Datasheet AD7675ASTRL, AD7675AST Datasheet (Analog Devices)

16-Bit, 100 kSPS,

a

FEATURES
Throughput: 100 kSPS
INL: ⴞ1.5 LSB Max (ⴞ0.0015% of Full-Scale)
16 Bits Resolution with No Missing Codes
S/(N+D): 94 dB Typ @ 45 kHz
THD: –110 dB Typ @ 45 kHz
Differential Input Range: ⴞ2.5 V
Both AC and DC Specifications
No Pipeline Delay
Parallel (8/16 Bits) and Serial 5 V/3 V Interface
SPI™/QSPI™/MICROWIRE™/DSP Compatible
Single 5 V Supply Operation
15 mW Typical Power Dissipation, 15 ␮W @ 100 SPS
Power-Down Mode: 7 ␮W Max
Package: 48-Lead Quad Flat Pack (LQFP)
Pin-to-Pin Compatible with the AD7660
Replacement of AD676, AD677

APPLICATIONS
CT Scanners
Data Acquisition
Instrumentation
Spectrum Analysis
Medical Instruments
Battery-Powered Systems
Process Control

FUNCTIONAL BLOCK DIAGRAM

AVDD AGND REF REFGND

IN+

IN–

PD

RESET

CONTROL LOGIC AND

CALIBRATION CIRCUITRY

SWITCHED

CAP DAC

Differential ADC

AD7675

DGNDDVD D

AD7675

SERIAL

PORT

16

CLOCK

CNVST

PARALLEL

INTERFACE

*

OVD D

OGND

SER/PAR

BUSY

DATA[15:0]

CS

RD

OB/2C

BYTESWAP

GENERAL DESCRIPTION

The AD7675 is a 16-bit, 100 kSPS, charge redistribution SAR,
fully differential analog-to-digital converter that operates from a
single 5 V power supply. The part contains a high-speed 16-bit
sampling ADC, an internal conversion clock, error correction
circuits, and both serial and parallel system interface ports.

The AD7675 is hardware factory calibrated and is comprehensively
tested to ensure such ac parameters as signal-to-noise ratio (SNR)
and total harmonic distortion (THD), in addition to the more
traditional dc parameters of gain, offset, and linearity.

It is fabricated using Analog Devices’ high-performance, 0.6
micron CMOS process and is available in a 48-lead LQFP with
operation specified from –40°C to +85°C.

*Patent pending

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

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.

PRODUCT HIGHLIGHTS

1. Excellent INL
The AD7675 has a maximum integral nonlinearity of 1.5 LSB
with no missing 16-bit code.

2. Superior AC Performances
The AD7675 has a minimum dynamic of 92 dB, 94 dB typical.

3. Fast Throughput
The AD7675 is a 100 kSPS, charge redistribution, 16-bit
SAR ADC with internal error correction circuitry.

4. Single-Supply Operation
The AD7675 operates from a single 5 V supply and typically
dissipates only 17 mW. Its power dissipation decreases
with the throughput to, for instance, only 15 µW at a 100 SPS
throughput. It consumes 7 µW maximum when in power-down.

5. Serial or Parallel Interface
Versatile parallel (8 or 16 bits) or 2-wire serial interface
arrangement compatible with either 3 V or 5 V logic.

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: [phone] www.analog.com
Fax: [phone] © Analog Devices, Inc., 2001

AD7675–SPECIFICATIONS

(–40ⴗC to +85ⴗC, AVDD = DVDD = 5 V, OVDD = 2.7 V to 5.25 V, unless otherwise noted.)

Parameter Conditions Min Typ Max Unit

RESOLUTION 16 Bits

ANALOG INPUT

Voltage Range V
Operating Input Voltage V
Analog Input CMRR f
Input Current 100 kSPS Throughput 1 µA

– V

IN+

IN–

to AGND –0.1 +3 V

IN+, VIN–

= 10 kHz 79 dB

IN

–V

REF

+V

REF

V

Input Impedance See Analog Input Section

THROUGHPUT SPEED

Complete Cycle 10 µs
Throughput Rate 0 100 kSPS

DC ACCURACY

Integral Linearity Error –1.5 +1.5 LSB

1

No Missing Codes 16 Bits
Transition Noise 0.35 LSB
+Full-Scale Error
–Full-Scale Error
Zero Error

2

2

2

–22 +22 LSB
–22 +22 LSB
–8 +8 LSB

Power Supply Sensitivity AVDD = 5 V ± 5% ±0.5 LSB

AC ACCURACY

Signal-to-Noise f

= 20 kHz 92 94 dB

IN

fIN = 45 kHz 94 dB

Spurious Free Dynamic Range fIN = 20 kHz 104.5 110 dB

fIN = 45 kHz 110 dB

Total Harmonic Distortion fIN = 20 kHz –110 –103.5 dB

fIN = 45 kHz –110 dB

Signal-to-(Noise+Distortion) fIN = 20 kHz 92 94 dB

fIN = 45 kHz 94 dB
fIN = 45 kHz, –60 dB Input 34 dB

3

3

3

3

3

3

3

3

3

–3 dB Input Bandwidth 3.9 MHz

SAMPLING DYNAMICS

Aperture Delay 2ns
Aperture Jitter 5 ps rms
Transient Response Full-Scale Step 8.75 µs

REFERENCE

External Reference Voltage Range 2.3 2.5 AVDD – 1.85 V
External Reference Current Drain 100 kSPS Throughput 35 µA

DIGITAL INPUTS

Logic Levels

V

IL

V

IH

I

IL

I

IH

–0.3 +0.8 V
+2.0 DVDD + 0.3 V
–1 +1 µA
–1 +1 µA

DIGITAL OUTPUTS

Data Format Parallel or Serial 16-Bit Conversion Results Available
Pipeline Delay Immediately After Completed Conversion

V

OL

V

OH

I

= 1.6 mA 0.4 V

SINK

I

= –100 µA OVDD – 0.6 V

SOURCE

POWER SUPPLIES

Specified Performance

AVDD 4.75 5 5.25 V
DVDD 4.75 5 5.25 V
OVDD 2.7 5.25 V

Operating Current 300 kSPS Throughput

AVDD 3mA

4

DVDD

4

OVDD

Power Dissipation

In Power-Down Mode

TEMPERATURE RANGE

Specified Performance T

NOTES

1

LSB means Least Significant Bit. With the ± 2.5 V input range, one LSB is 76.3 µV.

2

See Definition of Specifications section. These specifications do not include the error contribution from the external reference.

3

All specifications in dB are referred to a full-scale input FS. Tested with an input signal at 0.5 dB below full-scale unless otherwise specified.

4

Tested in parallel reading mode.

5

With all digital inputs forced to OVDD or OGND respectively.

6

Contact factory for extended temperature range.

Specifications subject to change without notice.

4

5

6

100 kSPS Throughput 17 25 mW
100 SPS Throughput 15 µW

MIN

to T

MAX

–40 +85 °C

–2–

750 µA

7.5 µA

7 µW

REV. 0

AD7675

TIMING SPECIFICATIONS

(–40ⴗC to +85ⴗC, AVDD = DVDD = 5 V, OVDD = 2.7 V to 5.25 V, unless otherwise noted.)

