Datasheet AD7650 Datasheet (Analog Devices)

16-Bit, 570 kSPS

a

FEATURES
Throughput

570 kSPS (Warp Mode)

500 kSPS (Normal Mode)
16 Bits Resolution
Analog Input Voltage Range: 0 V to 2.5 V
No Pipeline Delay
Parallel and Serial 5 V/3 V Interface

SPI™/QSPI™/MICROWIRE™/DSP
Single 5 V Supply Operation
Power Dissipation

77 mW Typical @ 444 kSPS (Impulse Mode)

21 ␮W @ 100 SPS
Power-Down Mode: 7 ␮W Max
Package: 48-Lead Quad Flat Pack (LQFP) or 48-Lead

Frame Chip-Scale Pack (LFCSP)
Pin-to-Pin Compatible with PulSAR ADCs

APPLICATIONS
Data Acquisition
Instrumentation
Digital Signal Processing
Spectrum Analysis
Medical Instruments
Battery-Powered Systems
Process Control

Compatible

FUNCTIONAL BLOCK DIAGRAM

AVDD AGND REF REFGND

IN+

IN–

PD

RESET

CALIBRATION CIRCUITRY

Low Cost CMOS ADC

AD7650

SWITCHED

CAP DAC

CLOCK

CONTROL LOGIC AND

CNVSTWARP IMPULSE

AD7650

DGNDDVDD

SERIAL

PORT

PARALLEL

INTERFACE

16

*

OVDD

OGND

SER/PAR

BUSY

DATA[15:0]

CS

RD

OB/2C

GENERAL DESCRIPTION

The AD7650 is a 16-bit, 570 kSPS, charge redistribution SAR,
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.

It features a very high sampling rate mode (Warp) and, for
asynchronous conversion rate applications, a fast mode (Normal)
and, for low power applications, a reduced power mode (Impulse)
where the power is scaled with the throughput.

It is fabricated using Analog Devices’ high-performance,

0.6 micron CMOS process and is available in a 48-lead LQFP
or in a tiny 48-lead Chip Scale package 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 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.

PRODUCT HIGHLIGHTS

1. Fast Throughput
The AD7650 is a 570 kSPS, charge redistribution, 16-bit
SAR ADC.

2. Single-Supply Operation
The AD7650 operates from a single 5 V supply. In impulse
mode, its power dissipation decreases with the throughput from
77 mW at 444 kSPS throughput to, for instance, only 21 µW
at a 100 SPS throughput. It consumes 7 µW maximum when
in power-down.

3. Serial or Parallel Interface
Versatile parallel or 2-wire serial interface arrangement com­patible with both 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., 2002

AD7650–SPECIFICATIONS

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

Parameter Condition Min Typ Max Unit

RESOLUTION 16 Bits

ANALOG INPUT

Voltage Range V
Operating Input Voltage V

Analog Input CMRR f

– V

IN+

IN–

IN+

V

IN–

= 10 kHz 62 dB

IN

0V

REF

V
–0.1 +3 V
–0.1 +0.5 V

Input Current 570 kSPS Throughput 7 µA
Input Impedance See Analog Input Section

THROUGHPUT SPEED

Complete Cycle In Warp Mode 1.75 µs
Throughput Rate In Warp Mode 1 570 kSPS
Time Between Conversions In Warp Mode 1 ms
Complete Cycle In Normal Mode 2 µs
Throughput Rate In Normal Mode 0 500 kSPS
Complete Cycle In Impulse Mode 2.25 µs
Throughput Rate In Impulse Mode 0 444 kSPS

DC ACCURACY

Integral Linearity Error –6 +6 LSB
No Missing Codes 15 Bits
Transition Noise 0.7 LSB
Full-Scale Error
Unipolar Zero Error

2

2

REF = 2.5 V ± 0.12 % of FSR

± 5 ± 25 LSB

Power Supply Sensitivity AVDD = 5 V ± 5% ± 3 LSB

AC ACCURACY

Signal-to-Noise f

= 100 kHz 86 dB

IN

4

Spurious Free Dynamic Range fIN = 100 kHz 98 dB
Total Harmonic Distortion f

Signal-to-(Noise + Distortion) f

= 45 kHz –98 dB

IN

f

= 100 kHz –96 dB

IN

= 100 kHz 86 dB

IN

–60 dB Input, f

= 100 kHz 30 dB

IN

–3 dB Input Bandwidth 18 MHz

SAMPLING DYNAMICS

Aperture Delay 2 ns
Aperture Jitter 5 ps rms
Transient Response Full-Scale Step 250 ns

REFERENCE

External Reference Voltage Range
External Reference Current Drain

570 kSPS Throughput 115 µA

2.3 2.5 AVDD – 1.85 V

DIGITAL INPUTS

Logic Levels

V

IL

V

IH

I

IL

I

IH

–0.3 +0.8 V

2.0 OVDD + 0.3 V
–1 +1 µA
–1 +1 µA

DIGITAL OUTPUTS
Data Format Parallel or Serial 16-Bit

Pipeline Delay Conversion Results Available

Immediately after Completed Conversion

I

V

OL

V

OH

= 1.6 mA 0.4 V

SINK

I

= –500 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

1

3

REV. 0

–2–

AD7650

AD7650

Parameter Condition Min Typ Max Unit

POWER SUPPLIES (continued)

Operating Current

AVDD 15.5 mA

6

DVDD

6

OVDD

Power Dissipation

TEMPERATURE RANGE

Specified Performance T

NOTES

1

LSB means Least Significant Bit. With the 0 V to 2.5 V input range, one LSB is 38.15 µV.

