register Data Width Selection Bit. When set to 1, the Channel Data registers will be

24 bits wide. When set to 0, then the Channel Data registers will be 16 bits wide.

0 CLAMP This bit determines the Channel Data register’s value when the analog input voltage is outside the nominal

input voltage range. When the CLAMP bit is set to 1, the Channel Data register will be digitally clamped
either to all zeros or all ones when the analog input voltage goes outside the nominal input voltage range.
When the CLAMP bit is reset to 0, the Data registers reflect the analog input voltage even outside the
nominal voltage range. See the Analog Inputs Extended Voltage Range section.

MD2 MD1 MD0 Mode Address Used for Mode Register Write Specify

000Idle Mode
001Continuous Conversion Mode The First Channel to Start Converting
010Single Conversion Mode Channel to Convert
011Power Down (Standby) Mode
100ADC Zero-Scale Self Calibration Channel Conversion Time Used for the ADC Self-Calibration
101For Future Use
110Channel Zero-Scale System Calibration Channel to Calibrate
111Channel Full-Scale System Calibration Channel to Calibrate

REV. 0–18–

MD2 MD1 MD0 Operating Mode

000Idle Mode

The default mode after Power-On or Reset.
The AD7738 returns to this mode automatically after any calibration or after a single conversion.

001Continuous Conversion Mode

The AD7738 performs a conversion on the specified channel. After the conversion is complete, the
relevant Channel Data register and Channel Status register are updated, the relevant RDY bit in the
ADC status register is set, and the AD7738 continues converting on the next enabled channel. The AD7738
will cycle through all enabled channels until put into another mode or reset. The cycle period will be the
sum of all enabled channels’ conversion times, set by corresponding Channel Conversion Time registers.

010Single Conversion Mode

The AD7738 performs a conversion on the specified channel. After the conversion is complete, the
relevant Channel Data register and Channel Status register are updated, the relevant RDY bit in the
ADC status register is set, the RDY pin goes low, the MD2, MD1, and MD0 bits are reset, and AD7738
returns to the Idle mode. Requesting a single conversion ignores the Channel Setup registers’ Enable bits
and a conversion will be performed even if that channel is disabled.

011Power-Down (Standby) Mode

The ADC and the analog front end (internal buffer) go into the power-down mode. The AD7738 digital
interface can still be accessed. The CLKDIS bit works separately, the MCLKOUT mode is not affected
by Power-Down (Standby) mode.

100ADC Zero-Scale Self-Calibration Mode

A zero-scale self-calibration is performed on internally shorted ADC inputs. After the calibration is
complete, the contents of the ADC Zero-Scale Calibration register are updated, all RDY bits in the ADC
status register are set, the RDY pin goes low, the MD2, MD1, and MD0 bits are reset, and the AD7738
returns to the Idle mode.

101For Future Use

110Channel Zero-Scale System Calibration Mode

A zero-scale system calibration is performed on the selected channel. An external system zero-scale
voltage should be provided at the AD7738 analog input and this voltage should remain stable for the
duration of the calibration. After the calibration is complete, the contents of the corresponding Channel
Zero Scale Calibration register are updated, all RDY bits in the ADC status register are set, the RDY pin
goes low, the MD2, MD1, and MD0 bits are reset, and AD7738 returns to the Idle mode.

111Channel Full-Scale System Calibration Mode

A full-scale system calibration is performed on the selected channel. An external system full-scale voltage
should be provided at the AD7738 analog input and this voltage should remain stable for the duration of
the calibration. After the calibration is complete, the contents of the corresponding Channel Full-Scale
Calibration register are updated, all RDY bits in the ADC status register are set, the RDY pin goes low,
the MD2, MD1, and MD0 bits are reset, and AD7738 returns to the Idle mode.

AD7738

REV. 0

–19–

AD7738

DIGITAL INTERFACE DESCRIPTION
Hardware

The AD7738 serial interface can be connected to the host
device via the serial interface in several different ways.

The CS pin can be used to select the AD7738 as one of several
circuits connected to the host serial interface. When the CS is
high, the AD7738 ignores the SCLK and DIN signals and
the DOUT pin goes to the high impedance state. When the CS
signal is not used, connect the CS pin to DGND.

