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Data Sheet

AD8229

Rev. B | Page 17 of 24

THEORY OF OPERATION

+IN

–IN

+VS

–VS

+VS

–VS

+VS

–VS

Ω

RG–

+VS

–VS

OUTPUT

REF

NODE 1

NODE 2

COMPENSATION

+VS

–VS

+VS

–VS

Ω

RG+

Ω

Figure 56. Simplified Schematic

ARCHITECTURE

The AD8229 is based on the classic 3-op-amp topology. This

topology has two stages: a preamplifier to provide differential

amplification followed by a difference amplifier that removes the

common-mode voltage and provides additional amplification.

Figure 56 shows a simplified schematic of the AD8229.

The first stage works as follows. To keep its two inputs matched,

Amplifier A1 must keep the collector of Q1 at a constant voltage. It

does this by forcing RG− to be a precise diode drop from –IN.

Similarly, A2 forces RG+ to be a constant diode drop from +IN.

Therefore, a replica of the differential input voltage is placed

across the gain setting resistor, RG. The current that flows

through this resistance must also flow through the R1 and R2

resistors, creating a gained differential signal between the A2

and A1 outputs.

The second stage is a G = 1 difference amplifier, composed of

Amplifier A3 and the R3 through R6 resistors. This stage removes

the common-mode signal from the amplified differential signal.

The transfer function of the AD8229 is

VOUT = G × (VIN+ − VIN−) + VREF

where:

kΩ

1 +

GAIN SELECTION

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

AD8229, which can be calculated by referring to Table 5 or by

using the following gain equation:

kΩ

−

Table 5. Gains Achieved Using 1% Resistors

1% Standard Table Value of RG (Ω)

Calculated Gain

6.04 k

1.993

1.5 k

5.000

665

10.02

316

19.99

121

50.59

60.4

100.34

30.1

200.34

12.1

496.9

6.04

994.4

3.01

1994.355

The AD8229 defaults to G = 1 when no gain resistor is used.

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

to the AD8229’s specifications to determine the total gain accuracy

of the system. When the gain resistor is not used, gain error and

gain drift are minimal.

RG Power Dissipation

The AD8229 duplicates the differential voltage across its inputs

onto the RG resistor. The RG resistor size should be chosen to

handle the expected power dissipation.

REFERENCE TERMINAL

The output voltage of the AD8229 is developed with respect to

the potential on the reference terminal. This is useful when the

output signal must be offset to a precise midsupply level. For

example, a voltage source can be tied to the REF pin to level-

shift the output so that the AD8229 can drive a single-supply

ADC. The REF pin is protected with ESD diodes and should

not exceed either +VS or −VS by more than 0.3 V.

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