2

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

3

Tested in warp mode.

4

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.

5

In warp mode.

6

Tested in parallel reading mode.

7

In impulse mode.

8

With all digital inputs forced to OVDD or OGND respectively.

9

Contact factory for extended temperature range.

Specifications subject to change without notice.

5

570 kSPS Throughput

4.2 mA

6

570 kSPS Throughput
444 kSPS Throughput
100 SPS Throughput
In Power-Down Mode

9

to T

MIN

MAX

5

7

7

8

–40 +85 °C

100 µA

115 mW
77 mW
21 µW

7 µW

TIMING SPECIFICATIONS

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

Parameter Symbol Min Typ Max Unit

REFER TO FIGURES 8 AND 9

Convert Pulsewidth t
Time Between Conversions t

1

2

5ns

1.75/2/2.25 Note 1 µs

(Warp Mode/Normal Mode/Impulse Mode)

CNVST LOW to BUSY HIGH Delay t
BUSY HIGH All Modes Except in t

3

4

30 ns

1.5/1.75/2 µs
Master Serial Read After Convert Mode
(Warp Mode/Normal Mode/Impulse Mode)

Aperture Delay t
End of Conversion to BUSY LOW Delay t
Conversion Time t

5

6

7

10 ns

2ns

1.5/1.75/2 µs
(Warp Mode/Normal Mode/Impulse Mode)

Acquisition Time t
RESET Pulsewidth t

8

9

250 ns
10 ns

REFER TO FIGURES 10, 11 AND 12

(Parallel Interface Modes)
CNVST LOW to DATA Valid Delay t

10

1.5/1.75/2 µs
(Warp Mode/Normal Mode/Impulse Mode)

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

REFER TO FIGURES 13 AND 14

(Master Serial Interface Modes)

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

2

2

CS LOW to SDOUT Delay t
CNVST LOW to SYNC Delay t

11

12

13

14

t

15

16

17

45 ns

40 ns

515ns

10 ns
10 ns
10 ns

25/275/525 ns

(Warp Mode/Normal Mode/Impulse Mode)

SYNC Asserted to SCLK First Edge Delay t
Internal SCLK Period t
Internal SCLK HIGH (INVSCLK Low)
Internal SCLK LOW (INVSCLK Low)

3

3

SDOUT Valid Setup Time t
SDOUT Valid Hold Time t

REV. 0

REV. 0

18

19

t

20

t

21

22

23

–3–

–3–

4ns
40 75 ns
30 ns

9.5 ns

4.5 ns
3ns

AD7650

TIMING SPECIFICATIONS

(continued)

Parameter Symbol Min Typ Max Unit

REFER TO FIGURES 13 AND 14 (continued)

SCLK Last Edge to SYNC Delay t

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 t

24

25

26

27

28

3

10 ns
10 ns
10 ns

2.75/3/3.25 µs

(Warp Mode/Normal Mode/Impulse Mode)

CNVST LOW to SYNC Asserted Delay t

29

1/1.25/1.5 µs

(Warp Mode/Normal Mode/Impulse Mode)

SYNC Deasserted to BUSY LOW Delay t

REFER TO FIGURES 15 AND 16

(Slave Serial Interface Modes)

2

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 warp mode only, the maximum time between conversions is 1 ms; otherwise, there is no required maximum time.

2

In serial interface modes, the SYNC, SCLK, and SDOUT timings are defined with a maximum load C

3

If the polarity of SCLK is inverted, the timing references of SCLK are also inverted.

Specifications subject to change without notice.

30

31

32

33

34

35

36

37

5ns
316ns
5ns
5ns
25 ns
10 ns
10 ns

of 10 pF; otherwise, the load is 60 pF maximum.

L

50 ns

1.6mA

TO OUTPUT

PIN

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

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

C

L

60pF*

C

L

500␮A

I

OL

1.4V

I

OH

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

= 10 pF

L

2V

0.8V

t

DELAY

2V

0.8V

t

DELAY

2V

0.8V

Figure 2. Voltage Reference Levels for Timing

–4–

REV. 0

AD7650

ABSOLUTE MAXIMUM RATINGS

Analog Inputs

2

IN+

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

Except the Data Bus D(7:4) . . . . –0.3 V to DVDD + 0.3 V

Data Bus Inputs D(7:4) . . . . . . . –0.3 V to OVDD + 0.3 V

Internal Power Dissipation

3

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

1

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: θJA = 91°C/W, θJC = 30°C/W.
48-Lead LFCSP: θJC = 26°C/W.

