AD622(RevC) 查看數據表（PDF） - Analog Devices

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AD622
(Rev.:RevC)

Low Cost Instrumentation Amplifier

Analog Devices 

AD622

THEORY OF OPERATION

The AD622 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

0.5% at G = 100) with only one resistor. Monolithic construc-

tion and laser wafer trimming allow the tight matching and

tracking of circuit components, thus insuring its performance.

The input transistors Q1 and Q2 provide a single differential-

pair bipolar input for high precision. Feedback through the

Q1-A1-R1 loop and the Q2-A2-R2 loop maintains 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 sub-

tracter 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 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 12 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 25.25 kΩ, allowing the gain to be programmed

accurately with a single external resistor.

Make vs. Buy: A Typical Application Error Budget

The AD622 offers a cost and performance advantages over

discrete “two op-amp” instrumentation amplifier designs along

with smaller size and less components. In a typical application

shown in Figure 14, a gain of 10 is required to receive and am-

plify a 0–20 mA signal from the AD694 current transmitter.

The current is converted to a voltage in a 50 Ω shunt. In appli-

cations where transmission is over long distances, line imped-

ance can be significant so that differential voltage measurement

is essential. Where there is no connection between the ground

returns of transmitter and receiver, there must be a dc path from

each input to ground, implemented in this case using two 1 kΩ

resistors. The error budget detailed in Table I shows how to

calculate the effect various error sources have on circuit accuracy.

The AD622 provides greater accuracy at lower cost. The higher

cost of the “homebrew” circuit is dominated in this case by the

matched resistor network. One could also realize a “homebrew”

design using cheaper discrete resistors which would be either

trimmed or hand selected to give high common-mode rejection.

This level of common-mode rejection would however degrade

significantly over temperature due to the drift mismatch of the

discrete resistors.

Note that for the homebrew circuit, the LT1013 specification

for noise has been multiplied by √2. This is because a “two op-

amp” type instrumentation amplifier has two op amps at its

inputs, both contributing to the overall noise.

AD694

0–20mA

TRANSMITTER

RL2

10⍀

0–20mA

RL2

10⍀

VIN

50⍀

1k⍀

RG

5.62k⍀

AD622

REFERENCE

1k⍀

1k⍀

1k⍀

9k⍀*

1/2

LT1013

1k⍀* 1k⍀*

1/2

LT1013

9k⍀*

*0.1% RESISTOR MATCH, 50ppm / C TRACKING

0–20 mA Current Loop

with 50 Ω Shunt Impedance

AD622 Monolithic

Instrumentation Amplifier,

G = 9.986

Figure 14. Make vs. Buy

“Homebrew” In Amp, G = 10

REV. C

–7–

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