Symbol Min Typ Max Unit

Refer to Figures 11 and 12

Convert Pulsewidth t
Time Between Conversions t
CNVST LOW to BUSY HIGH Delay t
BUSY HIGH All Modes Except in Master Serial Read t

1

2

3

4

5ns
10 µs

30 ns

1.25 µs

After Convert Mode
Aperture Delay t
End of Conversion to BUSY LOW Delay t
Conversion Time t
Acquisition Time t
RESET Pulsewidth t

5

6

7

8

9

10 ns

8.75 µs
10 ns

2ns

1.25 µs

Refer to Figures 13, 14, and 15 (Parallel Interface Modes)

CNVST LOW to DATA Valid Delay t
DATA Valid to BUSY LOW Delay t
Bus Access Request to DATA Valid t
Bus Relinquish Time t

Refer to Figures 16 and 17 (Master Serial Interface Modes)

1

CS LOW to SYNC Valid Delay t
CS LOW to Internal SCLK Valid Delay t
CS LOW to SDOUT Delay t
CNVST LOW to SYNC Delay t

SYNC Asserted to SCLK First Edge Delay
Internal SCLK Period
Internal SCLK HIGH
Internal SCLK LOW
SDOUT Valid Setup Time
SDOUT Valid Hold Time
SCLK Last Edge to SYNC Delay

2

2

2

2

2

2

2

CS HIGH to SYNC HI-Z t
CS HIGH to Internal SCLK HI-Z t
CS HIGH to SDOUT HI-Z t

BUSY HIGH in Master Serial Read After Convert

2

CNVST LOW to SYNC Asserted Delay t
SYNC Deasserted to BUSY LOW Delay t

10

11

12

13

14

15

16

17

t

18

t

19

t

20

t

21

t

22

t

23

t

24

25

26

27

t

28

29

30

45 ns

515ns

525 ns
3ns
25 40 ns
12 ns
7ns
4ns
2ns
3ns

See Table I µs

1.25 µs

25 ns

1.25 µs

40 ns

10 ns
10 ns
10 ns

10 ns
10 ns
10 ns

Refer to Figures 18 and 19 (Slave Serial Interface Modes)

External SCLK Setup Time t
External SCLK Active Edge to SDOUT Delay t
SDIN Setup Time t
SDIN Hold Time t
External SCLK Period t
External SCLK HIGH t
External SCLK LOW t

NOTES

1

In serial interface modes, the SYNC, SCLK, and SDOUT timings are defined with a maximum load CL of 10 pF; otherwise, the load is 60 pF maximum.

2

In serial master read during convert mode. See Table I for serial master read after convert mode.

Specifications subject to change without notice.

31

32

33

34

35

36

37

5ns
318ns
5ns
5ns
25 ns
10 ns
10 ns

REV. 0

–3–

AD7675

WARNING!

ESD SENSITIVE DEVICE

Table I. Serial Clock Timings in Master Read after Convert

DIVSCLK[1] 0011
DIVSCLK[0] 0101Unit

SYNC to SCLK First Edge Delay Minimum t
Internal SCLK Period Minimum t
Internal SCLK Period Typical t
Internal SCLK HIGH Minimum t
Internal SCLK LOW Minimum t
SDOUT Valid Setup Time Minimum t
SDOUT Valid Hold Time Minimum t
SCLK Last Edge to SYNC Delay Minimum t
Busy High Width Maximum t

18

19

19

20

21

22

23

24

28

3 171717ns
25 50 100 200 ns
40 70 140 280 ns
12 22 50 100 ns
7 214999ns
4 181818ns
2 4 3089ns
3 60 140 300 ns
2 2.5 3.5 5.75 µs

ABSOLUTE MAXIMUM RATINGS

Analog Inputs

2

, IN–2, REF, REFGND . . . . . . . . . . . . . . . . . . . . . . . .

IN+

1

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

Ground Voltage Differences

AGND, DGND, OGND . . . . . . . . . . . . . . . . . . . . . ±0.3 V

Supply Voltages

AVDD, DVDD, OVDD . . . . . . . . . . . . . . . . . . . . . . . . . 7 V

AVDD to DVDD,

AVDD to OVDD . . . . . . . . . . . . . . ± 7 V

DVDD to OVDD . . . . . . . . . . . . . . . . . . . . . . . . . . . . ± 7 V

Digital Inputs . . . . . . . . . . . . . . . . –0.3 V to DVDD + 0.3 V

Internal Power Dissipation

3

. . . . . . . . . . . . . . . . . . . . 700 mW

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

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

Lead Temperature Range

(Soldering 10 sec) . . . . . . . . . . . . . . . . . . . . . . . . . . 300°C

NOTES

1

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.

2

See Analog Input section.

3

Specification is for device in free air: 48-Lead LQFP: ␪JA = 91°C/W, ␪JC = 30°C/W.

ORDERING GUIDE

I

1.6mA

TO OUTPUT

PIN

C

L
1

60pF

500␮A

NOTE

1

IN SERIAL INTERFACE MODES, THE SYNC, SCLK, AND
SDOUT TIMINGS ARE DEFINED WITH A MAXIMUM LOAD
C

OF 10pF; OTHERWISE, THE LOAD IS 60pF MAXIMUM.

L

OL

1.4V

I

OH

Figure 1. Load Circuit for Digital Interface Timing, SDOUT,
SYNC, SCLK Outputs, C

0.8V

t

DELAY

L

2V

0.8V

= 10 pF

2V

t

DELAY

2V

0.8V

Figure 2. Voltage Reference Levels for Timing

Model Temperature Range Package Description Option

AD7675AST –40°C to +85°C Quad Flatpack (LQFP) ST-48
AD7675ASTRL –40°C to +85°C Quad Flatpack (LQFP) ST-48
EVAL-AD7675CB
EVAL-CONTROL BRD2

NOTES

1

This board can be used as a stand-alone evaluation board or in conjunction with the EVAL-CONTROL BRD2 for evaluation/
demonstration purposes.

2

This board allows a PC to control and communicate with all Analog Devices evaluation boards ending in the CB designators.

1

2

Evaluation Board
Controller Board

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 AD7675 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.

–4–

Package

REV. 0

AD7675

PIN FUNCTION DESCRIPTIONS

Pin No. Mnemonic Type Description

1 AGND P Analog Power Ground Pin
2 AVDD P Input Analog Power Pins. Nominally 5 V.
3, 6, 7, NC No Connect
40–42,
44–48
4 BYTESWAP DI Parallel Mode Selection (8/16 Bit). When LOW, the LSB is output on D[7:0] and the MSB is

output on D[15:8]. When HIGH, the LSB is output on D[15:8] and the MSB is output on D[7:0].

5 OB/2C DI Straight Binary/Binary Two’s Complement. When OB/2C is HIGH, the digital output is

straight binary; when LOW, the MSB is inverted resulting in a two’s complement output
from its internal shift register.

8 SER/PAR DI Serial/Parallel Selection Input. When LOW, the parallel port is selected; when HIGH, the

serial interface mode is selected and some bits of the DATA bus are used as a serial port.

9, 10 DATA[0:1] DO Bit 0 and Bit 1 of the Parallel Port Data Output Bus. When SER/PAR is HIGH, these outputs are

in high impedance.

11, 12 DATA[2:3] or DI/O When SER/PAR is LOW, these outputs are used as Bit 2 and Bit 3 of the Parallel Port

Data Output Bus.

DIVSCLK[0:1] When SER/PAR is HIGH, EXT/INT is LOW and RDC/SDIN is LOW which is the serial

master read after convert mode. These inputs, part of the serial port, are used to slow down, if
desired, the internal serial clock which clocks the data output. In the other serial modes, these
inputs are not used.