ORDERING GUIDE

Model Temperature Range Package Description Package Option

AD7650AST –40°C to +85°C Quad Flatpack (LQFP) ST-48
AD7650ASTRL –40°C to +85°C Quad Flatpack (LQFP) ST-48
AD7650ACP
AD7650ACPRL
EVAL-AD7650CB
EVAL-CONTROL BRD2

NOTES

1

Future Product. Contact Factory for availability.

2

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

3

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

1

1

2

3

–40°C to +85°C Quad Flatpack (LFCSP) CP-48
–40°C to +85°C Quad Flatpack (LFCSP) CP-48

1

1

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 AD7650 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.

WARNING!

ESD SENSITIVE DEVICE

REV. 0

–5–

AD7650

PIN CONFIGURATION

48-Lead LQFP and 48-Lead LFSCP

(ST-48 and CP-48)

AGND

AV DD

NC

DGND

OB/2C
WARP

IMPULSE
SER/PAR

D0

D1

D2

D3

NC = NO CONNECT

48

1

2

3

4

5

6

7

8

9

10

11

12

13 14

NCNCNCNCNC

46

47

PIN 1
IDENTIFIER

15 16 17 18

D4/EXT/INT

D5/INVSYNC

IN+NCNCNCIN–

45 44 39 38 3743 42 41 40

AD7650

TOP VIEW

(Not to Scale)

19 20

DVD D

OVD D

OGND

D6/INVSCLK

D7/RDC/SDIN

21 22

DGND

D8/SDOUT

REFGND

23 24

D9/SCLK

D10/SYNC

REF

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

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, 40–42, NC No Connect
44–48

4 DGND DI Must be tied to the ground where DVDD is referred.

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.

6 WARP DI Mode Selection. When HIGH and IMPULSE LOW, this input selects the fastest mode, the

maximum throughput is achievable, and a minimum conversion rate must be applied in order
to guarantee full specified accuracy. When LOW, full accuracy is maintained independent of
the minimum conversion rate.

7 IMPULSE DI Mode Selection. When HIGH and WARP LOW, this input selects a reduced power mode.

In this mode, the power dissipation is approximately proportional to the sampling rate.

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.

9–12 DATA[0:3] DO Bit 0 to Bit 3 of the Parallel Port Data Output Bus. These pins are always outputs, regardless

of the state of SER/PAR.

13 DATA[4] DI/O When SER/PAR is LOW, this output is used as 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 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. It is active in both master and slave mode. 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 Bit 6 of the Parallel Port Data Output Bus.

or INVSCLK 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.

–6–

REV. 0

AD7650

PIN FUNCTION DESCRIPTIONS (continued)

Pin No. Mnemonic Type Description

16 DATA[7] DI/O When SER/PAR is LOW, this output is used as 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

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 conversion 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 can be
output on SDOUT only when the conversion is complete.

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

host interface (5 V or 3 V).

19 DVDD P Digital Power. Nominally at 5 V.

20 DGND P Digital Power Ground
21 DATA[8] DO When SER/PAR is LOW, this output is used as 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 AD7650
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 a 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 clock.

33 RESET DI Reset Input. When set to a logic HIGH, reset the AD7650. Current conversion if any is aborted.

If not used, this pin could be tied to DGND.

34 PD DI Power-Down Input. When set to a logic HIGH, power consumption is reduced and conversions

are inhibited after the current one is completed.

REV. 0

–7–

AD7650

PIN FUNCTION DESCRIPTIONS (continued)

Pin No. Mnemonic Type Description

35 CNVST DI Start Conversion. A falling edge on CNVST puts the internal sample/hold into the hold state

and initiates a conversion. In impulse mode (IMPULSE HIGH and WARP LOW), if CNVST
is held low when the acquisition phase (t

the hold 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 Analog Input Ground
43 IN+ AI Primary Analog Input with a Range of 0 V to V

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

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

8

.

REF

DEFINITION OF SPECIFICATIONS

INTEGRAL NONLINEARITY ERROR (INL)

Linearity error refers to the deviation of each individual code
from a line drawn from “negative full scale” through “positive
full scale.” The point used as “negative full scale” occurs 1/2 LSB
before the first code transition. “Positive full scale” is defined as a
level 1 1/2 LSB beyond the last code transition. The deviation is
measured from the middle of each code to the true straight line.

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 (2.49994278 V for the 0 V–2.5 V range).
The full-scale error is the deviation of the actual level of the last
transition from the ideal level.

UNIPOLAR ZERO ERROR

The first transition should occur at a level 1/2 LSB above analog
ground (19.073 µV for the 0 V–2.5 V range). Unipolar zero
error is the deviation of the actual transition from that point.

SPURIOUS FREE DYNAMIC RANGE (SFDR)

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

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:

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.

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 AD7650 to achieve its rated accuracy
after a full-scale step function is applied to its input.

OVERVOLTAGE RECOVERY

The time required for the ADC to recover to full accuracy after
an analog input signal 150% of full-scale is reduced to 50% of
the full-scale value.

ENOB S N D

=+

and is expressed in bits.

