Datasheet AD621SQ-883B, AD621BR, AD621BN, AD621AR, AD621AN Datasheet (Analog Devices)

CONNECTION DIAGRAM

8-Lead Plastic Mini-DIP (N), Cerdip (Q)

and SOIC (R) Packages

TOP VIEW

(Not to Scale)

8

7

6

5

1

2

3

4

G = 10/100

–IN

+IN

G = 10/100

+V

S

OUTPUT

REF–V

S

AD621

REV. B

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
which may result from its use. No license is granted by implication or
otherwise under any patent or patent rights of Analog Devices.

a

Low Drift, Low Power

Instrumentation Amplifier

AD621

FEATURES
EASY TO USE
Pin-Strappable Gains of 10 and 100
All Errors Specified for Total System Performance
Higher Performance than Discrete In Amp Designs
Available in 8-Lead DIP and SOIC
Low Power, 1.3 mA Max Supply Current
Wide Power Supply Range (ⴞ2.3 V to ⴞ18 V)

EXCELLENT DC PERFORMANCE

0.15% Max, Total Gain Error
ⴞ5 ppm/ⴗC, Total Gain Drift
125 ␮V Max, Total Offset Voltage

1.0 ␮V/ⴗC Max, Offset Voltage Drift

LOW NOISE
9 nV/√Hz, @ 1 kHz, Input Voltage Noise

0.28 ␮V p-p Noise (0.1 Hz to 10 Hz)

EXCELLENT AC SPECIFICATIONS
800 kHz Bandwidth (G = 10), 200 kHz (G = 100)
12 ␮s Settling Time to 0.01%

APPLICATIONS
Weigh Scales
Transducer Interface and Data Acquisition Systems
Industrial Process Controls
Battery-Powered and Portable Equipment

PRODUCT DESCRIPTION

The AD621 is an easy to use, low cost, low power, high accu­racy instrumentation amplifier that is ideally suited for a wide
range of applications. Its unique combination of high perfor­mance, small size and low power, outperforms discrete in amp
implementations. High functionality, low gain errors, and low

SUPPLY CURRENT – mA

30,000

25,000

0

0205

TOTAL ERROR, ppm OF FULL SCALE

10 15

20,000

15,000

10,000

5,000

AD621A

3 OP AMP
IN AMP
(3

OP 07S)

Figure 1. Three Op Amp IA Designs vs. AD621

gain drift errors are achieved by the use of internal gain setting
resistors. Fixed gains of 10 and 100 can easily be set via external
pin strapping. The AD621 is fully specified as a total system,
therefore, simplifying the design process.

For portable or remote applications, where power dissipation,
size, and weight are critical, the AD621 features a very low
supply current of 1.3 mA max and is packaged in a compact
8-lead SOIC, 8-lead plastic DIP or 8-lead cerdip. The AD621
also excels in applications requiring high total accuracy, such
as precision data acquisition systems used in weigh scales and
transducer interface circuits. Low maximum error specifications
including nonlinearity of 10 ppm, gain drift of 5 ppm/°C, 50 µV
offset voltage, and 0.6 µV/°C offset drift (“B” grade), make
possible total system performance at a lower cost than has been
previously achieved with discrete designs or with other mono­lithic instrumentation amplifiers.

When operating from high source impedances, as in ECG and
blood pressure monitors, the AD621 features the ideal combina­tion of low noise and low input bias currents. Voltage noise is
specified as 9 nV/√Hz at 1 kHz and 0.28 µV p-p from 0.1 Hz to
10 Hz. Input current noise is also extremely low at 0.1 pA/√Hz.
The AD621 outperforms FET input devices with an input bias
current specification of 1.5 nA max over the full industrial tem­perature range.

SOURCE RESISTANCE – ⍀

10,000

0.1
1k 100M10k

TOTAL INPUT VOLTAGE NOISE, G = 100 – ␮Vp-p

(0.1 – 10Hz)