13 DATA[4] DI/O When SER/PAR is LOW, this output is used as the Bit 4 of the Parallel Port Data Output Bus.

or EXT/INT When SER/PAR is HIGH, this input, part of the serial port, is used as a digital select input for

choosing the internal or an external data clock. With EXT/INT tied LOW, the internal clock
is selected on SCLK output. With EXT/INT set to a logic HIGH, output data is synchronized
to an external clock signal connected to the SCLK input.

14 DATA[5] DI/O When SER/PAR is LOW, this output is used as the Bit 5 of the Parallel Port Data Output Bus.

or INVSYNC When SER/PAR is HIGH, this input, part of the serial port, is used to select the active state of

the SYNC signal. When LOW, SYNC is active HIGH. When HIGH, SYNC is active LOW.

15 DATA[6] DI/O When SER/PAR is LOW, this output is used as the Bit 6 of the Parallel Port Data Output Bus.

or INVSCLK

16 DATA[7] DI/O When SER/PAR is LOW, this output is used as the Bit 7 of the Parallel Port Data Output Bus.

or RDC/SDIN When SER/PAR is HIGH, this input, part of the serial port, is used as either an external data

17 OGND P Input/Output Interface Digital Power Ground
18 OVDD P Input/Output Interface Digital Power. Nominally at the same supply than the supply of the

19 DVDD P Digital Power. Nominally at 5 V.
20 DGND P Digital Power Ground

When SER/PAR is HIGH, this input, part of the serial port, is used to invert the SCLK signal.
It is active in both master and slave mode.

input or a read mode selection input depending on the state of EXT/INT.
When EXT/INT is HIGH, RDC/SDIN could be used as a data input to daisy chain the con-

version results from two or more ADCs onto a single SDOUT line. The digital data level on
SDIN is output on DATA with a delay of 16 SCLK periods after the initiation of the read
sequence. When EXT/INT is LOW, RDC/SDIN is used to select the read mode. When RDC/
SDIN is HIGH, the data is output on SDOUT during conversion. When RDC/SDIN is LOW,
the data is output on SDOUT only when the conversion is complete.

host interface (5 V or 3 V).

REV. 0

–5–

AD7675

PIN FUNCTION DESCRIPTIONS (continued)

Pin No. Mnemonic Type Description

21 DATA[8] DO When SER/PAR is LOW, this output is used as the Bit 8 of the Parallel Port Data Output Bus.

or SDOUT When SER/PAR is HIGH, this output, part of the serial port, is used as a serial data output

synchronized to SCLK. Conversion results are stored in an on-chip register. The AD7675
provides the conversion result, MSB first, from its internal shift register. The DATA
format is determined by the logic level of OB/2C. In serial mode, when EXT/INT is LOW,
SDOUT is valid on both edges of SCLK. In serial mode, when EXT/INT is HIGH: If
INVSCLK is LOW, SDOUT is updated on SCLK rising edge and valid on the next falling
edge. If INVSCLK is HIGH, SDOUT is updated on SCLK falling edge and valid on the next
rising edge.

22 DATA[9] DI/O When SER/PAR is LOW, this output is used as the Bit 9 of the Parallel Port Data Output Bus.

or SCLK When SER/PAR is HIGH, this pin, part of the serial port, is used as a serial data clock input

or output, dependent upon the logic state of the EXT/INT pin. The active edge where the
data SDOUT is updated depends upon the logic state of the INVSCLK pin.

23 DATA[10] DO When SER/PAR is LOW, this output is used as the Bit 10 of the Parallel Port Data Output Bus.

or SYNC When SER/PAR is HIGH, this output, part of the serial port, is used as a digital output frame

synchronization for use with the internal data clock (EXT/INT = Logic LOW). When a read
sequence is initiated and INVSYNC is LOW, SYNC is driven HIGH and remains HIGH
while SDOUT output is valid. When a read sequence is initiated and INVSYNC is HIGH,
SYNC is driven LOW and remains LOW while SDOUT output is valid.

24 DATA[11] DO When SER/PAR is LOW, this output is used as the Bit 11 of the Parallel Port Data Output Bus.

or RDERROR When SER/PAR is HIGH and EXT/INT is HIGH, this output, part of the serial port, is used

as an incomplete read error flag. In slave mode, when a data read is started and not complete
when the following conversion is complete, the current data is lost and RDERROR is pulsed high.

25–28 DATA[12:15] DO Bit 12 to Bit 15 of the Parallel Port Data output bus. These pins are always outputs regardless

of the state of SER/PAR.

29 BUSY DO Busy Output. Transitions HIGH when a conversion is started, and remains HIGH until the

conversion is complete and the data is latched into the on-chip shift register. The falling edge
of BUSY could be used as a data ready clock signal.

30 DGND P Must be tied to digital ground.
31 RD DI Read Data. When CS and RD are both LOW, the interface parallel or serial output bus is enabled.
32 CS DI Chip Select. When CS and RD are both LOW, the interface parallel or serial output bus is

enabled. CS is also used to gate the external serial clock.

33 RESET DI Reset Input. When set to a logic HIGH, reset the AD7675. Current conversion if any is aborted.
34 PD DI Power-Down Input. When set to a logic HIGH, power consumption is reduced and conver-

sions are inhibited after the current one is completed.

35 CNVST DI Start Conversion. If CNVST is HIGH when the acquisition phase (t

falling edge on CNVST puts the internal sample/hold into the hold state and initiates a
conversion. This mode is the most appropriate if low sampling jitter is desired. If CNVST is
LOW when the acquisition phase (t

) is complete, the internal sample/hold is put into the hold

8

state and a conversion is immediately started.
36 AGND P Must be tied to analog ground.
37 REF AI Reference Input Voltage
38 REFGND AI Reference Input Analog Ground
39 IN– AI Differential Negative Analog Input
43 IN+ AI Differential Positive Analog Input

NOTES
AI = Analog Input
DI = Digital Input
DI/O = Bidirectional Digital
DO = Digital Output
P = Power

) is complete, the next

8

–6–

REV. 0

PIN CONFIGURATION

48-Lead LQFP

(ST-48)

AD7675

NCNCNCNCNC

48 47 46 45 44 39 38 3743 42 41 40

1

AGND

AV DD

NC

BYTESWAP

OB/2C

NC

NC

SEP/PAR

D2/DIVSCLK[0]

D3/DIVSCLK[1]

NC = NO CONNECT

PIN 1

2

IDENTIFIER

3

4

5

6

7

8

9

D0

10

D1

11

12

13 14 15 16 17 18 19 20 21 22 23 24

D4/EXT/INT

AD7675

TOP VIEW

(Not to Scale)

OGND

D6/INVSCLK

D5/INVSYNC

D7/RDC/SDIN

DEFINITION OF SPECIFICATIONS

INTEGRAL NONLINEARITY ERROR (INL)

Integral Nonlinearity is the maximum deviation of a straight line
drawn through the transfer function of the actual ADC. The
deviation is measured from the middle of each code.

DIFFERENTIAL NONLINEARITY ERROR (DNL)

In an ideal ADC, code transitions are 1 LSB apart. Differential
nonlinearity is the maximum deviation from this ideal value. It
is often specified in terms of resolution for which no missing
codes are guaranteed.

