AD620 datasheet(10/16 Pages) AD

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AD620

REV. E

–10–

–VS

GAIN

SENSE

GAIN

SENSE

400

10k

REF

10k

+IN

– IN

20 A

400

OUTPUT

Figure 33. Simplified Schematic of AD620

THEORY OF OPERATION

The AD620 is a monolithic instrumentation amplifier based on

a modification of the classic three op amp approach. Absolute

value trimming allows the user to program gain accurately (to

0.15% at G = 100) with only one resistor. Monolithic construc-

tion and laser wafer trimming allow the tight matching and

tracking of circuit components, thus ensuring the high level of

performance inherent in this circuit.

The input transistors Q1 and Q2 provide a single differential-

pair bipolar input for high precision (Figure 33), yet offer 10

lower Input Bias Current thanks to Super

βeta processing. Feed-

back through the Q1-A1-R1 loop and the Q2-A2-R2 loop main-

tains constant collector current of the input devices Q1, Q2

thereby impressing the input voltage across the external gain

setting resistor RG. 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 sig-

nal, 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 transcon-

ductance 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 gain-

related errors. (b) The gain-bandwidth product (determined by

C1, C2 and the preamp transconductance) increases with pro-

grammed gain, thus optimizing frequency response. (c) The

input voltage noise is reduced to a value of 9 nV/

√Hz, deter-

mined mainly by the collector current and base resistance of the

input devices.

The internal gain resistors, R1 and R2, are trimmed to an abso-

lute value of 24.7 k

Ω, allowing the gain to be programmed

accurately with a single external resistor.

The gain equation is then

49.4 k

Ω

+ 1

so that

RG =

49.4 k

Ω

− 1

.... .... .... .... .... .... .... .... .... ....

Figure 31b. Gain Nonlinearity, G = 100, RL = 10 kΩ

(100

µV = 10 ppm)

.... .... .... .... .... .... .... .... .... ....

Figure 31c. Gain Nonlinearity, G = 1000, RL = 10 kΩ

(1 mV = 100 ppm)

AD620

VOUT

G=1

G=1000

49.9

10k *

10T

10k

499

G=10

G=100

5.49k

+VS

11k

100

100k

INPUT

10V p-p

–VS

*ALL RESISTORS 1% TOLERANCE

Figure 32. Settling Time Test Circuit

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