The RDY pin can be either polled for high to low transition or
can drive the host device interrupt input to indicate that the
AD7738 has finished the selected operation and/or new data
from the AD7738 are available. The host system can also wait a
designated time after a given command is written to the device
before reading. Alternatively, the AD7738 status can be polled.
When the RDY pin is not used in the system, it should be left as
an open circuit. (Note that the RDY pin is always an active
digital output, i.e., never goes into a high impedance state).

The RESET pin can be used to reset the AD7738. When not
used, connect this pin to DV

DD

.

The AD7738 interface can be reduced to just two wires con­necting DIN and DOUT pins to a single bidirectional data line.
The second signal in this 2-wire configuration is the SCLK
signal. The host system should change the data line direction
with reference to the AD7738 timing specification (see the Bus

Relinquish Time in the Timing Characteristics). The AD7738
cannot operate in the Continuous Read mode in 2-wire serial
interface configuration.

All the digital interface inputs are Schmitt-Triggered. There­fore, the AD7738 interface features higher noise immunity
and the AD7738 can be easily isolated from the host system via
optocouplers.

Figure 5 outlines some of the possible host device interfaces:
(a) SPI without using the CS signal, (b) DSP interface, and
(c) 2-wire configuration.

Reset

The AD7738 can be reset by the RESET pin or by writing a
reset sequence to the AD7738 serial interface. The reset
sequence is N ⫻ “0” + 32 ⫻ “1”, which could be the data
sequence 00h + FFh + FFh + FFh + FFh in a byte oriented
interface. The AD7738 also features a power-on reset with a
trip point of 2 V and goes to the defined default state after
power on.

It is the system designer’s responsibility to prevent an unwanted
write operation to the AD7738. The unwanted write operation
could happen when a spurious clock appears on the SCLK
while the CS pin is low. It should be noted that on system
power-on, if the AD7738 interface signals are floating or unde­fined, the part can be inadvertently configured into an unknown
state. This could be easily overcome by initiating either a HW
reset event or a 32 ones reset sequence as the first step in the
system configuration.

DV

AD7738

RESET

SCLK

DOUT

DIN

RDY

CS

DGND

DV

DD

DD

68HC11

SS

SCK

MISO

MOSI

INT

AD7738

RESET

SCLK

DOUT

DIN

RDY

CS

DV

DD

a. b. c.

Figure 5. AD7738 to Host Device Possible Interface

Access the AD7738 Registers

All communications to the part start with a write operation to
the Communications register followed by either reading or
writing the addressed register.

In a simultaneous read-write interface (such as SPI), write “0”
to the AD7738 while reading data.

Figure 6 shows the AD7738 interface read sequence for the
ADC Status register.

DV

DD

ADSP-2105

SCLK

DR

DT

INT

TFS

RFS

CS

SCLK

DIN

DOUT

AD7738

WRITE

COMMUNICATIONS

REGISTER

RESET

SCLK

DOUT

DIN

CS

DGND

READ

ADC STATUS

REGISTER

8xC51

P3.1/TXD

P3.0/RXD

Figure 6. The Serial Interface Signals—Register Access

REV. 0–20–

AD7738

Single Conversion and Reading Data

When the Mode register is being written, the ADC Status Byte
is cleared and the RDY pin goes high regardless of its previous
state. When the single conversion command is written to the
Mode register, the ADC starts the conversion on the channel
selected by the address of the Mode register. After the conversion
is completed, the Data register is updated, the Mode register is
changed to Idle mode, the relevant RDY bit is set, and the RDY
pin goes low. The RDY bit is reset and the RDY pin returns
high when the relevant Channel Data register is being read.

Figure 7 shows the digital interface signals executing a single
conversion on Channel 0, waiting for the RDY pin low, and
reading the Channel 0 Data register.

CS

SCLK

DIN

DOUT

RDY

COMMUNICATIONS

38h

WRITE

REGISTER

40h

WRITE
MODE

REGISTER

CONVERSION

TIME

Figure 7. Serial Interface Signals—Single Conversion Command and 16-Bit Data Reading

Dump Mode

When the DUMP bit in the Mode register is set to 1, the Chan­nel Status register will be read immediately by a read of the
Channel Data register regardless of whether the Status or the
Data register has been addressed through the Communication
register. The DIN pin should not be high while reading 24-bit
data in Dump mode. Otherwise the AD7738 will be reset.