−

[]

()

176 602./.

dB

–8–

REV. 0

Typical Performance Characteristics–

AD7650

4

3

2

1

0

INL – LSB

–1

–2

–3

–4

0 16384 65536

32768 49152

CODE

TPC 1. Integral Nonlinearity
vs. Code

8000

7000

6000

5000

4000

COUNTS

3000

2000

1000

0

7FFC 8000

7FFD 7FFF

12000014

7FFE

7396

7353

8001

CODE – Hexa

850759

8002

8003

8004

8005

3

2

1

0

DNL – LSB

–1

–2

–3

0 16384 65536

32768 49152

CODE

TPC 2. Differential Nonlinearity
vs. Code

0

f

= 571kSPS

–20

–40

–60

–80

–100

–120

AMPLITUDE – dB of Full Scale

–140

–160

0 57 228

114 171

FREQUENCY – kHz

S

f

= 45.01kHz

IN

SNR = 87dB
THD = –96dB
SFDR = 98dB
SINAD = 86.5dB

285

10000

9000

8000

7000

6000

5000

COUNTS

4000

3000

2000

1000

0 0 79 151 1 0 0

0

7FFD 8001

9514

3336

7FFF

7FFE 8000

3303

8002

CODE – Hexa

8003

8004

8005

8006

TPC 3. Histogram of 16,384
Conversions of a DC Input at the
Code Transition

100

95

90

85

SINAD

80

SNR AND S/[N+D] – dB

75

70

110 1k

ENOB

SNR

FREQUENCY – kHz

100

15.0

14.5

14.0

13.5

13.0

12.5

12.0

ENOB – Bits

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

–60

THD, HARMONICS – dB

–65

–70

–75

–80

–85

–90

–95

–100

–105

–110

–115

SFDR

2ND HARMONIC

3RD HARMONIC

0

THD

10 100 1k

FREQUENCY – kHz

TPC 7. THD, Harmonics, and SFDR
vs. Frequency

TPC 5. FFT Plot

115

110

105

100

95

90

85

80

75

70

65

60

SFDR – dB

100k

AVDD, WARP/NORMAL

10k

DVDD, WARP/NORMAL

1k

100

10

1

0.1

OPERATING CURRENT – ␮A

0.01

0.001

0.1

AVDD, IMPULSE

DVDD, IMPULSE

1 10 100 1k 10k 100k 1M

SAMPLING RATE – SPS

TPC 8. Operating Currents
vs. Sample Rate

OVDD, ALL MODES

TPC 6. SNR, S/(N+D), and ENOB
vs. Frequency

50

OVDD = 2.7V, 85ⴗC

OVDD = 5V, 25ⴗC

50

100 150

CL – pF

OVDD = 2.7V, 25ⴗC

OVDD = 5V, 85ⴗC

DELAY – ns

t

40

30

20

12

10

0

0

TPC 9. Typical Delay vs. Load
Capacitance C

L

200

REV. 0

–9–

AD7650

CIRCUIT INFORMATION

The AD7650 is a very fast, low power, single supply, precise
16-bit analog-to-digital converter (ADC). The AD7650 features
different modes to optimize performances according to the
applications.

In warp mode, the AD7650 is capable of converting 570,000
samples per second (570 kSPS).

The AD7650 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 AD7650 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 48-lead
LQFP or in a tiny 48-LFCSP packages that save space and allows
flexible configurations as either serial or parallel interface. The
AD7650 is pin-to-pin compatible with the AD7664.

CONVERTER OPERATION

The AD7650 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 an array of 16 binary weighted capacitors and an additional
“LSB” capacitor. The comparator’s negative input is connected to
a “dummy” capacitor of the same value as the capacitive DAC array.

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

. All independent switches are connected to the analog input

A

IN+. Thus, the capacitor array is used as a sampling capacitor
and acquires the analog signal on IN+ input. Similarly, the
“dummy” capacitor acquires the analog signal on IN– input.

When the CNVST input goes low, a conversion phase is initiated.
When the conversion phase begins, SW

and SWB are opened

A

first. The capacitor array and the “dummy” capacitor are then
disconnected from the inputs and connected to the REFGND
input. Therefore, the differential voltage between 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 binary­weighted voltage steps (V

REF

/2, V

/4,...V

REF

/65536). The

REF

control logic toggles these switches, starting with the MSB first,
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.

Modes of Operation

The AD7650 features three modes of operations, Warp, Normal,
and Impulse. Each of these modes is more suitable for specific
applications.

The Warp mode allows the fastest conversion rate up to 570 kSPS.
However, in this mode, and this mode only, the full specified
accuracy is guaranteed only when the time between conversion
does not exceed 1 ms. If the time between two consecutive
conversions is longer than 1 ms, for instance, after power-up,
the first conversion result should be ignored. This mode makes
the AD7650 ideal for applications where both high accuracy and
fast sample rate are required.

The normal mode is the fastest mode (500 kSPS) without any
limitation about the time between conversions. This mode makes the
AD7650 ideal for asynchronous applications such as data acqui­sition systems, where both high accuracy and fast sample rate are
required. It is selected when both IMPULSE and WARP are low.

The impulse mode, the lowest power dissipation mode, allows
power saving between conversions. When operating at 100 SPS,
for example, it typically consumes only 21 µW. This feature
makes the AD7650 ideal for battery-powered applications.

