AD8226ARMZ-R7 datasheet(20/29 Pages) AD

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Data Sheet

AD8226

Rev. C | Page 19 of 28

THEORY OF OPERATION

24.7kΩ

–IN

+IN

+VS

–VS

50kΩ

+VS

–VS

VOUT

REF

NODE 1

NODE 2

VBIAS

+VS

–VS

+VS

–VS

NODE 4

NODE 3

50kΩ

DIFFERENCE

AMPLIFIER STAGE

GAIN STAGE

ESD AND

OVERVOLTAGE

PROTECTION

ESD AND

OVERVOLTAGE

PROTECTION

–VS

Figure 58. Simplified Schematic

ARCHITECTURE

The AD8226 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 to remove the

common-mode voltage. Figure 58 shows a simplified schematic

of the AD8226.

The first stage works as follows: in order to maintain a constant

voltage across the bias resistor RB, A1 must keep Node 3 a con-

stant diode drop above the positive input voltage. Similarly, A2

keeps Node 4 at a constant diode drop above the negative input

voltage. Therefore, a replica of the differential input voltage is

placed across the gain-setting resistor, RG. The current that

flows across this resistance must also flow through the R1

and R2 resistors, creating a gained differential signal between

the A2 and A1 outputs. Note that, in addition to a gained

differential signal, the original common-mode signal, shifted

a diode drop up, is also still present.

The second stage is a difference amplifier, composed of A3 and

four 50 kΩ resistors. The purpose of this stage is to remove the

common-mode signal from the amplified differential signal.

The transfer function of the AD8226 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

AD8226, which can be calculated by referring to Table 7 or by

using the following gain equation:

kΩ

49.4





Table 7. 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 AD8226 defaults to G = 1 when no gain resistor is used.

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

to the AD8226 specifications to determine the total gain accu-

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

error and gain drift are minimal.

If a gain of 5 is required and minimal gain drift is important,

consider using the AD8227. The AD8227 has a default gain of 5

that is set with internal resistors. Because all resistors are internal,

the gain drift is extremely low (<5 ppm/°C maximum).

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