Figure 8 shows the digital interface signals executing a single
conversion on Channel 0, waiting for for the RDY pin low, and
reading the Channel 0 Status register and Data register in the
Dump mode.

48h

WRITE

COMMUNICATIONS

REGISTER

(00h) (00h)

DATA

READ DATA

REGISTER

DATA

CS

SCLK

DIN

DOUT

RDY

COMMUNICATIONS

38h

WRITE

REGISTER

48h

WRITE

MODE

REGISTER

CONVERSION

TIME

48h

WRITE

COMMUNICATIONS

REGISTER

(00h) (00h)

CH. STAT

READ

CHANNEL

STATUS

DATA

READ DATA

REGISTER

(00h)

DATA

Figure 8. Serial Interface Signals—Single Conversion Command, 16-Bit Data Reading, Dump Mode

REV. 0

–21–

AD7738

Continuous Conversion Mode

When the Mode register is being written, the ADC Status Byte
is cleared and the RDY pin goes high regardless of its previous
state. When the continuous conversion command is written to
the Mode register, the ADC starts conversion on the channel
selected by the address of the Mode register.

After the conversion is complete, the relevant Channel Data
register and Channel Status register are updated, the

relevant
RDY bit in the ADC Status register is set, and the AD7738
continues converting on the next enabled channel. The
AD7738 will cycle through all enabled channels until put
into

another mode or reset. The cycle period will be the sum of
all enabled channels’ conversion times, set by corresponding
Channel Conversion Time registers.

The RDY bit is reset when the relevant Channel Data register

is

being read. The behavior of the RDY pin depends on the
RDYFN bit in the I/O Port register. When RDYFN bit is 0, the
RDY pin goes low when any channel has unread data. When
this bit is set to 1 the RDY pin will only go low if all enabled
channels have unread data.

CONTINUOUS
CONVERSION

SERIAL

INTERFACE

RDY

START

READ

DATA

CH0

If an ADC conversion result has not been read before a new
ADC conversion is completed, then the new result will overwrite
the previous one. The relevant RDY bit goes low and the RDY
pin goes high for at least 163 MCLK cycles (~26.5 µs), indicating
when the Data register is updated and the previous conversion
data is lost.

If the Data register is being read as an ADC conversion completes,
then the Data Register will not be updated with the new result (to
avoid data corruption) and the new conversion data is lost.

Figure 9 shows the digital interface signals sequence for the
Continuous Conversion mode with Channels 0 and 1 enabled
and the RDYFN bit set to 0. The RDY pin goes low and the
Data Register is read after each conversion. Figure 10 shows a
similar sequence, but with the RDYFN bit set to 1. The RDY
pin goes low and the Data register is read after all conversions
are completed. Figure 11 shows the RDY pin when no data are
read from the AD7738.

READ

DATA

CH1

READ

DATA

CH0

READ

DATA

CH1

SERIAL

INTERFACE

RDY

CONTINUOUS
CONVERSION

SERIAL

INTERFACE

RDY

START
CONTINUOUS
CONVERSION

START

CH0 CONVERSION CH1 CONVERSION CH0 CONVERSION CH1 CONVERSION CH0 CONVERSION

Figure 9. Continuous Conversion, CH0 and CH1, RDYFN = 0

READ

READ

DATA

DATA

CH0

CH1

CH0 CONVERSION CH1 CONVERSION CH0 CONVERSION CH1 CONVERSION CH0 CONVERSION

Figure 10. Continuous Conversion, CH0 and CH1, RDYFN = 1

CH0 CONVERSION CH1 CONVERSION CH0 CONVERSION CH1 CONVERSION CH0 CONVERSION

Figure 11. Continuous Conversion, CH0 and CH1, No Data Read

READ

DATA

CH0

READ

DATA

CH1

REV. 0–22–

CS

SCLK

AD7738

DIN

DOUT

RDY

38h

WRITE
COMM .

REGISTER

48h

WRITE

MODE

REGISTER

CONVERSION

ON CH0

COMPLETE

48h

WRITE

COMM .

REGISTER

00h

CH.STAT.