100k 10M

1,000

100

10

1

1M

TYPICAL STANDARD
BIPOLAR INPUT
IN AMP

AD621 SUPER␤ETA
BIPOLAR INPUT
IN AMP

Figure 2. Total Voltage Noise vs. Source Resistance

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

AD621–SPECIFICATIONS

Gain = 10

AD621A AD621B AD621S

1

Model Conditions Min Typ Max Min Typ Max Min Typ Max Unit

GAIN

Gain Error V

OUT

= ±10 V 0.15 0.05 0.15 %

Nonlinearity,

V

OUT

= –10 V to +10 V RL = 2 kΩ 2 10 2 10 2 10 ppm of FS

Gain vs. Temperature –1.5 ± 5 –1.5 ± 5–1±5 ppm/°C

TOTAL VOLTAGE OFFSET

Offset (RTI) VS = ±15 V 75 250 50 125 75 250 µV

Over Temperature V

S

= ±5 V to ±15 V 400 215 500 µV

Average TC V

S

= ±5 V to ±15 V 1.0 2.5 0.6 1.5 1.0 2.5 µV/°C

Offset Referred to the

Input vs. Supply (PSR)2VS = ±2.3 V to ± 18 V 95 120 100 120 95 120 dB

Total NOISE

Voltage Noise (RTI) 1 kHz 13 17 13 17 13 17 nV/√Hz

RTI 0.1 Hz to 10 Hz 0.55 0.55 0.8 0.55 0.8 µV p-p

Current Noise f = 1 kHz 100 100 100 fA/√Hz

0.1 Hz–10 Hz 10 10 10 pA p-p

INPUT CURRENT V

S

= ±15 V

Input Bias Current 0.5 2.0 0.5 1.0 0.5 2 nA

Over Temperature 2.5 1.5 4 nA
Average TC 3.0 3.0 8.0 pA/°C

Input Offset Current 0.3 1.0 0.3 0.5 0.3 1.0 nA

Over Temperature 1.5 0.75 2.0 nA
Average TC 1.5 1.5 8.0 pA/°C

INPUT

Input Impedance

Differential 10储210储210储2GΩ储pF
Common-Mode 10储210储210储2GΩ储pF

Input Voltage Range

3

VS = ±2.3 V to ±5 V –VS + 1.9 +VS – 1.2 –VS + 1.9 +VS – 1.2 –VS + 1.9 +VS – 1.2 V

Over Temperature –V

S

+ 2.1 +VS – 1.3 –VS + 2.1 +VS – 1.3 –VS + 2.1 +VS – 1.3 V

V

S

= ±5 V to ±18 V –VS + 1.9 +VS – 1.4 –VS + 1.9 +VS – 1.4 –VS + 1.9 +VS – 1.4 V

Over Temperature –V

S

+ 2.1 +VS – 1.4 –VS + 2.1 +VS – 1.4 –VS + 2.3 +VS – 1.4 V

Common-Mode Rejection

Ratio DC to 60 Hz with
1 kΩ Source Imbalance VCM = 0 V to ± 10 V 93 110 100 110 93 110 dB

OUTPUT

Output Swing RL = 10 kΩ,

V

S

= ±2.3 V to ±5 V –VS + 1.1 +VS – 1.2 –VS + 1.1 +VS – 1.2 –VS + 1.1 +VS – 1.2 V

Over Temperature –V

S

+ 1.4 +VS – 1.3 –VS + 1.4 +VS – 1.3 –VS + 1.6 +VS – 1.3 V

V

S

= ±5 V to ±18 V –VS + 1.2 +VS – 1.4 –VS + 1.2 +VS – 1.4 –VS + 1.2 +VS – 1.4 V

Over Temperature –V

S

+ 1.6 +VS – 1.5 –VS + 1.6 +VS – 1.5 –VS + 2.3 +VS – 1.5 V

Short Current Circuit ± 18 ±18 ± 18 mA

DYNAMIC RESPONSE

Small Signal,

–3 dB Bandwidth 800 800 800 kHz

Slew Rate 0.75 1.2 0.75 1.2 0.75 1.2 V/µs
Settling Time to 0.01% 10 V Step 12 12 12 µs

REFERENCE INPUT

R

IN

20 20 20 kΩ

I

IN

VIN +, V

REF

= 0 50 60 50 60 +50 +60 µA

Voltage Range –V

S

+ 1.6 +VS – 1.6 –VS + 1.6 +VS – 1.6 VS + 1.6 +VS – 1.6 V

Gain to Output 1 ± 0.0001 1 ± 0.0001 1 ± 0.0001

POWER SUPPLY

Operating Range ±2.3 ± 18 ±2.3 ± 18 ±2.3 ±18 V
Quiescent Current V

S

= ±2.3 V to ±18 V 0.9 1.3 0.9 1.3 0.9 1.3 mA

Over Temperature 1.1 1.6 1.1 1.6 1.1 1.6 mA

TEMPERATURE RANGE

For Specified Performance –40 to +85 –40 to +85 –55 to +125 °C

NOTES

1

See Analog Devices’ military data sheet for 883B tested specifications.

2

This is defined as the supply range over which PSRR is defined.

3

Input Voltage Range = CMV + (Gain × V

DIFF

).

Specifications subject to change without notice.

(Typical @ 25ⴗC, VS = ⴞ15 V, and RL = 2 k⍀, unless otherwise noted.)

REV. B

–2–

AD621A AD621B AD621S

1

Model Conditions Min Typ Max Min Typ Max Min Typ Max Unit

GAIN

Gain Error V

OUT

= ±10 V 0.15 0.05 0.15 %

Nonlinearity,

V

OUT

= –10 V to +10 V RL = 2 kΩ 2 10 2 10 2 10 ppm of FS

Gain vs. Temperature –1 ± 5–1±5–1±5 ppm/°C

TOTAL VOLTAGE OFFSET

Offset (RTI) VS = ±15 V 35 125 25 50 35 125 µV

Over Temperature V

S

= ±5 V to ±15 V 185 215 225 µV

Average TC V

S

= ±5 V to ±15 V 0.3 1.0 0.1 0.6 0.3 1.0 µV/°C

Offset Referred to the

Input vs. Supply (PSR)2VS = ±2.3 V to ±18 V 110 140 120 140 110 140 dB

Total NOISE

Voltage Noise (RTI) 1 kHz 9 13 9 13 9 13 nV/√Hz

RTI 0.1 Hz to 10 Hz 0.28 0.28 0.4 0.28 0.4 µV p-p

Current Noise f = 1 kHz 100 100 100 fA/√Hz

0.1 Hz–10 Hz 10 10 10 pA p-p

INPUT CURRENT V

S

= ±15 V

Input Bias Current 0.5 2.0 0.5 1.0 0.5 2 nA

Over Temperature 2.5 1.5 4 nA
Average TC 3.0 3.0 8.0 pA/°C

Input Offset Current 0.3 1.0 0.3 0.5 0.3 1.0 nA

Over Temperature 1.5 0.75 2.0 nA
Average TC 1.5 1.5 8.0 pA/°C

INPUT

Input Impedance

Differential 10储210储210储2GΩ储pF
Common-Mode 10储210储210储2GΩ储pF

Input Voltage Range

3

VS = ±2.3 V to ±5 V –VS + 1.9 +VS – 1.2 –VS + 1.9 +VS – 1.2 –VS + 1.9 +VS – 1.2 V

Over Temperature –V

S

+ 2.1 +VS – 1.3 –VS + 2.1 +VS – 1.3 –VS + 2.1 +VS – 1.3 V

V

S

= ±5 V to ±18 V –VS + 1.9 +VS – 1.4 –VS + 1.9 +VS – 1.4 –VS + 1.9 +VS – 1.4 V

Over Temperature –V

S

+ 2.1 +VS – 1.4 –VS + 2.1 +VS – 1.4 –VS + 2.3 +VS – 1.4 V

Common-Mode Rejection

Ratio DC to 60 Hz with
1 kΩ Source Imbalance VCM = 0 V to ±10 V 110 130 120 130 110 130 dB

