AD8227ARMZ-R71 datasheet(20/25 Pages) AD

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AD8227

Rev. 0 | Page 19 of

THEORY OF OPERATION

8kΩ

–IN

+IN

+VS

–VS

50kΩ

10kΩ

+VS

–VS

VOUT

REF

NODE 1

NODE 2

+VS

–VS

VBIAS

+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 AD8227 is based on the classic three 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 amplifica-

tion. Figure 58 shows a simplified schematic of the AD8227.

The first stage works as follows. To maintain a constant voltage

across the bias resistor, RB, Amplifier A1 must keep Node 3 at a

constant diode drop above the positive input voltage. Similarly,

Amplifier 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

Amplifier A3 and the R3 through R6 resistors. This stage

removes the common-mode signal from the amplified

differential signal and gains it by 5.

The transfer function of the AD8227 is

VOUT = G × (VIN+ − VIN−) + VREF

where:

kΩ

5 +

GAIN SELECTION

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

AD8227. The gain can be calculated by referring to Table 7 or

by using the following gain equation:

kΩ

−

Table 7. Gains Achieved Using Common Resistor Values

Standard Table Value of RG

Calculated Gain

No resistor

100 kΩ

5.8

49.9 kΩ

6.6

26.7 kΩ

20 kΩ

16 kΩ

10 kΩ

5.36 kΩ

19.9

2 kΩ

1.78 kΩ

49.9

1 kΩ

845 Ω

99.7

412 Ω

199

162 Ω

499

80.6 Ω

998

The AD8227 defaults to G = 5 when no gain resistor is used.

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

to the specifications of the AD8227 to determine the total gain

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

error and gain drift are minimal.

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