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Data Sheet

AD8222

Rev. B | Page 15 of 24

THEORY OF OPERATION

+VS

+IN

–IN

–VS

10kΩ

400Ω

10kΩ

REF

OUTPUT

IB COMPENSATION

+VS

–VS

+VS

–VS

+VS

R1 24.7kΩ

24.7kΩ

–VS

+VS

–VS

+VS

Figure 45. Simplified Schematic

AMPLIFIER ARCHITECTURE

The two instrumentation amplifiers of the AD8222 are based

on the classic 3-op-amp topology. Figure 45 shows a simplified

schematic of one of the amplifiers. The input transistors, Q1

and Q2, are biased at a fixed current. Any differential input

signal forces the output voltages of A1 and A2 to change so that

the differential voltage also appears across RG. The current that

flows through RG must also flow through R1 and R2, resulting

in a precisely amplified version of the differential input signal

between the outputs of A1 and A2. Topologically, Q1 + A1 + R1

and Q2 + A2 + R2 can be viewed as precision current feedback

amplifiers. The common-mode signal and the amplified differen-

tial signal are applied to a difference amplifier that rejects the

common-mode voltage. The difference amplifier employs innova-

tions that result in low output offset voltage as well as low output

offset voltage drift.

Because the input amplifiers employ a current feedback architec-

ture, the gain-bandwidth product of the AD8222 increases

with gain, resulting in a system that does not suffer from the

expected bandwidth loss of voltage feedback architectures at

higher gains.

The transfer function of the AD8222 is

VOUT = G(V+IN − V−IN) + VREF

where:

kΩ

49.4

1 



GAIN SELECTION

Placing a resistor across the RG terminals sets the gain of the

AD8222, which can be calculated by referring to Table 8 or by

using the following gain equation:

kΩ

49.4





Table 8. Gains Achieved Using 1% Resistors

1% Standard Table Value of RG (Ω)

Calculated Gain

49.9 k

1.990

12.4 k

4.984

5.49 k

9.998

2.61 k

19.93

1.00 k

50.40

499

100.0

249

199.4

100

495.0

49.9

991.0

The AD8222 defaults to G = 1 when no gain resistor is used.

The tolerance and gain drift of the RG resistor should be added

to the AD8222 specifications to determine the total gain

accuracy of the system. When the gain resistor is not used,

gain error and gain drift are kept to a minimum.

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