IN+

REF

REFGND

IN–

MSB

32,768C

16,384C 4C 2C C C

Figure 3. ADC Simplified Schematic

65,536C

LSB

SW

SW

A

COMP

B

SWITCHES

CONTROL

CONTROL

LOGIC

CNVST

BUSY

OUTPUT

CODE

–10–

REV. 0

AD7650

Transfer Functions

Using the OB/2C digital input, the AD7650 offers two output
codings: straight binary and two’s complement. The LSB size is

/65536, which is about 38.15 µV. The ideal transfer char-

V

REF

acteristic for the AD7650 is shown in Figure 4 and Table I.

111...111

111...110

111...101

ADC CODE – Straight Binary

000...010

000...001

000...000

0.5 LSB

1 LSB = V

1 LSB0V

/65536

REF

ANALOG INPUT

V

REF

V

REF

– 1.5 LSB

– 1 LSB

Figure 4. ADC Ideal Transfer Function

Table I. Output Codes and Ideal Input Voltages

Digital Output Code (Hex)

Analog Straight Two’s

Description Input Binary Complement

FSR – 1 LSB 2.499962 V FFFF

1

7FFF

1

FSR – 2 LSB 2.499923 V FFFE 7FFE
Midscale + 1 LSB 1.250038 V 8001 0001
Midscale 1.25 V 8000 0000
Midscale – 1 LSB 1.249962 V 7FFF FFFF
–FSR + 1 LSB 38 µV 0001 8001
–FSR 0 V 0000

NOTES

1

This is also the code for overrange analog input (V

2

This is also the code for underrange analog input (V

IN+

2

– V

IN–

below V

IN+

8000

above V

REF

IN–

– V

).

2

REFGND

).

TYPICAL CONNECTION DIAGRAM

Figure 5 shows a typical connection diagram for the AD7650.

ANALOG

SUPPLY

(5V)

2.5V REF
NOTE 1

ANALOG INPUT

(0V TO 2.5V)

100⍀

+

+

C

REF

10␮F

1

100nF

REF

1␮F

REFGND

+

10␮F

AGNDAVDD DGND DVDD OVDD OGND

AD7650

NOTE 2 U1

15⍀

C

C

NOTES

1. THE ADR421 IS RECOMMENDED WITH C

2. THE AD8021 IS RECOMMENDED WITH A COMPENSATION CAPACITOR C

3. OPTIONAL LOW JITTER CNVST.

4.7nF

IN+

IN–

PD RESET

REF

= 47␮F.

Figure 5. Typical Connection Diagram

100nF

CS

+

100nF

SCLK

SDOUT

BUSY

CNVST

OB/2C

SER/PAR

WARP

IMPULSE

RD

= 10pF, TYPE CERAMIC NPO.

C

NOTE 3

10␮F

SERIAL

D

DVD D

DIGITAL SUPPLY
(3.3V OR 5V)

PORT

CLOCK

␮C/␮P/DSP

REV. 0

–11–

AD7650

Analog Input

Figure 6 shows an equivalent circuit of the input structure of the
AD7650.

AV DD

IN+

OR IN–

AGND

Figure 6. Equivalent Analog Input Circuit

C1

D1

D2

R1

C2

The two diodes, D1 and D2, provide ESD protection for the
analog inputs IN+ and IN–. Care must be taken to ensure that the
analog input signal never exceeds the supply rails by more than

0.3 V. This will cause these diodes to become forward-biased and
start conducting current. These diodes can handle a forward­biased current of 100 mA maximum. For instance, these conditions
could eventually occur when the input buffer’s (U1) 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 allows the sampling of the differential
signal between IN+ and IN–. Unlike other converters, the IN–
input is sampled at the same time as the IN+ input. By using
this differential input, small signals common to both inputs are
rejected. For instance, by using IN– to sense a remote signal
ground, difference of ground potentials between the sensor and
the local ADC ground are eliminated.

During the acquisition phase, the impedance of the analog input
IN+ can be modeled as a parallel combination of capacitor C1
and the network formed by the series connection of R1 and C2.
Capacitor C1 is primarily the pin capacitance. The resistor R1 is
typically 140 Ω and is a lumped component made up of some
serial resistors and the on resistance of the switches. The capacitor
C2 is typically 60 pF and is mainly the ADC sampling capacitor.
During the conversion phase, where the switches are opened,
the input impedance is limited to C1. The R1, C2 makes a one­pole low-pass filter that reduces undesirable aliasing effect and
limits the noise.

When the source impedance of the driving circuit is low, the
AD7650 can be driven directly. Large source impedances will
significantly affect the ac performances, especially the total
harmonic distortion.

Driver Amplifier Choice

Although the AD7650 is easy to drive, the driver amplifier needs
to meet at least the following requirements:

•

The driver amplifier and the AD7650 analog input circuit

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

•

The noise generated by the driver amplifier needs to be kept

as low as possible to preserve the SNR and transition noise
performance of the AD7650. The noise coming from the
driver is filtered by the AD7650 analog input circuit one-pole
low-pass filter made by R1 and C2 or the external filter if any
are used.