+FULL-SCALE ERROR

The last transition (from 011 . . . 10 to 011 . . . 11 in two’s
complement coding) should occur for an analog voltage 1 1/2 LSB

±

below the nominal +full scale (+2.499886 V for the

2.5 V range).
The +full-scale error is the deviation of the actual level of the
last transition from the ideal level.

IN+NCNCNCIN–

DVD D

OVD D

DGND

D8/SDOUT

REFGND

REF

D9/SCLK

D10/SYNC

D11/RDERROR

36

35

34

33

32

31

30

29

28

27

26

25

AGND

CNVST

PD

RESET

CS
RD

DGND

BUSY

D15

D14

D13

D12

EFFECTIVE NUMBER OF BITS (ENOB)

ENOB is a measurement of the resolution with a sine wave
input. It is related to S/(N+D) by the following formula:

ENOB S N D

=+

/–./.176 602

[]

()

dB

and is expressed in bits.

TOTAL HARMONIC DISTORTION (THD)

THD is the ratio of the rms sum of the first five harmonic
components to the rms value of a full-scale input signal and is
expressed in decibels.

SIGNAL-TO-NOISE RATIO (SNR)

SNR is the ratio of the rms value of the actual input signal to the
rms sum of all other spectral components below the Nyquist
frequency, excluding harmonics and dc. The value for SNR is
expressed in decibels.

–FULL-SCALE ERROR

The first transition (from 100 . . . 00 to 100 . . . 01 in two’s
complement coding) should occur for an analog voltage 1 1/2 LSB

±

above the nominal –full scale (–2.499962 V for the

2.5 V range).
The –full-scale error is the deviation of the actual level of the
last transition from the ideal level.

BIPOLAR ZERO ERROR

The bipolar zero error is the difference between the ideal midscale
input voltage (0 V) and the actual voltage producing the midscale
output code.

SPURIOUS FREE DYNAMIC RANGE (SFDR)

The difference, in decibels (dB), between the rms amplitude of
the input signal and the peak spurious signal.

REV. 0

–7–

SIGNAL-TO-(NOISE + DISTORTION) RATIO (S/[N+D])

S/(N+D) is the ratio of the rms value of the actual input signal
to the rms sum of all other spectral components below the
Nyquist frequency, including harmonics but excluding dc. The
value for S/(N+D) is expressed in decibels.

APERTURE DELAY

Aperture delay is a measure of the acquisition performance and
is measured from the falling edge of the CNVST input to when
the input signal is held for a conversion.

TRANSIENT RESPONSE

The time required for the AD7675 to achieve its rated accuracy
after a full-scale step function is applied to its input.

AD7675

–Typical Performance Characteristics

1.00

0.75

0.50

0.25

0.00

INL – LSB

–0.25

–0.50

–0.75

–1.00

0 16384 32768 49152 65536

CODE

TPC 1. Integral Nonlinearity vs. Code

9000

8000

7000

6000

5000

4000

COUNTS

3000

2000

1000

0000

0

7FFB

7FFC07FFD07FFE67FFF

8246

8118

8000

CODE IN HEXA

80011480020800308004

16000

14000

12000

10000

8000

COUNTS

6000

4000

2000

0000

7FF8

0

7FF9

0

7FFA07FFB07FFC

14687

887

CODE IN HEXA

810

7FFD 7FFE

0

7FFF080000800108002

TPC 4. Histogram of 16,384 Conversions of a DC Input at
the Code Center

18

16

14

12

10

8

6

NUMBER OF UNITS

4

2

0

0

–0.8

–0.6 –0.4 –0.2 0.0–1.0

NEGATIVE INL – LSB

TPC 2. Histogram of 16,384 Conversions of a DC Input at
the Code Transition

18

16

14

12

10

8

6

NUMBER OF UNITS

4

2

0

0.2

0.4 0.6 0.8 1.00

POSITIVE INL – LSB

TPC 3. Typical Positive INL Distribution (40 Units)

TPC 5. Typical Negative INL Distribution (40 Units)

0

–20

–40

–60

–80

–100

–120

–140

AMPLITUDE – dB of Full Scale

–160

–180

0 102030 50

f

= 100 kSPS

S

f

= 45.01kHz

IN

SNR = 94dB
THD = –110dB
SFDR = 110dB
SINAD = 93.9dB

FREQUENCY – kHz

40

TPC 6. FFT Plot

–8–

REV. 0

AD7675

CL – pF

50

t

12 DELAY – ns

0

20

2000

40

10

10050 150

30

OVDD = 5.0V @ 25ⴗC

OVDD = 5.0V @ 85ⴗC

OVDD = 2.7V @ 25ⴗC

OVDD = 2.7V @ 85ⴗC

TEMPERATURE – ⴗC

250

POWER-DOWN OPERATING CURRENTS – nA

0

100

–15 105–55 45

150

50

DVD D

–35 5 8525 65

200

OVD D

AV DD

100

95

90

SNR

85

SINAD

80

SNR AND S/[N+D] – dB

75

70

10 10001 100

FREQUENCY – kHz

ENOB

TPC 7. SNR, S/(N+D), and ENOB vs. Frequency

96

SNR

94

92

SINAD

16.0

15.5

15.0

14.5

14.0

13.5

13.0

ENOB – Bits

TPC 10. Typical Delay vs. Load Capacitance C

100k

10k

1k

100

10

AV DD

DVD D

L

1

90

SNR (REFERRED TO FULL SCALE) – dB

88

–40 0–60 –20

–50 –30 –10

INPUT LEVEL – dB

TPC 8. SNR and S/(N+D) vs. Input Level

96

SNR

93

90

SNR – dB

87

84

–35 45 10525–15 65

TPC 9. SNR, THD vs. Temperature

THD

0 125–55 85

TEMPERATURE – ⴗC

–104

–106

–108

THD – dB

–110

–112

TPC 12. Power-Down Operating Currents vs. Temperature

0.1

OPERATING CURRENTS – ␮A

0.01

0.001
10

SAMPLING RATE – SPS

1k100 10k

TPC 11. Operating Currents vs. Sample Rate

OVD D

100k

REV. 0

–9–

AD7675

IN+

REF

REFGND

IN–

32,768C 16,384C

MSB

32,768C 16,384C

MSB

4C 2C C C

4C 2C C C

Figure 3. ADC Simplified Schematic

LSB

LSB

SW

COMP

SW

+

–

SWITCHES

CONTROL

CONTROL

LOGIC

CNVST

BUSY

OUTPUT

CODE

CIRCUIT INFORMATION

The AD7675 is a fast, low-power, single-supply, precise 16-bit
analog-to-digital converter (ADC). The AD7675 is capable of
converting 100,000 samples per second (100 kSPS) and allows
power saving between conversions. When operating at 100 SPS,
for example, it consumes typically only 15 µW. This feature
makes the AD7675 ideal for battery-powered applications.

The AD7675 provides the user with an on-chip track/hold,
successive approximation ADC that does not exhibit any pipe­line or latency, making it ideal for multiple multiplexed channel
applications.

The AD7675 can be operated from a single 5 V supply and be
interfaced to either 5 V or 3 V digital logic. It is housed in a
48-lead LQFP package that combines space savings and allows
flexible configurations as either serial or parallel interface. The
AD7675 is pin-to-pin compatible with the AD7660.

CONVERTER OPERATION

The AD7675 is a successive approximation analog-to-digital
converter based on a charge redistribution DAC. Figure 3 shows
the simplified schematic of the ADC. The capacitive DAC con­sists of two identical arrays of 16 binary-weighted capacitors that
are connected to the two comparator inputs.

During the acquisition phase, terminals of the array tied to the
comparator’s input is connected to AGND via SW

and SW–.

+

All independent switches are connected to the analog inputs.
Thus, the capacitor arrays are used as sampling capacitors and
acquire both analog signals.