OUTPUT

Output Swing RL = 10 kΩ,

V

S

= ±2.3 V to ±5 V –VS + 1.1 +VS – 1.2 –VS + 1.1 +VS – 1.2 –VS + 1.1 +VS – 1.2 V

Over Temperature –V

S

+ 1.4 +VS – 1.3 –VS + 1.4 +VS – 1.3 –VS + 1.6 +VS – 1.3 V

V

S

= ±5 V to ±18 V –VS + 1.2 +VS – 1.4 –VS + 1.2 +VS – 1.4 –VS + 1.2 +VS – 1.4 V

Over Temperature –V

S

+ 1.6 +VS – 1.5 –VS + 1.6 +VS – 1.5 –VS + 2.3 +VS – 1.5 V

Short Current Circuit ± 18 ±18 ± 18 mA

DYNAMIC RESPONSE

Small Signal,

–3 dB Bandwidth 200 200 200 kHz

Slew Rate 0.75 1.2 0.75 1.2 0.75 1.2 V/µs
Settling Time to 0.01% 10 V Step 12 12 12 µs

REFERENCE INPUT

R

IN

20 20 20 kΩ

I

IN

VIN +, V

REF

= 0 5060 5060 5060µA

Voltage Range –V

S

+ 1.6 +VS – 1.6 –VS + 1.6 +VS – 1.6 VS + 1.6 +VS – 1.6 V

Gain to Output 1 ± 0.0001 1 ± 0.0001 1 ± 0.0001

POWER SUPPLY

Operating Range ±2.3 ± 18 ±2.3 ± 18 ±2.3 ±18 V
Quiescent Current V

S

= ±2.3 V to ±18 V 0.9 1.3 0.9 1.3 0.9 1.3 mA

Over Temperature 1.1 1.6 1.1 1.6 1.1 1.6 mA

TEMPERATURE RANGE

For Specified Performance –40 to +85 –40 to +85 –55 to +125 °C

NOTES

1

See Analog Devices’ military data sheet for 883B tested specifications.

2

This is defined as the supply range over which PSEE is defined.

3

Input Voltage Range = CMV + (Gain × V

DIFF

).

Specifications subject to change without notice.

Gain = 100

(Typical @ 25ⴗC, VS = ⴞ15 V, and RL = 2 k⍀, unless otherwise noted.)

AD621

REV. B

–3–

AD621

REV. B

–4–

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

Specification is for device in free air:

8-Lead Plastic Package: θJA = 95°C/W
8-Lead Cerdip Package: θJA = 110°C/W
8-Lead SOIC Package: θJA = 155°C/W

ABSOLUTE MAXIMUM RATINGS

1

Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ± 18 V

Internal Power Dissipation

2

. . . . . . . . . . . . . . . . . . . . 650 mW

Input Voltage (Common Mode) . . . . . . . . . . . . . . . . . . . . ±V

S

Differential Input Voltage . . . . . . . . . . . . . . . . . . . . . . . ±25 V

Output Short Circuit Duration . . . . . . . . . . . . . . . . Indefinite

Storage Temperature Range (Q) . . . . . . . . . –65°C to +150°C

Storage Temperature Range (N, R) . . . . . . . –65°C to +125°C

Operating Temperature Range

AD621 (A, B) . . . . . . . . . . . . . . . . . . . . . . –40°C to +85°C

AD621 (S) . . . . . . . . . . . . . . . . . . . . . . . . – 55°C to +125°C

Lead Temperature Range

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

ESD SUSCEPTIBILITY

ESD (electrostatic discharge) sensitive device. Electrostatic
charges as high as 4000 volts, which readily accumulate on the
human body and on test equipment, can discharge without
detection. Although the AD621 features proprietary ESD pro­tection circuitry, permanent damage may still occur on these
devices if they are subjected to high energy electrostatic dis­charges. Therefore, proper ESD precautions are recommended
to avoid any performance degradation or loss of functionality.

ORDERING GUIDE

Temperature Package Package

Model Range Description Option

1

AD621AN –40°C to +85°C 8-Lead Plastic DIP N-8
AD621BN –40°C to +85°C 8-Lead Plastic DIP N-8
AD621AR –40°C to +85°C 8-Lead Plastic SOIC R-8
AD621BR – 40°C to +85°C 8-Lead Plastic SOIC R-8
AD621SQ/883B

2

–55°C to +125°C 8-Lead Cerdip Q-8

AD621ACHIPS –40°C to +85°C Die

NOTES

1

N = Plastic DIP; Q = Cerdip; R = SOIC.

2

See Analog Devices’ military data sheet for 883B specifications.

METALIZATION PHOTOGRAPH

Dimensions shown in inches and (mm).

Contact factory for latest dimensions.