•

The driver needs to have a THD performance suitable to that

of the AD7650.

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

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

Voltage Reference Input

The AD7650 uses an external 2.5 V voltage reference. The volt­age reference input REF of the AD7650 has a dynamic input
impedance. Therefore, it should be driven by a low impedance
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 low ESR tantalum capacitor
connected to the REF and REFGND inputs with minimum para­sitic inductance. 47 µF is an appropriate value for 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 AD7650s, it is more effective to
buffer the reference voltage with a low-noise, very stable op amp
such as the AD8031.

Care should also be taken with the reference temperature coeffi­cient 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.

Note that V

, as mentioned in the specification table, could

REF

be increased to AVDD –1.85 V. Since the input range is defined
in terms of V

, this would essentially increase the range to

REF

make it a 0 V to 3 V input range with a reference voltage of 3 V.
The AD780 can be selected with a 3 V reference voltage.

Power Supply

The AD7650 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 AD7650 is independent of
power supply sequencing and thus free from supply voltage
induced latchup.

–12–

REV. 0

AD7650

POWER DISSIPATION VS. THROUGHPUT

Operating currents are very low during the acquisition phase,
which allows a significant power saving when the conversion
rate is reduced as shown in Figure 7. This power saving depends
on the mode used. In impulse mode, the AD7650 automatically
reduces its power consumption at the end of each conversion
phase. This feature makes the AD7650 ideal for very low power
battery applications. It should be noted that the digital interface
remains active even during the acquisition phase. To reduce the
operating digital supply currents even further, the digital inputs
need to be driven close to the power supply rails (i.e., DVDD or
DGND for all inputs except EXT/INT, INVSYNC, INVSCLK,
RDC/SDIN, and OVDD or OGND for these last four inputs).

100k

POWER DISSIPATION – ␮W

10k

100

0.1

1k

10

1

0.1

WARP/NORMAL

IMPULSE

100k1k101 100 10k 1M

SAMPLING RATE – SPS

Figure 7. Power Dissipation vs. Sampling Rate

CONVERSION CONTROL

Figure 8 shows the detailed timing diagrams of the conversion
process. The AD7650 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

t

1

CNVST

power-up, CNVST should be brought low once to initiate the
conversion process. In this mode, the AD7650 could sometimes
run slightly faster then the guaranteed limits in the impulse mode
of 444 kSPS. This feature does not exist in warp or normal modes.

t

9

RESET

BUSY

DATA

t

8

CNVST

Figure 9. RESET Timing

Although CNVST is a digital signal, it should be designed with
special care with fast, clean edges, and levels with minimum
overshoot and undershoot or ringing.

It is a good thing to shield the CNVST trace with ground and
also to add a low value serial resistor (i.e., 50 V) termination
close to the output of the component that drives this line.

For applications where the SNR is critical, CNVST signal should
have a very low jitter. Some solutions to achieve that is 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.

DIGITAL INTERFACE

The AD7650 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
AD7650 digital interface also accommodates both 3 V or 5 V logic
by simply connecting the OVDD supply pin of the AD7650 to
the host system interface digital supply. Finally, by using the
OB/2C input pin, both 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 AD7650 in
multicircuits applications and is held low in a single AD7650
design. RD is generally used to enable the conversion result on
the data bus.

BUSY

t

3

t

5

MODE

ACQUIRE CONVERT ACQUIRE CONVERT

t

4

t

6

t

7

t

8

Figure 8. Basic Conversion Timing

In impulse mode, conversions can be automatically initiated.
If CNVST is held low when BUSY is low, the AD7650 controls
the acquisition phase and then automatically initiates a new
conversion. By keeping CNVST low, the AD7650 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

REV. 0

–13–

CS = RD = 0

t

1

CNVST

t

10

BUSY

DATA

BUS

t

3

PREVIOUS CONVERSION DATA NEW DATA

t

4

t

11

Figure 10. Master Parallel Data Timing for Reading
(Continuous Read)

AD7650

PARALLEL INTERFACE

The AD7650 is configured to use the parallel interface 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 11
and Figure 12. When the data is read during the conversion,
however, it is recommended that it is 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 11. Slave Parallel Data Timing for Reading
(Read After Convert)

CS = 0

t

CNVST,

RD

BUSY

DATA

BUS

t

3

t

12

1

PREVIOUS

CONVERSION

t

t

4

13

Figure 12. Slave Parallel Data Timing for Reading
(Read During Convert)

SERIAL INTERFACE

The AD7650 is configured to use the serial interface when the
SER/PAR is held high. The AD7650 outputs 16 bits of data,
MSB first, on the SDOUT pin. This data is synchronized with
the 16 clock pulses provided on SCLK pin. The output data is
valid on both the rising and falling edge of the data clock.

MASTER SERIAL INTERFACE
Internal Clock

The AD7650 is configured to generate and provide the serial data
clock SCLK when the EXT/

INT

pin is held low. The AD7650
also generates a SYNC signal to indicate to the host when the
serial data is valid. The serial clock SCLK and the SYNC signal
can be inverted if desired. Depending on RDC/SDIN input, the
data can be read after each conversion or during the following
conversion. Figure 13 and Figure 14 show the detailed timing
diagrams of these two modes.