When the acquisition phase is complete and the CNVST input
goes or is low, a conversion phase is initiated. When the con­version phase begins, SW

and SW– are opened first. The two

+

capacitor arrays are then disconnected from the inputs and

connected to the REFGND input. Therefore, the differential
voltage between the output of IN+ and IN– captured at the
end of the acquisition phase is applied to the comparator inputs,
causing the comparator to become unbalanced.

By switching each element of the capacitor array between
REFGND or REF, the comparator input varies by binary
weighted voltage steps (V

REF

/2, V

/4...V

REF

/65536). The

REF

control logic toggles these switches, starting with the MSB first,
in order to bring the comparator back into a balanced condition.
After the completion of this process, the control logic generates
the ADC output code and brings BUSY output low.

Transfer Functions

Using the OB/2C digital input, the AD7675 offers two output
codings: straight binary and two’s complement. The ideal trans­fer characteristic for the AD7675 is shown in Figure 4.

111...111

111...110

111...101

ADC CODE – Straight Binary

000...010

000...001

000...000

–FS + 1 LSB–FS

–FS + 0.5 LSB

+FS – 1 LSB

+FS – 1.5 LSB

ANALOG INPUT

Figure 4. ADC Ideal Transfer Function

–10–

REV. 0

AD7675

ANALOG

SUPPLY

ADR421

2.5V REF

NOTE 1

ANALOG INPUT+

ANALOG INPUT–

(5V)

1M⍀

100nF

NOTE 4

AD8021

NOTE 4

AD8021

NOTE 3

–

+

–

+

50⍀

U1

U2

50⍀

50k⍀

100⍀

+

C

+

REF

NOTE 2

15⍀

C

C

15⍀

C

C

10␮F

NOTE 5

100nF

1␮F

2.7nF

2.7nF

NOTE 5

NOTE 5

AV DD AGND DGND

REF

REFGND

IN+

+

10␮F 100nF

AD7675

IN–

DVD D

DVD D

OVD D OGND

SCLK

SDOUT

BUSY

CNVST

OB/2C

SER/PAR

BYTESWAP

RESET

100nF

CS

RD

PD

+

10␮F

50⍀

NOTE 7

DVD D

DIGITAL SUPPLY
(3.3V OR 5V)

SERIAL PORT

D

CLOCK

␮C/␮P/DSP

NOTES

1. SEE VOLTAGE REFERENCE INPUT SECTION.

2. WITH THE RECOMMENDED VOLTAGE REFERENCES, C

3. OPTIONAL CIRCUITRY FOR HARDWARE GAIN CALIBRATION.

4. THE AD8021 IS RECOMMENDED. SEE DRIVER AMPLIFIER CHOICE SECTION.

5. SEE ANALOG INPUT SECTION.

6. OPTION, SEE POWER SUPPLY SECTION

7. OPTIONAL LOW JITTER CNVST, SEE CONVERSION CONTROL SECTION.

IS 47␮F. SEE CHAPTER VOLTAGE REFERENCE INPUT SECTION

REF

Figure 5. Typical Connection Diagram. (±2.5 V Range Shown)

TYPICAL CONNECTION DIAGRAM

Figure 5 shows a typical connection diagram for the AD7675.
Different circuitry shown on this diagram are optional and are
discussed below.

Analog Inputs

Figure 6 shows a simplified analog input section of the AD7675.

the input buffer’s (U1) or (U2) supplies are different from
AVDD. In such case, an input buffer with a short-circuit cur­rent limitation can be used to protect the part.

This analog input structure is a true differential structure. By
using these differential inputs, signals common to both inputs
are rejected, as shown in Figure 7, which represents the typical
CMRR over frequency.

AV DD

IN+

IN–

AGND

R+ = 684⍀

R– = 684⍀

C

S

C

S

Figure 6. AD7675 Simplified Analog Input

The diodes shown in Figure 6 provide ESD protection for the
inputs. Care must be taken to ensure that the analog input sig­nal never exceeds the absolute ratings on these inputs. This will
cause these diodes to become forward-biased and start conducting
current. These diodes can handle a forward-biased current of
120 mA maximum. This condition could eventually occur when

85

80

75

70

65

60

CMRR – dB

55

50

45

40

10k 10M1k 1M

100k

FREQUENCY – Hz

Figure 7. Analog Input CMRR vs. Frequency

REV. 0

–11–

AD7675

During the acquisition phase, for ac signals, the AD7675 behaves
like a one-pole RC filter consisting of the equivalent resistance
R+, R–, and C

. The resistors R+ and R– are typically 684 Ω

S

and are lumped components made up of some serial resistors
and the on-resistance of the switches. The capacitor C

is typically

S

60 pF and is mainly the ADC sampling capacitor. This one pole
filter with a typical –3 dB cutoff frequency of 3.9 MHz reduces
undesirable aliasing effect and limits the noise coming from
the inputs.

Because the input impedance of the AD7675 is very high, the
AD7675 can be driven directly by a low impedance source
without gain error. That allows users to put, as shown in
Figure 5, an external one-pole RC filter between the output
of the amplifier output and the ADC analog inputs to even
further improve the noise filtering done by the AD7675 analog
input circuit. However, the source impedance has to be kept
low because it affects the ac performances, especially the total
harmonic distortion. The maximum source impedance depends
on the amount of total harmonic distortion (THD) that can be
tolerated. The THD degrades proportionally to the source
impedance.

Single to Differential Driver

For applications using unipolar analog signals, a single-ended to
differential driver will allow for a differential input into the part.
The schematic is shown in Figure 8.

ANALOG INPUT

2.5V REF

(UNIPOLAR)

590⍀

590⍀

U1

590⍀

U2

AD8021

C

C

590⍀

AD8021

C

C

IN+

IN–

AD7675

REF

2.5V REF

Figure 8. Single-Ended to Differential Driver Circuit

This configuration, when provided an input signal of 0 to V

REF

,

will produce a differential ±2.5 V with a common mode at 1.25 V.

If the application can tolerate more noise, the AD8138 can be used.

Driver Amplifier Choice

Although the AD7675 is easy to drive, the driver amplifier needs
to meet at least the following requirements:

•

The driver amplifier and the AD7675 analog input circuit
have to be able to settle for a full-scale step of the capaci­tor array at a 16-bit level (0.0015%). In the amplifier’s data
sheet, the settling at 0.1% or 0.01% is more commonly speci­fied. It could significantly differ from the settling time at
16-bit level and, therefore, it should be verified prior to the
driver selection. The tiny op amp AD8021 which combines
ultra low noise and a high gain bandwidth meets this settling
time requirement even when used with a high gain up to 13.

•

The noise generated by the driver amplifier needs to be kept
as low as possible in order to preserve the SNR and transi­tion noise performance of the AD7675. The noise coming
from the driver is filtered by the AD7675 analog input circuit

one-pole, low-pass filter made by R+, R–, and C

. The SNR

S

degradation due to the amplifier is:

LOSS

20

=

SNR LOG






784




28

π

fNe

+

3

–

dB N

4

()






2




where

f

is the –3 dB input bandwidth of the AD7675 (3.9 MHz)

–3 dB

or the cutoff frequency of the input filter if any is used.

N is the noise factor of the amplifier (1 if in buffer con­figuration)

e

is the equivalent input noise voltage of the op amp in

N

1/2

nV/(Hz)

.

For instance, in the case of a driver with an equivalent input
noise of 2 nV/√Hz like the AD8021 and configured as a buffer,
thus with a noise gain of +1, the SNR degrades by only 0.04 dB
with the filter in Figure 5, and 0.07 dB without.