1.125 (3.57)

0.0708
(2.545)

5

REFERENCE

RG 1

RG 8

+V

S

7

4 –V

S

2

–IN

3

+IN

OUTPUT

6

Typical Performance Characteristics–AD621

INPUT OFFSET VOLTAGE – ␮V

50

40

0

–200 –100

PERCENTAGE OF UNITS

0 +100 +200

30

20

10

SAMPLE SIZE = 90

TPC 1. Typical Distribution of V

OS,

Gain = 10

INPUT OFFSET VOLTAGE – ␮V

50

40

0

–80 –40

PERCENTAGE OF UNITS

0 +40 +80

30

20

10

SAMPLE SIZE = 90

TPC 2. Typical Distribution of VOS, Gain = 100

INPUT OFFSET CURRENT – pA

50

40

0

–400 –200

PERCENTAGE OF UNITS

0 +200 +400

30

20

10

SAMPLE SIZE = 90

TPC 3. Typical Distribution of Input Offset Current

REV. B

–5–

INPUT BIAS CURRENT – pA

50

40

0

–800 –400

PERCENTAGE OF UNITS

0 +400 +800

30

20

10

SAMPLE SIZE = 90

TPC 4. Typical Distribution of Input Bias Current

WARM-UP TIME – Minutes

2.0

0

051

CHANGE IN OFFSET VOLTAGE – ␮V

23

1.5

1.0

0.5

4

TPC 5. Change in Input Offset Voltage vs. Warm-Up Time

FREQUENCY – Hz

1000

100

1

1 100k10

VOLTAGE NOISE – nV/ Hz

100 1k 10k

10

GAIN = 10

GAIN = 100

TPC 6. Voltage Noise Spectral Density

AD621

REV. B

–6–

FREQUENCY – Hz

1000

100

1

10

CURRENT NOISE – nV/ Hz

100

1000

10

TPC 7. Current Noise Spectral Density vs. Frequency

TIME – 1 sec/div

RTI NOISE – 0.2␮V/div

TPC 8a. 0.1 Hz to 10 Hz RTI Voltage Noise, Gain = 10

TIME – 1 sec/div

RTI NOISE – 0.1␮V/div

TPC 8b. 0.1 Hz to 10 Hz RTI Voltage Noise, G = 100

10

0%

100

90

1s

100mV

TPC 9. 0.1 Hz to 10 Hz Current Noise, 5 pA per Vertical
Div, 1 Second per Horizontal Div

100

1000

AD621A

FET INPUT
IN AMP

SOURCE RESISTANCE – ⍀

TOTAL DRIFT FROM 25ⴗC TO 85ⴗC, RTI – ␮V

100,000

10

1k 10M

10,000

10k 1M100k

TPC 10. Total Drift vs. Source Resistance

FREQUENCY – Hz

0.1 1M1 10 100 1k 10k 100k

160

0

CMR – dB

140

80

60

40

20

120

100

GAIN = 100

GAIN = 10

TPC 11. CMR vs. Frequency, RTI, for a Zero to 1 k

Ω

Source Imbalance

AD621

REV. B

–7–

FREQUENCY – Hz

PSR – dB

160

1M

80

40

1

60

0.1

140

100

120

100k10k1k10010

20

G = 100

G = 10

180

TPC 12. Positive PSR vs. Frequency

FREQUENCY – Hz

PSR – dB

160

1M

80

40

1

60

0.1

140

100

120

100k10k1k10010

20

G = 100

G = 10

180

TPC 13. Negative PSR vs. Frequency

1000

100 10M

100

1

1k

10

100k 1M10k

FREQUENCY – Hz

CLOSED-LOOP GAIN – V/V

0.1

TPC 14. Closed-Loop Gain vs. Frequency

OUTPUT VOLTAGE – Volts p-p

FREQUENCY – Hz

35

0

1M

15

5

10k

10

1k

30

20

25

100k

G = 10 & 100

TPC 15. Large Signal Frequency Response

INPUT VOLTAGE LIMIT – Volts

(REFERRED TO SUPPLY VOLTAGES)

20

+1.0

+0.5

50

+1.5

–1.5

–1.0

–0.5

1510

SUPPLY VOLTAGE ⴞ Volts

–0.0

+0.0

+V

S

–V

S

TPC 16. Input Voltage Range vs. Supply Voltage

INPUT VOLTAGE LIMIT – Volts

(REFERRED TO SUPPLY VOLTAGES)

20

+1.0

+0.5

50

+1.5

–1.5

–1.0

–0.5

1510

SUPPLY VOLTAGE ⴞ Volts

–0.0

+0.0

+V

S

–V

S

RL = 10k⍀

RL = 2k⍀

RL = 10k⍀

RL = 2k⍀

TPC 17. Output Voltage Swing vs. Supply Voltage,
G = 10

AD621

REV. B

–8–

OUTPUT VOLTAGE SWING – Volts p-p

LOAD RESISTANCE – ⍀

30

0

0

10k

20

10

100 1k

VS = ⴞ 15V
G = 10

TPC 18. Output Voltage Swing vs. Resistive Load

100

90

10␮s

5V 1mV

10

0%

TPC 19. Large Signal Pulse Response and Settling
Time Gain, G = 10 (0.5 mV = 0.01%), R

L

= 1 kΩ,

C

L

= 100 pF

10

100

90

10␮s

20mV

0%

TPC 20. Small Signal Pulse Response, G = 10,
R

L

= 1 kΩ, CL = 100 pF

10

0%

100

90

10␮s

5V 1mV

TPC 21. Large Signal Pulse Response and Settling
Time, G = 100 (0.5 mV = 0.1%), R

L

= 2 kΩ, CL = 100 pF

10

0%

100

90

10␮s

20mV

TPC 22. Small Signal Pulse Response, G = 100,
R

L

= 2 kΩ, CL = 100 pF

OUTPUT STEP SIZE – Volts

SETTLING TIME – ␮s

20

0

020

15

5

5

10

10 15

TO 0.01%

TO 0.1%

TPC 23. Settling Time vs. Step Size, G = 10

AD621

REV. B

–9–

OUTPUT STEP SIZE – Volts

SETTLING TIME – ␮s

20

0

020

15

5

5

10

10

15

TO 0.01%

TO 0.1%

TPC 24. Settling Time vs. Step Size, Gain = 100

TEMPERATURE – ⴗC

INPUT CURRENT – nA

+I

B

–I

B

2.0

–2.0

175

–1.0

–1.5

–75

–0.5

0

0.5

1.0

1.5

1257525–25

–125

TPC 25. Input Bias Current vs. Temperature

10

0%

100

90

100␮V

0PW 0

20 WFM AQR WARNING

0 WFM

VZR 0

2V

TPC 26. Gain Nonlinearity, G = 100, RL = 10 kΩ,
C

L

= 0 pF. Vertical Scale: 100 µV/Div = 100 ppm/Div

Horizontal Scale: 2 Volts/Div

10

0%

100

90

2V

100␮V

TPC 27. Gain Nonlinearity, G = 10, RL = 10 kΩ, Vertical
Scale: 100

µ

V/Div = 100 ppm/Div, Horizontal Scale:

2 Volts/Div

+

–

AD621

1k⍀

10T

+V

S

–V

S

G = 10

G = 100

G = 10 G = 100

INPUT

20V p-p

10k⍀

1%

10k⍀

1%

100k⍀

1%

V

OUT

11k⍀

0.1%

1k⍀

0.1%

TPC 28. Settling Time Test Circuit

AD621

REV. B

–10–

+V

S

–V

S

I1 20␮A

A1

C1 C2

R1 25k⍀

R5

5555.6⍀

Q1

G = 100

R6

555.6⍀

R3

400⍀

–IN

2

4

1

R4

400⍀

3

+IN

10k⍀

10k⍀

OUTPUT

5

6

A3

–

+–+

–

+

I220␮A

V

B

A2

R2 25k⍀

10k⍀

10k⍀

Q2

G = 100

8

REF

7

Figure 3. Simplified Schematic of AD621

THEORY OF OPERATION

The AD621 is a monolithic instrumentation amplifier based on
a modification of the classic three op amp circuit. Careful layout
of the chip, with particular attention to thermal symmetry builds
in tight matching and tracking of critical components, thus
preserving the high level of performance inherent in this circuit,
at a low price.

On chip gain resistors are pretrimmed for gains of 10 and 100.
The AD621 is preset to a gain of 10. A single external jumper
(between Pins 1 and 8) is all that is needed to select a gain of

100. Special design techniques assure a low gain TC of 5 ppm/°C
max, even at a gain of 100.

Figure 3 is a simplified schematic of the AD621. The input
transistors Q1 and Q2 provide a single differential-pair bipolar
input for high precision, yet offer 10× lower Input Bias Current,
thanks to Superβeta processing. Feedback through the Q1-A1-R1
loop and the Q2-A2-R2 loop maintains constant collector cur­rent of the input devices Q1 and Q2, thereby impressing the
input voltage across the gain-setting resistor, RG, which equals

R5 at a gain of 10 or the parallel combination of R5 and R6 at a
gain of 100.

This creates a differential gain from the inputs to the A1/A2
outputs given by G = (R1 + R2) / RG + 1. The unity-gain
subtracter A3 removes any common-mode signal, yielding a
single-ended output referred to the REF pin potential.

The value of RG also determines the transconductance of the
preamp stage. As RG is reduced for larger gains, the transcon­ductance increases asymptotically to that of the input transistors.
This has three important advantages: (a) Open-loop gain is
boosted for increasing programmed gain, thus reducing gain­related errors. (b) The gain-bandwidth product (determined by
C1, C2 and the preamp transconductance) increases with pro­grammed gain, thus optimizing frequency response. (c) The
input voltage noise is reduced to a value of 9 nV/√Hz, deter­mined mainly by the collector current and base resistance of the
input devices.

Make vs. Buy: A Typical Bridge Application Error Budget

The AD621 offers improved performance over discrete three op
amp IA designs, along with smaller size, fewer components and
10 times lower supply current. In the typical application, shown
in Figure 4, a gain of 100 is required to amplify a bridge output of
20 mV full scale over the industrial temperature range of –40°C to
+85°C. The error budget table below shows how to calculate
the effect various error sources have on circuit accuracy.

Regardless of the system it is being used in, the AD621 provides
greater accuracy, and at low power and price. In simple systems,
absolute accuracy and drift errors are by far the most significant
contributors to error. In more complex systems with an intelligent
processor, an autogain/autozero cycle will remove all absolute
accuracy and drift errors leaving only the resolution errors of
gain nonlinearity and noise, thus allowing full 14-bit accuracy.

Note that for the discrete circuit, the OP07 specifications for
input voltage offset and noise have been multiplied by 2. This is
because a three op amp type in amp has two op amps at its inputs,
both contributing to the overall input error.

OP07D

OP07D

–

+

10k⍀*10k⍀*

10k⍀*10k⍀*

OP07D

–

+

10k⍀**

10k⍀**

+

–

3 OP AMP, IN AMP, G = 100

* 0.02% RESISTOR MATCH, 3PPM/ⴗC TRACKING
** DISCRETE 1% RESISTOR, 100PPM/ⴗC TRACKING

SUPPLY CURRENT = 15mA MAX

+

–

AD621A

REFERENCE

AD621A MONOLITHIC
INSTRUMENTATION
AMPLIFIER, G = 100

SUPPLY CURRENT = 1.3mA MAX

10V

R = 350⍀

R = 350⍀

R = 350⍀

R = 350⍀

PRECISION BRIDGE TRANSDUCER

100k⍀**

Figure 4. Make vs. Buy

AD621

REV. B

–11–

–

+

AD705

5V

3k⍀

3k⍀

3k⍀

3k⍀

AD621B

ADC

REF

IN

AGND

DIGITAL
DATA
OUTPUT

20k⍀

10k⍀

20k⍀

–

+

0.6mA
MAX

0.10mA1.3mA

MAX

1.7mA

Figure 5. A Pressure Monitor Circuit which Operates on a 5 V Power Supply

Pressure Measurement

Although useful in many bridge applications such as weigh-scales,
the AD621 is especially suited for higher resistance pressure
sensors powered at lower voltages where small size and low
power become more even significant.