READ

CH0

STAT US

Figure 12. Continuous Conversion CH0 and CH1, Continuous Read

Continuous Read (Continuous Conversion) Mode

When the Continuous RD bit in the Mode register is set, the
first write “48h” to the communication register starts the Continu­ous Read mode. As shown in Figure 12, subsequent accesses
to

the part sequentially reads the Channel Status and Data
registers of the last completed conversion without any further
configuration of the Communication register being required.

Note that the Continuous Conversion bit in the Mode register
should be set when entering the Continuous Read mode.

Note that the Continuous Read mode is “Dump Mode” reading
of the Channel Status and Data register regardless of the Dump
bit value. Use the Channel bits in the Channel Status register to
check/recognize which channel data is actually being shifted out.

Note that the last completed conversion result is being read.
Therefore, the RDYFN bit in the I/O Port register should be 0,
and reading the result should always start before the next con­version is completed.

The AD7738 will stay in Continuous Read mode as long as the
DIN pin is low while the CS pin is low. Therefore, write 0 to
the AD7738 while reading in Continuous Read mode. To
exit Continuous Read mode, take the DIN pin high for at least
100 ns after a read is complete. (Write “80h” to the AD7738 to
exit continuous reading.)

The Continuous RD bit in the Mode register is not changed
by taking the DIN pin high. Therefore, the next write “48h”
starts the Continuous Read mode again. To completely stop
the continuous read mode, write to the Mode register to clear
the Continuous RD bit.

CIRCUIT DESCRIPTION

The AD7738 is a sigma-delta A/D converter, intended for the
measurement of wide dynamic range, low frequency signals in
industrial process control, instrumentation, PLC, and DSC.

It contains a multiplexer, an input buffer, a sigma-delta (or
charge-balancing) ADC, digital filter, clock oscillator, digital I/O
port, and a serial communications interface.

Analog Front End

The AD7738 has nine analog input pins connected to the
ADC through the internal multiplexer. The analog front end
can be configured as eight single-ended inputs four differen­tial inputs,

or any combination of these. Selection of ADC
inputs is determined via the COM0 and COM1 bits in
the Channel Setup registers.

00h

DATA

READ CH0

DATA REGISTER

00h

DATA

CONVERSION

ON CH1

COMPLETE

00h

CH.STAT.

READ

CH1

STAT US

00h

DATA

READ CH1 DATA

00h

DATA

REGISTER

The AD7738 contains a wide bandwidth, fast settling time
differential input buffer capable of driving the dynamic load of a
high speed sigma-delta modulator. With the internal buffer
enabled, the analog inputs feature relatively high input imped­ance. However, if chopping is enabled and/or when switching
between channels, there is a dynamic current charging the
capacitance of the multiplexer, capacitance of the pins, and any
additional capacitance connected to the MUXOUT. In typical
configurations with MUXOUT connected directly to the
ADCIN, this capacitance could be approximately 20 pF. The
AD7738 has been designed to provide adequate settling time
after a multiplexer switch and before the actual sampling starts
only if the analog inputs resistive source impedance does not
exceed 10 kΩ.

An RC connected to the analog inputs may convert the dynamic
charging currents to a dc voltage and cause additional gain or
offset errors. The recommended low-pass RC filter on the
AD7738 analog inputs is 20 Ω and 100 nF.

The multiplexer output and the ADC input are pinned out
externally. This facilitates shared signal conditioning between
the multiplexer and the ADC. Please note that if chop is enabled
and/or when switching between channels, any circuit connected
between MUXOUT and ADCIN should be fully settled within
the settling time provided by the AD7738. See the Multiplexer,
Conversion, and Data Output Timing section.

- ADC

The AD7738 core consists of a charge balancing sigma-delta
modulator and a digital filter. The architecture is optimized for
fast fully settled conversion. This allows for fast channel-to-channel
switching while maintaining inherently excellent linearity, high
resolution, and low noise.

Chopping

With chopping enabled, the multiplexer repeatedly reverses the
ADC inputs. Every output data result is then calculated as an
average of two conversions, the first with positive and the sec­ond with negative offset term included. This effectively removes
any offset error of the input buffer and sigma-delta modulator,
resulting in excellent dc offset and offset drift specifications.

Figure 13 shows the channel signal chain with chopping enabled.