CS, RD

CNVST

BUSY

SYNC

SCLK

SDOUT

EXT/INT = 0 RDC/SDIN = 0 INVSCLK = INVSYNC = 0

t

3

t

28

t

29

t

14

t

15

X

t

16

t

22

t

18

t

19

t

20

t

21

123 141516

D15 D14 D2 D1 D0

t

23

t

t

24

Figure 13. Master Serial Data Timing for Reading (Read After Convert)

–14–

30

t

25

t

26

t

27

REV. 0

AD7650

EXT/INT = 0 RDC/SDIN = 1 INVSCLK = INVSYNC = 0

CS, RD

t

1

CNVST

t

3

BUSY

t

17

SYNC

SCLK

SDOUT

t

14

t

15

t

18

t

16

t

22

t

19

t20 t

21

12 3 141516

D15 D14 D2 D1 D0X

t

23

t

24

Figure 14. Master Serial Data Timing for Reading (Read Previous Conversion During Convert)

t

25

t

26

t

27

Usually, because the AD7650 is used with a fast throughput, the
mode master, read during conversion is the most recommended
serial mode when it can be used.

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

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.

SLAVE SERIAL INTERFACE
External Clock

The AD7650 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. When CS and
RD are both low, 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 15
and Figure 16 show the detailed timing diagrams of these methods.

While the AD7650 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 AD7650 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

Though the maximum throughput cannot be achieved using this
mode, it is the most recommended of the serial slave modes.
Figure 15 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 rising and falling edge of the clock.

Among the advantages of this method, the conversion perfor­mance is not degraded because there are no voltage transients
on the digital interface during the conversion process.

Another advantage is to be able 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 AD7650 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 desired as, for instance, in
isolated multiconverter applications.

An example of the concatenation of two devices is shown in
Figure 17. Simultaneous sampling is possible by using a common
CNVST signal. It should be noted that the RDC/SDIN input is
latched on the edge of SCLK opposite to 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.

REV. 0

–15–

AD7650

BUSY

SCLK

SDOUT

SDIN

CS

CNVST

BUSY

CS

EXT/INT = 1 INVSCLK = 0

t

35

t36 t

37

1 2 3 14151617 18

t

31

X

D15 D14 D1

t

16

X15 X14 X13 X1 X0 Y15 Y14

t

33

t

32

D13

t

34

RD = 0

D0

X15 X14

Figure 15. Slave Serial Data Timing for Reading (Read After Convert)

EXT/INT = 1 INVSCLK = 0

t

3

t

t36 t

35

37

RD = 0

SCLK

SDOUT

t

16

123 141516

t

31

X

D15 D14 D13

t

32

Figure 16. Slave Serial Data Timing for Reading (Read Previous Conversion During Convert)

BUSY OUT

BUSYBUSY

AD7650

(UPSTREAM)

RDC/SDIN SDOUT

SCLK IN

CS IN

CNVST IN

#2

CNVST

CS

SCLK

AD7650

#1

(DOWNSTREAM)

SDOUTRDC/SDIN

CNVST

SCLK

DATA OUT

CS

Figure 17. Two AD7650s in a “Daisy-Chain” Configuration

D1

D0

External Clock Data Read During Conversion

Figure 16 shows the detailed timing diagrams of this method.
During a conversion, while both CS and RD are both 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 edge 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, at least 18 MHz, when impulse mode is
used, 25 MHz when normal mode is used or 40 MHz when
warp mode is used, is recommended to ensure that all the bits
are read during the first half of the conversion phase. It is also
possible to begin to read the data after conversion and continue to
read the last bits even after a new conversion has been initiated.
That allows the use of a slower clock speed like 14 MHz in impulse
mode, 18 MHz in normal mode and 25 MHz in warp mode.

–16–

REV. 0

AD7650

MICROPROCESSOR INTERFACING

The AD7650 is ideally suited for traditional dc measurement appli­cations supporting a microprocessor, and ac signal processing
applications interfacing to a digital signal processor. The AD7650
is designed to interface either with a parallel 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 AD7650 to
prevent digital noise from coupling into the ADC. The following
sections illustrate the use of the AD7650 with an SPI-equipped
microcontroller, the ADSP-21065L and ADSP-218x signal
processors.

SPI Interface (MC68HC11)

Figure 18 shows an interface diagram between the AD7650 and an
SPI-equipped microcontroller like the MC68HC11. To accom­modate the slower speed of the microcontroller, the AD7650 acts
as a slave device and data must be read after conversion. This mode
allows also 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 to 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
Interrupt Enable (SPIE = 1) by writing to the SPI Control Reg­ister (SPCR). The IRQ is configured for edge-sensitive-only
operation (IRQE = 1 in OPTION register).

OVD D

DVD D

AD7650*

SER/PAR

EXT/INT

BUSY

CS
RD

INVSCLK

SDOUT

SCLK

CNVST

*

ADDITIONAL PINS OMITTED FOR CLARITY

MC68HC11

IRQ

MISO/SDI

SCK

I/O PORT

*

ADSP-21065L in Master Serial Interface

As shown in Figure 19, the AD7650 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 number of wire connections and being able to read
the data during or after conversion at user convenience.