Figure 5 shows a 3 kΩ pressure transducer bridge powered from
5 V. In such a circuit, the bridge consumes only 1.7 mA. Adding
the AD621 and a buffered voltage divider allows the signal to be
conditioned for only 3.8 mA of total supply current.

Small size and low cost make the AD621 especially attractive for
voltage output pressure transducers. Since it delivers low noise
and drift, it will also serve applications such as diagnostic non­invasion blood pressure measurement.

Wide Dynamic Range Gain Block Suppresses Large Common­Mode and Offset Signals

The AD621 is especially useful in wide dynamic range applica­tions such as those requiring the amplification of signals in the

presence of large, unwanted common-mode signals or offsets.
Many monolithic in amps achieve low total input drift and noise
errors only at relatively high gains (~100). In contrast the AD621’s
low output errors allow such performance at a gain of 10, thus
allowing larger input signals and therefore greater dynamic
range. The circuit of Figure 6 (±15 V supply, G = 10) has
only 2.5 µV/°C max. V

OS

drift and 0.55 µ/V p-p typical 0.1 Hz
to 10 Hz noise, yet will amplify a ±0.5 V differential signal while
suppressing a ±10 V common-mode signal, or it will amplify a
±1.25 V differential signal while suppressing a 1 V offset by use
of the DAC driving the reference pin of the AD621. An added
benefit, the offsetting DAC connected to the reference pin allows
removal of a dc signal without the associated time-constant
of ac coupling. Note the representations of a differential and
common-mode signal shown in Figure 6 such that a single-ended
(or normal mode) signal of 1 V would be composed of a 0.5 V
common-mode component and a 1 V differential component.

Table I. Make vs. Buy Error Budget

AD621 Circuit Discrete Circuit Error, ppm of Full Scale

Error Source Calculation Calculation AD621 Discrete

ABSOLUTE ACCURACY at TA = +25°C

Input Offset Voltage, µV 125 µV/20 mV (150 µV × 2/20 mV 16,250 15,000
Output Offset Voltage, µV N/A ((150 µV × 2)/100)/20 mV N/A 12,150
Input Offset Current, nA 2 nA × 350 Ω/20 mV (6 nA × 350 Ω)/20 mV 12,118 121,53
CMR, dB 110 dB→3.16 ppm, × 5 V/20 mV (0.02% Match × 5 V)/20 mV 12,791 14,988

Total Absolute Error 17,558 20,191

DRIFT TO +85°C

Gain Drift, ppm/°C 5 ppm × 60°C 100 ppm/°C Track × 60°C 13,300 12,600
Input Offset Voltage Drift, µV/°C1µV/°C × 60°C/20 mV (2.5 µV/°C × 2 × 60°C)/20 mV 13,000 15,000
Output Offset Voltage Drift, µV/°C N/A (2.5 µV/°C × 2 × 60°C)/100/20 mV N/A 12,150

Total Drift Error 13,690 15,750

RESOLUTION

Gain Nonlinearity, ppm of Full Scale 40 ppm 40 ppm 12,140 12,140
Typ 0.1 Hz–10 Hz Voltage Noise, µV p-p 0.28 µV p-p/20 mV (0.38 µV p-p × √2)120 mV 121,14 12,127

Total Resolution Error 121,54 121,67

Grand Total Error 11,472 36,008

G = 100, VS = ±15 V.
(All errors are min/max and referred to input.)

AD621

REV. B

–12–

AD548

TO

REF

C

R

TO

V

OUT1

–

+

–

+

ⴛ10

AD621

DAC

0 TO ⴞ10V

–

+

V

DIFF

ⴞ0.5V

INPUT A:
ⴞ10V CM

V

COM

ⴞ10V–

+

–

+

INPUT B:

ⴞ1V

OFFSET

V

DIFF

+ V

OFFSET

ⴞ(1.25V + 1V)

V

OUT1

G = 10

–

+

ⴛ10

AD621

V

OUT2

TOTAL GAIN = 100

10k⍀

10k⍀

OPTIONAL

USE THIS IN PLACE OF THE DAC FOR ZERO SUPPRESSION FUNCTION.

Figure 6. Suppressing a Large Common-Mode or Offset Voltage in Order to Measure a Small Differential Signal
(V

S

= ±15 V)

The AD621, as well as many other monolithic instrumentation
amplifiers, is based on the “three op amp” in amp circuit (Fig­ure 7) amplifier. Since the input amplifiers (A1 and A2) have a
common-mode gain of unity and a differential gain equal to the
set gain of the overall in amp, the voltages V1 and V2 are defined
by the equations

V

1

= VCM + G × V

DIFF

/2

V

2

= VCM – G × V

DIFF

/2

The common-mode voltage will drive the outputs of amplifiers
A1 and A2 to the differential-signal voltage, multiplied by the
gain, spreads them apart. For a 10 V common-mode 0.1 V
differential input, V1 would be at 10.5 V and V2 at 9.5 V.

A1

A3

–

+

10k⍀

10k⍀

10k⍀

10k⍀

A2

–

+

20k⍀

20k⍀

+

–

V1

V2

INPUT AMPLIFIER

DIFFERENTIAL GAIN = 10
COMMON MODE GAIN = 1

OUTPUT AMPLIFIER

DIFFERENTIAL GAIN = 1

COMMON MODE GAIN = 1/1000

4.44k⍀

Figure 7. Typical Three Op Amp Instrumentation
Amplifier, Differential Gain = 10

The AD621’s input amplifiers can provide output voltage within

2.5 V of the supplies. To avoid saturation of the input amplifier
the input voltage must therefore obey the equations:

V

CM

+ G × V

DIFF

/2 ≤ (Upper Supply – 2.5 V)

V

CM

– G × V

DIFF

/2 ≥ (Lower Supply + 2.5 V)

Figure 8 shows the trade-off between common-mode and
differential-mode input for ±15 V supplies and G = 10.

