AD8222ACPZ-R7 datasheet(15/24 Pages) AD

15 / 24 page

AD8222

Rev. 0 | 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 44. Simplified Schematic

AMPLIFIER ARCHITECTURE

The two instrumentation amplifiers of the AD8222 are based on

the classic three op amp topology. Figure 44 shows a simplified

schematic of one of the amplifiers. 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, and R1 and Q2,

A2, and R2 can be viewed as precision current feedback

amplifiers. The common-mode signal and the amplified

differential signal are applied to a difference amplifier that

rejects the common-mode voltage. The difference amplifier

employs innovations that result in low output offset voltage as

well as low output offset voltage drift.

Because the input amplifiers employ a current feedback

architecture, 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(VIN+ − VIN−) + 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’s 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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