The AD7650 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 as a timer. The serial port on the ADSP-21065L 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.

DVDD

OVDD

OR

OGND

AD7650*

SER/PAR

RDC/SDIN

RD
EXT/INT
CS

INVSCLK

SYNC

SDOUT

SCLKINVSYNC

CNVST

*

ADDITIONAL PINS OMITTED FOR CLARITY

ADSP-21065L*

SHARC

RFS

DR

RCLK

FLAG OR TFS

Figure 19. Interfacing to the ADSP-21065L Using the
Serial Master Mode

Figure 18. Interfacing the AD7650 to SPI Interface

REV. 0

–17–

AD7650

APPLICATION HINTS
Bipolar and Wider Input Ranges

In some applications, it is desired to use a bipolar or wider ana­log input range like, for instance, ±10 V, ± 5 V or 0 V to 5 V.
Although the AD7650 has only one unipolar range, by simple
modifications of the input driver circuitry, bipolar and wider
input ranges can be used without any performance degradation.

Figure 20 shows a connection diagram which allows that. Compo­nents values required and resulting full-scale ranges are shown
in Table II.

R1

U2

R2

R3 R4

C

REF

5V

U1

10nF

100nF

1mF

IN+

AD7650

IN–

REF

REFGND

ANALOG

INPUT

2.5V REF

Figure 20. Using the AD7650 in 16-Bit Bipolar
and/or Wider Input Ranges

Table II. Component Values and Input Ranges

Input Range R1 R2 R3 R4

± 10 V 250 Ω 2 kΩ 10 kΩ 8 kΩ
± 5 V 500 Ω 2 kΩ 10 kΩ 6.67 kΩ

0 V to –5 V 1 kΩ 2 kΩ None 0 Ω

For applications where accurate gain and offset are desired, they
can be calibrated by acquiring a ground and a voltage reference
using an analog multiplexer, U2, as shown for bipolar input
ranges in Figure 20.

Layout

The AD7650 has very good immunity to noise on the power
supplies as can be seen in Figure 9. However, care should still
be taken with regard to grounding layout.

The printed circuit board that houses the AD7650 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 AD7650 or, at least, as close as possible to the
AD7650. If the AD7650 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 AD7650.

It is recommended to avoid running digital lines under the device
as these will couple noise onto the die. The analog ground plane
should be allowed to run under the AD7650 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 supplies lines to the AD7650 should use as large
trace as possible to provide low impedance paths and reduce the
effect of glitches on the power supplies lines. Good decoupling
is also important to lower the supplies impedance presented to
the AD7650 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 AD7650 can either be a separate
supply or come from the analog supply AVDD or the digital
interface supply OVDD. When the system digital supply is noisy,
or fast switching digital signals are present, it is recommended
that if no separate supply is available, 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 AD7650 has five different ground pins: IN–, REFGND,
AGND, DGND, and OGND. IN– is used to sense the analog
input signal. 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.

Evaluating the AD7650 Performance

A recommended layout for the AD7650 is outlined in the
evaluation board for the AD7650. 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 Board.

–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

AD7650

37

36

0.276 (7.0)
BSC SQ

PIN 1
INDICATOR

TOP VIEW

(PINS DOWN)

COPLANARITY

0.003 (0.08)

0.008 (0.2)

0.004 (0.09)

CONTROLLING DIMENSIONS ARE IN MILLIMETERS; INCH DIMENSIONS
ARE ROUNDED-OFF MILLIMETER EQUIVALENTS FOR REFERENCE
ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN

08
MIN

7ⴗ
0ⴗ

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

0.276
(7.00)

BSC

SQ

25

24

48-Lead Frame Chip Scale Flatpack (LFCSP)

(CP-48)

0.024 (0.60)

0.017 (0.42)

0.009 (0.24)

37

36

BOTTOM

VIEW

TOP

VIEW

0.266 (6.75)
BSC SQ

0.024 (0.60)

0.017 (0.42)

0.009 (0.24)

0.057 (1.45)

0.053 (1.35)

48

1

0.215 (5.45)

0.209 (5.30) SQ

0.203 (5.15)

REV. 0

128 MAX

0.039 (1.00) MAX

0.033 (0.85)
NOM

0.020 (0.50)

0.016 (0.40)

0.031 (0.80) MAX

0.026 (0.65) NOM

0.020 (0.50)
BSC

CONTROLLING DIMENSIONS ARE IN MILLIMETERS; INCH DIMENSIONS
ARE ROUNDED-OFF MILLIMETER EQUIVALENTS FOR REFERENCE
ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN

0.008 (0.20)
REF

0.012 (0.30)

0.002 (0.05)

0.0004 (0.01)

0.0 (0.0)

25

24

0.012 (0.30)

0.009 (0.23)

0.007 (0.18)

–19–

1
2

13

PADDLE CONNECTED TO AGND

C02770–0–1/02(0)

–20–

PRINTED IN U.S.A.