REV. 0

–23–

AD7738

AIN(+)

AIN(– )

MULTIPLEXER

CHOP

MUXOUT ADCIN

BUFFER

f

MCLK

MODULATOR

/2 f

-

MCLK

DIGITAL

FILTER

/2

Figure 13. Channel Signal Chain Diagram with Chopping Enabled

Multiplexer, Conversion, and Data Output Timing

The specified “Conversion Time” includes one or two “Settling”
and “Sampling” periods and a “Scaling” time.

With chopping enabled (Figure 14), a conversion cycle starts
with a “Settling” time of 43 or 44 MCLK cycles (~7 µs with

6.144 MHz MCLK) to allow the circuits following the multiplexer
to settle. Then the sigma-delta modulator samples the analog
signals, and the digital filter processes the digital data stream.
The “Sampling” time depends on FW, i.e., on the Channel
Conversion Time register contents. After another “Settling” of
42 MCLK cycles (~6.8 µs), the “Sampling” time is repeated
with a reversed (chopped) analog input signal. Then, during the
“Scaling” time of 163 MCLK cycles (~26.5 µs), the two results
from the digital filter are averaged, scaled using the Calibration
registers, and written into the Channel Data register.

With chopping disabled (Figure 15), there is only one “Sampling”
time preceded by a “Settling” time of 43 or 44 MCLK cycles
and followed by a “Scaling” time of 163 MCLK cycles.

The RDY pin goes high during the “Scaling time” regardless of
its previous state. The relevant RDY bit is set in the ADC Sta­tus register, and in the Channel Status register the RDY pin
goes low when the Channel Data register is updated and the

+

CHOP

SCALING

ARITHMETIC

-

(CALIBRATIONS)

DIGITAL

INTERFACE

OUTPUT DATA
AT THE SELECTED
DATA RATE

channel conversion cycle is finished. If in Continuous Conversion
mode, the part will automatically continue with a conversion
cycle on the next enabled channel.

Note, that every channel can be configured independently for
conversion time and chopping mode. The overall cycle and effec­tive per channel data rate depends on all enabled channel settings.

Frequency Response

The sigma-delta modulator runs at 1/2 of MCLK frequency,
which is effectively the sampling frequency. Therefore, the
Nyquist frequency is 1/4 of the MCLK frequency. The digital
filter, in association with the modulator, features frequency
response of a first order low-pass filter. The –3 dB point is close
to the frequency of 1/Channel Conversion Time. The roll-off is
–20 dB/dec up to the Nyquist frequency. If chopping is enabled,
the input signal is resampled by chopping. Therefore, the over­all frequency response features notches close to the frequency of
1/Channel Conversion Time. The top envelope is again the
ADC response of –20 dB/dec.

The typical frequency response plots are in Figure 16. The plots
are normalized to 1/Channel Conversion Time.

MULTIPLEXER

RDY

-

CHANNEL 0

SETTLING

TIME

+CHANNEL 1

SAMPLING

TIME

SETTLING

TIME

CONVERSION TIME

-

CHANNEL 1

SAMPLING

TIME

SCALING

TIME

+CHANNEL 2

Figure 14. Multiplexer and Conversion Timing—Continuous Conversion on Several Channels with Chopping Enabled

MULTIPLEXER

RDY

+CHANNEL 0 +CHANNEL 2

SETTLING

TIME

+CHANNEL 1

SAMPLING

TIME

CONVERSION TIME

SCALING

TIME

Figure 15. Multiplexer and Conversion Timing—Continuous Conversion on Several Channels with Chopping Disabled

REV. 0–24–

AD7738

0

–10

CHOP = 1

–20

–30

GAIN – dB

–40

–50

–60

0.1 101
NORMALIZED INPUT FREQUENCY

(INPUT FREQUENCY  CONVERSION TIME)

a. Chopping Enabled

Figure 16. Typical ADC Frequency Response

Analog Inputs Voltage Range

The absolute input voltage range with input the buffer enabled
is restricted from AGND + 200 mV to AV

– 300 mV, which

DD

also places restrictions on the common-mode range. Care must
be taken in setting up the common-mode voltage and input
voltage range so that these limits are not exceeded, otherwise
there will be degradation in linearity performance.