•

The driver needs to have a THD performance suitable to
that of the AD7675.

The AD8021 meets these requirements and is usually appropri­ate for almost all applications. The AD8021 needs an external
compensation capacitor of 10 pF. This capacitor should have
good linearity as an NPO ceramic or mica type.

The AD8022 could also be used where dual version is needed
and gain of 1 is used.

The AD8132 or the AD8138 could also be used to generate a differ­ential signal from a single-ended signal. When using the AD8138
with the filter in Figure 5, the SNR degrades by only 0.9 dB.

The AD829 is another alternative where high-frequency (above
100 kHz) performances are not required. In gain of 1, it requires
an 82 pF compensation capacitor.

The AD8610 is also another option where low-bias current is
needed in low-frequency applications.

The AD8519, OP162, or the OP184 could also be used.

Voltage Reference Input

The AD7675 uses an external 2.5 V voltage reference.

The voltage reference input REF of the AD7675 has a dynamic
input impedance. Therefore, it should be driven by a low­impedance source with an efficient decoupling between REF
and REFGND inputs. This decoupling depends on the choice
of the voltage reference but usually consists of a 1 µF

ceramic
capacitor and a low ESR tantalum capacitor connected to the
REF and REFGND inputs with minimum parasitic induc­tance. 47 µF is an appropriate value for the tantalum capacitor
when used with one of the recommended reference voltages:

•

The low-noise, low temperature drift ADR421 and AD780
voltage references

•

The low-power ADR291 voltage reference

•

The low-cost AD1582 voltage reference

For applications using multiple AD7675s, it is more effective to
buffer the reference voltage with a low-noise, very stable op amp
like the AD8031.

–12–

REV. 0

Care should also be taken with the reference temperature coeffi-

SAMPLING RATE – SPS

POWER DISSIPATION – ␮W

0.1

10k

100 100k10 10k

100

1k

1

100k

1k

10

1M

cient of the voltage reference which directly affects the full-scale
accuracy if this parameter matters. For instance, a ±15 ppm/°C
tempco of the reference changes the full scale by ±1 LSB/°C.

V

, as mentioned in the specification table, could be increased

REF

to AVDD – 1.85 V. The benefit here is the increased SNR
obtained as a result of this increase. Since the input range is
defined in terms of V

, this would essentially increase the

REF

range to make it a ±3 V input range with an AVDD above

4.85 V. The theoretical improvement as a result of this increase
in reference is 1.58 dB (20 log [3/2.5]). Due to the theoretical
quantization noise, however, the observed improvement is
approximately 1 dB. The AD780 can be selected with a 3 V
reference voltage.

Power Supply

The AD7675 uses three sets of power supply pins: an analog
5 V supply AVDD, a digital 5 V core supply DVDD, and a
digital input/output interface supply OVDD. The OVDD supply
allows direct interface with any logic working between 2.7 V and

5.25 V. To reduce the number of supplies needed, the digital
core (DVDD) can be supplied through a simple RC filter from
the analog supply as shown in Figure 5. The AD7675 is inde­pendent of power supply sequencing and thus free from supply
voltage induced latchup. Additionally, it is very insensitive to
power supply variations over a wide frequency range as shown
in Figure 9.

75

AD7675

Figure 10. Power Dissipation vs. Sample Rate

CONVERSION CONTROL

Figure 11 shows the detailed timing diagrams of the conversion
process. The AD7675 is controlled by the signal CNVST which
initiates conversion. Once initiated, it cannot be restarted or
aborted, even by the power-down input PD, until the conver­sion is complete. The CNVST signal operates independently of
CS and RD signals.

t

2

CNVST

t

1

70

BUSY

t

3

t

5

MODE ACQUIRE CONVERT ACQUIRE CONVERT

t

4

t

6

t

7

t

8

Figure 11. Basic Conversion Timing

For true sampling applications, the recommended operation of

PSRR – dB

65

60

55

50

45

40

the CNVST signal is as follows:

35

POWER DISSIPATION

The AD7675 automatically reduces its power consumption at
the end of each conversion phase. During the acquisition phase,
the operating currents are very low which allows a significant
power saving when the conversion rate is reduced as shown in
Figure 10. This feature makes the AD7675 ideal for very low­power battery applications.

It should be noted that the digital interface remains active even

10k 10M1k 1M

100k

FREQUENCY – Hz

Figure 9. PSRR vs. Frequency

CNVST must be held high from the previous falling edge of
BUSY, and during a minimum delay corresponding to the
acquisition time t

; then, when CNVST is brought low, a

8

conversion is initiated and BUSY signal goes high until the
completion of the conversion. Although CNVST is a digital
signal, it should be designed with this special care with fast,
clean edges and levels, with minimum overshoot and under­shoot or ringing.

For applications where the SNR is critical, the CNVST signal should
have a very low jitter. Some solutions to achieve that are to use a
dedicated oscillator for CNVST generation or, at least, to clock
it with a high-frequency low-jitter clock, as shown in Figure 5.

during the acquisition phase. To reduce the operating digital
supply currents even further, the digital inputs need to be driven
close to the power rails (i.e., DVDD and DGND) and OVDD
should not exceed DVDD by more than 0.3 V.

REV. 0

–13–

AD7675

t

9

RESET

BUSY

DATA

t

8

CNVST

Figure 12. RESET Timing

For other applications, conversions can be automatically initi­ated. If CNVST is held low when BUSY is low, the AD7675
controls the acquisition phase and then automatically initiates a
new conversion. By keeping CNVST low, the AD7675 keeps
the conversion process running by itself. It should be noted that
the analog input has to be settled when BUSY goes low. Also, at
power-up, CNVST should be brought low once to initiate the
conversion process. In this mode, the AD7675 could sometimes
run slightly faster than the guaranteed limit of 100 kSPS.

DIGITAL INTERFACE

The AD7675 has a versatile digital interface; it can be interfaced
with the host system by using either a serial or parallel interface.
The serial interface is multiplexed on the parallel data bus. The
AD7675 digital interface also accommodates both 3 V or 5 V
logic by simply connecting the OVDD supply pin of the AD7675
to the host system interface digital supply. Finally, by using the
OB/2C input pin, either two’s complement or straight binary
coding can be used.

The two signals CS and RD control the interface. When at least
one of these signals is high, the interface outputs are in high
impedance. Usually, CS allows the selection of each AD7675 in
multicircuits applications and is held low in a single AD7675
design. RD is generally used to enable the conversion result on
the data bus.

CS = RD = 0

CNVST

t

1

PARALLEL INTERFACE

The AD7675 is configured to use the parallel interface (Figure 13)
when the SER/PAR is held low. The data can be read either
after each conversion, which is during the next acquisition
phase, or during the following conversion as shown, respectively,
in Figure 14 and Figure 15. When the data is read during the
conversion, however, it is recommended that it be read-only
during the first half of the conversion phase. That avoids any
potential feedthrough between voltage transients on the digital
interface and the most critical analog conversion circuitry.