By cascading with use of the optional AD621, the circuit of
Figure 6 will provide ±1 V of zero suppression at gains of 10
and 100 (at V

OUT1

and V

OUT2

respectively) with maximum TCs

of ±4 ppm/°C and ± 8 ppm/°C, respectively. Therefore, depend­ing on the magnitude of the differential input signal, either
V

OUT1

or V

OUT2

may be used as the output.

VCM – ⴞVolts

1.2

0.2

0102

V

DIFF

– ⴞVolts

48

1.0

0.8

0.6

0.4

6

0

12

Figure 8. Trade-Off Between VCM and V

DIFF

Range (VS =

±

15 V, G = 10), for Reference Pin at Ground

AD621

REV. B

–13–

Precision V-I Converter

The AD621 along with another op amp and two resistors make
a precision current source (Figure 9). The op amp buffers the
reference terminal to maintain good CMR. The output voltage
V

X

of the AD621 appears across R1 which converts it to a cur­rent. This current less only the input bias current of the op amp
then flows out to the load.

+V

S

V

IN–

V

IN+

AD621

+V

X–

R1

I

L

AD705

LOAD

–V

S

IL =

V

X

R1

(V

IN+

) – (V

IN–

) G

R1

=

Figure 9. Precision Voltage to Current Converter
(Operates on 1.8 mA,

±

3 V)

INPUT AND OUTPUT OFFSET VOLTAGE

The AD621 is fully specified for total input errors at gains of 10
and 100. That is, effects of all error sources within the AD621
are properly included in the guaranteed input error specs, elimi­nating the need for separate error calculation.

Total Error RTI = Input Error + (Output Error/G)

Total Error RTO = (Input Error × G) + Output Error

REFERENCE TERMINAL

Although usually grounded, the reference terminal may be used
to offset the output of the AD621. This is useful when the load
is “floating” or does not share a ground with the rest of the system.
It also provides a direct means of injecting a precise offset.

Another benefit of having a reference terminal is that it can be
quite effective in eliminating ground loops and noise in a circuit
or system.

V

OL

+V

S

AD621

V

OUT

–V

S

V

OL

R

P

R

P

GAIN = 10 OR 100

Figure 10. Input Overload Protection

INPUT OVERLOAD CONSIDERATIONS

Failure of a transducer, faults on input lines, or power supply
sequencing can subject the inputs of an instrumentation ampli­fier to voltages well beyond their linear range, or even the supply
voltage, so it is essential that the amplifier handle these over­loads without being damaged.

The AD621 will safely withstand continuous input overloads of
±3.0 volts (±6.0 mA). This is true for gains of 10 and 100, with
power on or off.

The inputs of the AD621 are protected by high current capacity
dielectrically isolated 400 Ω thin-film resistors R3 and R4 (Fig­ure 3) and by diodes which protect the input transistors Q1 and
Q2 from reverse breakdown. If reverse breakdown occurred, there
would be a permanent increase in the amplifier’s input current.

The input overload capability of the AD621 can be easily increased
while only slightly degrading the noise, common-mode rejection
and offset drift of the device by adding external resistors in series
with the amplifier’s inputs as shown in Figure 10.

Table II summarizes the overload voltages and total input
noise for a range of range of r values. Note that a 2 kΩ resis­tor in series with each input will protect the AD621 from a
±15 volt continuous overload, while only increasing input noise
to 13 nV√Hz—about the same level as would be expected from
a typical unprotected 3 op amp in amp.

Table II. Input Overload Protection vs. Value of Resistor R

P

Total Input Noise Maximum Continuous

Value of in nV√Hz @ 1 kHz Overload Voltage, V

OL

Resistor RPG = 10 G = 100 In Volts

01493
499 Ω 14 10 6

1.00 kΩ 14 11 9

2.00 kΩ 15 13 15

3.01 kΩ* 16 14 21

4.99 kΩ* 17 16 33

*1/4 watt, 1% metal-film resistor. All others are 1/8 watt, 1% RN55

or equivalent.

AD621

REV. B

–14–

Gain Selection

The AD621 has accurate, low temperature coefficient (TC),
gains of 10 and 100 available. The gain of the AD621 is nomi­nally set at 10; this is easily changed to a gain of 100 by simply
connecting a jumper between Pins 1 and 8.

AD621

555.5

⍀

5,555.5

⍀

R

EXT

Figure 11. Programming the AD621 for Gains Between
10 and 100

As shown in Figure 11, the device can be programmed for any
gain between 10 and 100 by connecting a single external resistor
between Pins 1 and 8. Note that adding the external resistor will
degrade both the gain accuracy and gain TC. Since the gain
equation of the AD621 yields:

G = 1 +

9(R

X

+ 6,111.111)

(R

X

+ 555. 555)

This can be solved for the nominal value of external resistor for
gains between 10 and 100:

RX=

(G – 1) 555.555 – 55,000

(10 – G )

Table III gives practical 1% resistor values for several com­mon gains.

Table III. Practical 1% External Resistor
Values for Gains Between 10 and 100

Desired Recommended Temperature
Gain 1% Resistor Value Gain Error Coefficient (TC)

10 ∞ (Pins 1 and 8 Open)

*

5 ppm/°C max

20 4.42 kΩ±10% ≈0.4 (50 ppm/°C

+ Resistor TC)

50 698 Ω±10% ≈0.4 (50 ppm/°C

+ Resistor TC)

100 0 (Pins 1 and 8 Shorted)

*

5 ppm/°C max

*Factory trimmed–exact value depends on grade.

A High Performance Programmable Gain Amplifier

The excellent performance of the AD621 at a gain of 10 makes
it a good choice to team up with the AD526 programmable gain
amplifier (PGA) to yield a differential input PGA with gains of
10, 20, 40, 80, 160. As shown in Figure 12, the low offset of the
AD621 allows total circuit offset to be trimmed using the offset
null of the AD526, with only a negligible increase in total drift
error. The total gain TC will be 9 ppm/°C max, with 2 µV/°C
typical input offset drift. Bandwidth is 600 kHz to gains of 10 to
80, and 350 kHz at G = 160. Settling time is 13 µs to 0.01%
for a 10 V output step for all gains.