The analog inputs on the AD7738 can accept either unipolar or
bipolar input voltage ranges. Bipolar input ranges do not imply
that the part can handle negative voltages with respect to system
ground on its analog inputs. Unipolar and bipolar signals on the
AIN(+) input are referenced to the voltage on the respective
AIN(–) input.

For example, if AINCOM is 2.5 V and CH0 is configured to
measure AIN0 – AINCOM, 0 V to 1.25 V, the input voltage
range on the AIN0 input is 2.5 V to 3.75 V. If CH0 is config­ured to measure AIN0 – AINCOM, ±1.25 V, the input voltage
range on the AIN0 input is 1.25 V to 3.75 V.

Analog Inputs Extended Voltage Range

The AD7738 output data code span corresponds to the nominal
input voltage range. However, the correct operation of the ADC
is guaranteed within the min/max input voltage range.

When the CLAMP bit of the Mode register is set to 1, the
Channel Data register will be digitally clamped either to all
zeros or all ones when the analog input voltage goes outside
the nominal input voltage range.

As shown in Tables XIII and XIV, when CLAMP = 0, the data
reflect the analog input voltage outside the nominal voltage
range. In this case, the SIGN and OVR bits in the Channel
Status register should be considered along with the Data register
value to decode the actual conversion result.

0

–10

CHOP = 0

–20

–30

GAIN – dB

–40

–50

–60

0.1 10001

(INPUT FREQUENCY  CONVERSION TIME)

NORMALIZED INPUT FREQUENCY

10 100

b. Chopping Disabled

Table XIII. Input Voltage Range 1.25 V, 16 Bits, CLAMP = 0

Input (V) Data (Hex) SIGN OVR

+1.45000 147B 0 1
+1.25008 0001 0 1
+1.25004 0000 0 1
+1.25000 FFFF 0 0
+0.00004 8001 0 0

0.00000 8000 0 0
–0.00004 7FFF 1 0
–1.25000 0000 1 0
–1.25004 FFFF 1 1
–1.25008 FFFE 1 1
–1.45000 EB85 1 1

Table XIV. Input Voltage Range 0 V to 1.25 V, 16 Bits,
CLAMP = 0

Input (V) Data (Hex) SIGN OVR

1.45000 28F5 0 1

1.25004 0001 0 1

1.25002 0000 0 1

1.25000 FFFF 0 0

0.00002 0001 0 0

0.00000 0000 0 0
–0.00002 0000 1 1

REV. 0

–25–

AD7738

Voltage Reference Inputs

The AD7738’s reference inputs, REFIN(+) and REFIN(–),
provide a differential reference input capability. The common­mode range for these differential inputs is from AGND to AV

DD

.
The nominal reference voltage for specified operation is 2.5 V.
Both reference inputs feature a high impedance, dynamic load.
Because the input impedance on each reference input is dynamic,
external resistance/capacitance combinations may result in gain
errors on the part.

The output noise performance outlined in Tables I to VI is for
an analog input of 0 V and is unaffected by noise on the reference.
To obtain the same noise performance as shown in the noise tables
over the full input range requires a low noise reference source for
the AD7738. If the reference noise in the bandwidth of interest
is excessive, it will degrade the performance of the AD7738.

Recommended reference voltage sources for the AD7738 include
the ADR421, AD780, REF43, and REF192. It is generally
recommended to decouple the output of these references to
further reduce the noise level.

AV

DD

+

10F

20

0.1F

AIN0

0.1F

ANALOG
INPUTS

20

20

AV

DD

+

10F

VIN

0.1F

ADR421

GND

VOUT

0.1F

AIN7

0.1F

AINCOM

0.1F

REFIN(+)

REFIN(–)

AV

MUXOUT -

DD

MUX

Reference Detect

The AD7738 includes on-chip circuitry to detect if the part has
a valid reference for conversions.

If the voltage between the REFIN(+) and REFIN(–) pins goes
below the NOREF Trigger Voltage (0.5 V typ) and the AD7738 is
performing conversion, the NOREF bit in the Channel Status
register is set.

I/O Port

The AD7738 Pin SYNC/P1 can be used as a general-purpose
digital I/O pin or to synchronize the AD7738 with other devices
in the system. When the SYNC bit in the I/O Port register is set
and the SYNC pin is low, the AD7738 doesn’t process any
conversion. If it is put into single conversion mode, Continuous
Conversion mode, or any Calibration mode, the AD7738 waits
until the SYNC pin goes high and then starts operation. This
allows the user to start conversion from a known point in time,
i.e., the rising edge of the SYNC pin.