CS

RD

BUSY

DATA

BUS

t

12

CURRENT

CONVERSION

t

13

Figure 14. Slave Parallel Data Timing for Reading
(Read after Convert)

CS = 0

CNVST,

RD

BUSY

DATA

BUS

t

3

t

12

t

1

PREVIOUS

CONVERSION

t

4

t

13

Figure 15. Slave Parallel Data Timing for Reading (Read
During Convert)

The BYTESWAP pin allows a glueless interface to an 8-bit bus.
As shown in Figure 16, the LSB byte is output on D[7:0] and
the MSB is output on D[15:8] when BYTESWAP is low. When
BYTESWAP is high, the LSB and MSB bytes are swapped and
the LSB is output on D[15:8] and the MSB is output on D[7:0].
By connecting BYTESWAP to an address line, the 16-bit data
can be read in two bytes on either D[15:8] or D[7:0].

t

10

BUSY

DATA

BUS

t

3

PREVIOUS CONVERSION DATA NEW DATA

t

4

t

11

Figure 13. Master Parallel Data Timing for Reading
(Continuous Read)

–14–

RD

BYTE

PINS D[15:8]

PINS D[7:0]

CS

HI-Z

HI-Z

HIGH BYTE LOW BYTE

t

12

LOW BYTE HIGH BYTE

Figure 16. 8-Bit Parallel Interface

HI-Z

t

12

t

HI-Z

13

REV. 0

AD7675

SERIAL INTERFACE

The AD7675 is configured to use the serial interface when the
SER/PAR is held high. The AD7675 outputs 16 bits of data,
MSB first, on the SDOUT pin. This data is synchronized with
the 16 clock pulses provided on the SCLK pin.

MASTER SERIAL INTERFACE
Internal Clock

The AD7675 is configured to generate and provide the serial
data clock SCLK when the EXT/INT pin is held low. The AD7675
also generates a SYNC signal to indicate to the host when the

CS, RD

CNVST

BUSY

SYNC

SCLK

SDOUT

EXT/INT = 0

t

3

t

29

t

14

t

20

t

15

t

16

t

22

RDC/SDIN = 0 INVSCLK = INVSYNC = 0

t

18

t

19

t

21

123 141516

D15 D14 D2 D1 D0X

t

23

serial data is valid. The serial clock SCLK and the SYNC signal
can be inverted if desired. The output data is valid on both the
rising and falling edge of the data clock. Depending on RDC/
SDIN input, the data can be read after each conversion, or
during the following conversion. Figure 17 and Figure 18 show
the detailed timing diagrams of these two modes.

Usually, because the AD7675 has a longer acquisition phase
than the conversion phase, the data is read immediately after
conversion. That makes the mode master, read after conversion,
the most recommended serial mode when it can be used.

t

28

t

30

t

25

t

24

t

26

t

27

Figure 17. Master Serial Data Timing for Reading (Read after Convert)

CS, RD

CNVST

BUSY

SYNC

SCLK

SDOUT

EXT/INT = 0

t

1

t

3

t

17

t

14

t

15

t

18

t

16

t

22

t

19

t20t

21

123 141516

D15 D14 D2 D1 D0X

RDC/SDIN = 1 INVSCLK = INVSYNC = 0

t

23

t

25

t

24

t

26

t

27

Figure 18. Master Serial Data Timing for Reading (Read Previous Conversion during Convert)

REV. 0

–15–

AD7675

In read-after-conversion mode, it should be noted that, unlike in
other modes, the signal BUSY returns low after the 16 data bits
are pulsed out and not at the end of the conversion phase which
results in a longer BUSY width.

In read-during-conversion mode, the serial clock and data toggle
at appropriate instances which minimizes potential feedthrough
between digital activity and the critical conversion decisions.

To accommodate slow digital hosts, the serial clock can be
slowed down by using DIVSCLK.

SLAVE SERIAL INTERFACE
External Clock

The AD7675 is configured to accept an externally supplied
serial data clock on the SCLK pin when the EXT/INT pin is
held high. In this mode, several methods can be used to read the
data. The external serial clock is gated by CS and the data are
output when both CS and RD are low. Thus, depending on
CS, the data can be read after each conversion or during the
following conversion. The external clock can be either a con­tinuous or discontinuous clock. A discontinuous clock can be
either normally high or normally low when inactive. Figure 19
and Figure 20 show the detailed timing diagrams of these meth­ods. Usually, because the AD7675 has a longer acquisition
phase than the conversion phase, the data are read immediately
after conversion.

While the AD7675 is performing a bit decision, it is important
that voltage transients not occur on digital input/output pins or
degradation of the conversion result could occur. This is particu­larly important during the second half of the conversion phase
because the AD7675 provides error-correction circuitry that can
correct for an improper bit decision made during the first half of

the conversion phase. For this reason, it is recommended that when
an external clock is being provided, it is a discontinuous clock
that is toggling only when BUSY is low or, more importantly,
that it does not transition during the latter half of BUSY high.

External Discontinuous Clock Data Read after Conversion

This mode is the most recommended of the serial slave modes.
Figure 19 shows the detailed timing diagrams of this method.
After a conversion is complete, indicated by BUSY returning
low, the result of this conversion can be read while both CS and
RD are low. The data is shifted out, MSB first, with 16 clock
pulses and is valid on both the rising and falling edge of the clock.

One of the advantages of this method is that the conversion
performance is not degraded because there is no voltage transient
on the digital interface during the conversion process.

Another advantage is the ability to read the data at any speed up
to 40 MHz, which accommodates both slow digital host inter­face and the fastest serial reading.

Finally, in this mode only, the AD7675 provides a “daisy chain”
feature using the RDC/SDIN input pin for cascading multiple
converters together. This feature is useful for reducing compo­nent count and wiring connections when it is desired as it is, for
instance, in isolated multiconverters applications.

An example of the concatenation of two devices is shown in
Figure 21. Simultaneous sampling is possible by using a com­mon CNVST signal. It should be noted that the RDC/SDIN
input is latched on the opposite edge of SCLK of the one used
to shift out the data on SDOUT. Hence, the MSB of the
“upstream” converter just follows the LSB of the “downstream”
converter on the next SCLK cycle.

CS

BUSY

SCLK

SDOUT

SDIN

EXT/INT = 1 RD = 0

t

35

t

t

36

37

123 1415161718

t

31

D15 D14 D1 D0D13

t

16

X15 X14 X13 X1 X0 Y15 Y14

t

33

t

32

t

34

INVSCLK = 0

Figure 19. Slave Serial Data Timing for Reading (Read after Convert)

X15 X14X

–16–

REV. 0

AD7675

External Clock Data Read During Conversion

Figure 20 shows the detailed timing diagrams of this method.
During a conversion, while both CS and RD are low, the result
of the previous conversion can be read. The data is shifted out,
MSB first, with 16 clock pulses, and is valid on both rising and
falling edges of the clock. The 16 bits have to be read before the
current conversion is complete. If that is not done, RDERROR is
pulsed high and can be used to interrupt the host interface to
prevent incomplete data reading. There is no “daisy chain”
feature in this mode, and RDC/SDIN input should always be
tied either high or low.

To reduce performance degradation due to digital activity, a fast
discontinuous clock of 18 MHz at least is recommended to ensure
that all the bits are read during the first half of the conversion
phase. For this reason, this mode is more difficult to use.

MICROPROCESSOR INTERFACING

The AD7675 is ideally suited for traditional dc measurement
applications supporting a microprocessor, and ac signal process­ing applications interfacing to a digital signal processor. The
AD7675 is designed to interface either with a parallel 8-bit or

EXT/INT = 1 RD = 0

CS

16-bit wide interface or with a general purpose serial port or I/O
ports on a microcontroller. A variety of external buffers can be
used with the AD7675 to prevent digital noise from coupling
into the ADC. The following sections illustrate the use of the
AD7675 with an SPI-equipped microcontroller, and the
ADSP-21065L and ADSP-218x signal processors.