–

+

–V

S

AD621

+V

S

INPUTS

–

+

0.1␮F

G = 10

–

+

–V

S

AD526

+V

S

0.1␮F

20k⍀

0.1␮F

0.1␮F

2

OUTPUT

Figure 12. A High Performance Programmable Gain
Amplifier

COMMON-MODE REJECTION

Instrumentation amplifiers like the AD621 offer high CMR
which is a measure of the change in output voltage when both
inputs arc changed by equal amounts. These specifications are
usually given for a full-range input voltage change and a speci­fied source imbalance.

For optimal CMR, the reference terminal should be tied to a
low impedance point, and differences in capacitance and resis­tance should be kept to a minimum between the two inputs. In
many applications shielded cables are used to minimize noise,
and for best CMR over frequency the shield should he properly
driven. Figures 13 and 14 show active data guards that are config­ured to improve ac common-mode rejections by “bootstrapping”
the capacitances of input cable shields, thus minimizing the
capacitance mismatch between the inputs.

–INPUT

+INPUT

100⍀

100⍀

100k⍀

100k⍀

–V

S

–V

S

+V

S

V

OUT

REFERENCE

AD621

AD648

–

+

Figure 13. Differential Shield Driver, G = 10

AD548

100⍀

– INPUT

+ INPUT

REFERENCE

V

OUT

AD621

4

–V

S

+V

S

8

3

1

2

7

5

6

Figure 14. Common-Mode Shield Driver, G = 100

AD621

REV. B

–15–

GROUNDING

Since the AD621 output voltage is developed with respect to the
potential on the reference terminal, it can solve many ground­ing problems by simply tying the REF pin to the appropriate
“local ground.”

In order to isolate low level analog signals from a noisy digital
environment, many data-acquisition components have separate
analog and digital ground pins (Figure 15). It would be conve­nient to use a single ground line; however, current through
ground wires and PC runs of the circuit card can cause hundreds
of millivolts of error. Therefore, separate ground returns should
be provided to minimize the current flow from the sensitive
points to the system ground. These ground returns must be tied
together at some point, usually best at the ADC package as shown.

DIGITAL P.S.

+5V

C

ANALOG P.S.

+15VC–15V

AD574A

+

AD621

AD585

S/H

ADC

5

9

11

15

6

2

4

7

1

11

7

6

4

0.1␮F
1␮F

3

DIGITAL
DATA
OUTPUT

0.1␮F
1␮F1␮F

Figure 15. Basic Grounding Practice

GROUND RETURNS FOR INPUT BIAS CURRENTS

Input bias currents are those currents necessary to bias the input
transistors of an amplifier. There must be a direct return path
for these currents; therefore when amplifying “floating” input
sources such as transformers, or ac-coupled sources, there must
be a dc path from each input to ground as shown in Figures 16a
through 16c. Refer to the Instrumentation Amplifier Application
Guide (free from Analog Devices) for more information regard­ing in amp applications.

+V

S

AD621

LOAD

–V

S

REFERENCE

TO POWER SUPPLY GROUND

+INPUT

–INPUT

V

OUT

Figure 16a. Ground Returns for Bias Currents when Using
Transformer Input Coupling

+V

S

AD621

LOAD

–V

S

REFERENCE

TO POWER SUPPLY GROUND

+INPUT

–INPUT

V

OUT

Figure 16b. Ground Returns for Bias Currents when Using
a Thermocouple Input

100k⍀ 100k⍀

–INPUT

AD621

+INPUT

+V

S

–V

S

V

OUT

LOAD

REFERENCE

TO POWER SUPPLY GROUND

Figure 16c. Ground Returns for Bias Currents when Using
AC Input Coupling

AD621

REV. B

–16–

OUTLINE DIMENSIONS

Dimensions shown in inches and (mm).

Plastic DIP (N-8) Package

0.125 (3.18)
MIN

0.165 0.01
(4.19

0.25)

0.39 (9.91)
MAX

0.25

(6.35)

4

5

8

1

0.035 0.01

(0.89

0.25)

0.018

0.003

(0.46

0.08)

0.30 (7.62)
REF

0 - 15

0.10

(2.54)

TYP

0.011 0.003
(4.57

0.76)

SEATING PLANE

0.31

(7.87)

0.18

0.03

(4.57

0.76)

0.033

(0.84)

NOM

Cerdip (Q-8) Package

0.005 (0.13) MIN 0.055 (1.4) MAX

0.405 (10.29) MAX

0.150
(3.81)
MIN

0.200

(5.08)

MAX

0.310 (7.87)

0.220 (5.59)

0.070 (1.78)

0.030 (0.76)

0.200 (5.08)

0.125 (3.18)

0.023 (0.58)

0.014 (0.36)

0.320 (8.13)

0.290 (7.37)

0 - 15

0.015 (0.38)

0.008 (0.20)

0.100 (2.54)
BSC

SEATING PLANE

0.060 (1.52)

0.015 (0.38)

41

58

SOIC (R-8) Package

0.181 (4.60)

0.205 (5.20)

0.020 (0.50)

0.045 (1.15)

0.007 (0.18)

0.015 (0.38)

0.100 (2.59)

0.094(2.39)

0.004 (0.10)

0.010 (0.25)

1

4

5

8

0.188 (4.77)

0.198 (5.03)

0.150 (3.80)

0.158 (4.00)

0.228 (5.80)

0.244 (6.200)

0.014 (0.36)

0.018 (0.46)

0.050 (1.27)
TYP

C00776–0–1/01 (rev. B)

PRINTED IN U.S.A.