DV

6.144MHz

HOST
SYSTEM

DD

+

10F

ADCIN

AD7738

BUFFER

CLOCK

GENERATOR

24-BIT

- ADC

SERIAL

INTERFACE

AND

CONTROL

LOGIC

DGNDAGND

0.1F

DV

DD

MCLKIN

MCLKOUT

33pF 33pF

DV

DD

RESET

SCLK

DIN

DOUT

RDY

CS

Figure 17. Typical Connection for the AD7738 Application

REV. 0–26–

AD7738

CALIBRATION

The AD7738 provides zero-scale self-calibration, and zero and
full system calibration capability, which can effectively reduce
the offset error and gain error to the order of the noise. After
each conversion, the ADC conversion result is scaled using the
ADC Calibration Registers and the relevant Channel Calibration
registers before being written to the Data register. See the equa­tions shown below.

For unipolar ranges:

Data = ((ADC result – ADC ZS Cal. reg.)  ADC FS reg./
200000h – Ch. ZS Cal. reg.)  Ch. FS Cal. reg./200000h

For bipolar ranges:

Data = ((ADC result – ADC ZS Cal. reg.)  ADC FS reg./
400000h + 800000h – Ch. ZS Cal. reg.)  Ch. FS Cal. reg./
200000h

Where the ADC result is in the range of 0 to FFFFFFh.

Note that the Channel ZS Calibration register has the format of
a sign bit + 22 bits Channel offset value.

It is strongly recommended that the user does not change the
ADC FS register.

To start any calibration, write the relevant mode bits to the
AD7738 Mode register. After the calibration is complete, the
contents of the corresponding Calibration registers are updated, all
RDY bits in the ADC Status register are set, the RDY pin goes
low, and the AD7738 reverts to Idle mode.

The calibration duration is the same as conversion time configured
on the selected channel. The longer conversion time gives less
noise and yields a more exact calibration. Therefore, use at least
the default conversion time to initiate any calibration.

ADC Zero-Scale Self-Calibration

The ADC Zero-Scale Self-Calibration can effectively remove
the offset error in Chopping Disabled mode. If repeated after a
temperature change, it can also remove the offset drift error in
Chopping Disabled mode.

The zero-scale self-calibration is performed on internally shorted
ADC inputs. The negative Analog Input terminal on the selected
channel is used to set the ADC ZS Calibration common mode.
Therefore, either the negative terminal on selected differential pair
or AINCOM on single-ended channel configuration should be
driven to a proper commwon-mode voltage.

It is strongly recommended that the ADC ZS Calibration register
should only be updated as part of a zero-scale self-calibration.

Per Channel System Calibration

If the per channel system calibrations are used, these should be
initiated in the following order: first a Channel ZS System Cali­bration followed by a Channel FS System Calibration.

The System Calibration is affected by the ADC ZS and FS Cali­bration registers; therefore, if both Self-Calibration and System
Calibration are used in a system, an ADC Self-Calibration cycle
should be performed first followed by a System Calibration cycle.

While executing a system calibration, the fully settled system
zero-scale voltage signal or system full-scale voltage signal must
be connected to the selected channel analog inputs.

The per channel Calibration registers can be read, stored, or
modified and written back to the AD7738. Note, when writing
the Calibration registers the AD7738 must be in the idle mode.
Note that outside the specified calibration range, the calibration
is possible but the performance may degrade. (See the System
Calibration section in the specification pages of this data sheet.)

REV. 0

–27–

AD7738

OUTLINE DIMENSIONS

28-Lead Thin Shrink Small Outline Package (TSSOP)

(RU-28)

Dimensions shown in millimeters

9.80

9.70

9.60

28

PIN 1

0.15

0.05

COPLANARITY

0.10

15

4.50

4.40

4.30

141

0.65

BSC

0.30

0.19

COMPLIANT TO JEDEC STANDARDS MO-153AE

SEATING
PLANE

1.20

MAX

6.40 BSC

0.20

0.09

C03072–0–11/02(0)

8
0

0.75

0.60

0.45

–28–

PRINTED IN U.S.A.

REV. 0