SPI Interface (MC68HC11)

Figure 22 shows an interface diagram between the AD7675 and
an SPI-equipped microcontroller like the MC68HC11. To
accommodate the slower speed of the microcontroller, the
AD7675 acts as a slave device and data must be read after con­version. This mode also allows the “daisy chain” feature. The
convert command could be initiated in response to an internal
timer interrupt. The reading of output data, one byte at a time if
necessary, could be initiated in response to the end-of-conversion
signal (BUSY going low) using an interrupt line of the micro­controller. The Serial Peripheral Interface (SPI) on the MC68HC11
is configured for master mode (MSTR) = 1, Clock Polarity Bit
(CPOL) = 0, Clock Phase Bit (CPHA) = 1 and SPI
enable (SPIE) = 1 by writing to the SPI Control Register

interrupt

(SPCR).
The IRQ is configured for edge-sensitive-only operation
(IRQE = 1 in OPTION register).

INVSCLK = 0

CNVST

BUSY

SCLK

SDOUT

t

3

t

16

t

35

t36t

37

12 3 141516

t

31

t

32

D1 D0X D15 D14 D13

Figure 20. Slave Serial Data Timing for Reading (Read Previous Conversion during Convert)

BUSY
OUT

BUSY BUSY

AD7675 #2

(UPSTREAM)

RDC/SDIN SDOUT

CNVST

CS

SCLK

AD7675 #1

(DOWNSTREAM)

RDC/SDIN SDOUT

CNVST

SCLK

CS

DATA
OUT

REV. 0

SCLK IN

CS IN

CNVST IN

Figure 21. Two AD7675s in a “Daisy Chain” Configuration

–17–

AD7675

DVD D

AD7675*

SER/PAR
EXT/INT

CS
RD

INVSCLK

BUSY

SDOUT

SCLK

CNVST

*ADDITIONAL PINS OMITTED FOR CLARITY

MC68HC11*

IRQ

MISO/SDI

SCK

I/O PORT

Figure 22. Interfacing the AD7675 to SPI Interface

ADSP-21065L in Master Serial Interface

As shown in Figure 23, the AD7675 can be interfaced to the
ADSP-21065L using the serial interface in master mode without any
glue logic required. This mode combines the advantages of reducing
the wire connections and the ability to read the data during or after
conversion maximum speed transfer (DIVSCLK[0:1] both low).

The AD7675 is configured for the internal clock mode (EXT/
INT low) and acts, therefore, as the master device. The convert
command can be generated by either an external low jitter oscil­lator or, as shown, by a FLAG output of the ADSP-21065L or
by a frame output TFS of one serial port of the ADSP-21065L
which can be used like a timer. The serial port on the ADSP­21065L is configured for external clock (IRFS = 0), rising edge
active (CKRE = 1), external late framed sync signals (IRFS = 0,
LAFS = 1, RFSR = 1) and active high (LRFS = 0). The serial
port of the ADSP-21065L is configured by writing to its receive
control register (SRCTL)—see ADSP-2106x SHARC User’s
Manual. Because the serial port within the ADSP-21065L will
be seeing a discontinuous clock, an initial word reading has to
be done after the ADSP-21065L has been reset to ensure that
the serial port is properly synchronized to this clock during each
following data read operation.

DVD D

AD7675*

SER/PAR

RDC/SDIN

RD
EXT/INT

CS

INVSYNC

INVSCLK

SYNC

SDOUT

SCLK

CNVST

*ADDITIONAL PINS OMITTED FOR CLARITY

ADSP-21065L*

SHARC

RFS

DR

RCLK

FLAG OR TFS

Figure 23. Interfacing to the ADSP-21065L Using
the Serial Master Mode

APPLICATION HINTS
Layout

The AD7675 has very good immunity to noise on the power
supplies as can be seen in Figure 21. However, care should still
be taken with regard to grounding layout.

The printed circuit board that houses the AD7675 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. Digital and analog

ground planes should be joined in only one place, preferably
underneath the AD7675, or, at least, as close as possible to the
AD7675. If the AD7675 is in a system where multiple devices
require analog to digital ground connections, the connection
should still be made at one point only, a star ground point,
which should be established as close as possible to the AD7675.

It is recommended that running digital lines under the device
should be avoided as these will couple noise onto the die. The
analog ground plane should be allowed to run under the AD7675
to avoid noise coupling. Fast switching signals like CNVST or
clocks should be shielded with digital ground to avoid radiating
noise to other sections of the board, and should never run near
analog signal paths. Crossover of digital and analog signals
should be avoided. Traces on different but close layers of the
board should run at right angles to each other. This will reduce the
effect of feedthrough through the board.

The power supply lines to the AD7675 should use as large a
trace as possible to provide low impedance paths and reduce the
effect of glitches on the power supply lines. Good decoupling is
also important to lower the supplies impedance presented to
the AD7675 and reduce the magnitude of the supply spikes.
Decoupling ceramic capacitors, typically 100 nF, should be
placed on each power supplies pins AVDD, DVDD, and OVDD
close to, and ideally right up against these pins and their corre­sponding ground pins. Additionally, low ESR 10 µF capacitors
should be located in the vicinity of the ADC to further reduce
low-frequency ripple.

The DVDD supply of the AD7675 can be either a separate
supply or come from the analog supply, AVDD, or from the
digital interface supply, OVDD. When the system digital supply
is noisy, or fast switching digital signals are present, it is recom­mended if no separate supply is available, to connect the DVDD
digital supply to the analog supply AVDD through an RC filter
as shown in Figure 5, and connect the system supply to the inter­face digital supply OVDD and the remaining digital circuitry.
When DVDD is powered from the system supply, it is useful to
insert a bead to further reduce high-frequency spikes.

The AD7675 has four different ground pins: REFGND, AGND,
DGND, and OGND. REFGND senses the reference voltage and
should be a low-impedance return to the reference because it
carries pulsed currents. AGND is the ground to which most
internal ADC analog signals are referenced. This ground must
be connected with the least resistance to the analog ground
plane. DGND must be tied to the analog or digital ground plane
depending on the configuration. OGND is connected to the
digital system ground.

The layout of the decoupling of the reference voltage is important.
The decoupling capacitor should be close to the ADC and conne cte d
with short and large traces to minimize parasitic inductances.

Evaluating the AD7675 Performance

A recommended layout for the AD7675 is outlined in the evalu­ation board for the AD7675. The evaluation board package
includes a fully assembled and tested evaluation board, docu­mentation, and software for controlling the board from a PC
via the Eval-Control BRD2.

–18–

REV. 0

OUTLINE DIMENSIONS

(

)

Dimensions shown in inches and (mm).

48-Lead Quad Flatpack (LQFP)

(ST-48)

0.063 (1.60)

0.030 (0.75)

0.018 (0.45)

MAX

0.354 (9.00) BSC SQ

48

1

AD7675

37

36

COPLANARITY

0.003 (0.08)

0.004 (0.09)

0.008 (0.2)

0ⴗ
MIN

7ⴗ
0ⴗ

TOP VIEW

(PINS DOWN)

12

13

0.019 (0.5)
BSC

0.006 (0.15)

0.002

0.05

0.011 (0.27)

0.006 (0.17)

SEATING
PLANE

24

25

0.276
(7.00)

BSC

SQ

0.057 (1.45)

0.053 (1.35)

REV. 0

–19–

C02689–.8–10/01(0)

–20–

PRINTED IN U.S.A.