AD8295BCPZ-R7 datasheet(19/28 Pages) AD

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AD8295

Rev. 0 | Page 19 of

THEORY OF OPERATION

As shown in Figure 52, the AD8295 contains a precision

instrumentation amplifier, two uncommitted op amps, and a

precision resistor array. These components allow many common

applications to be wired using simple pin-strapping, directly at

the IC. This not only saves printed circuit board (PCB) space

but also improves circuit performance because both temperature

drift and resistor tolerance errors are reduced.

A1 OUT

A1 R2

A2 +IN

A2 –IN

–IN

+VS

–VS

OUT

REF

+IN

20kΩ

A2 OUT

AD8295

A1 +IN

A1 R1

A1 –IN

Figure 52. Functional Block Diagram

UNCOMMITTED OP AMPS

The AD8295 has two uncommitted op amps that can be used

independently. These op amps allow simple pin-strapping for

many common applications circuits.

Op Amp A1 has its inverting input connected to a precision 2:1

voltage divider resistor network. Because this network is internal

to the IC, these resistors are closely matched and also track each

other, with temperature variations. Op Amp A1 and the associated

resistor network can be used to create either a noninverting gain

stage of 2 or an inverting gain stage of −1 with excellent gain

accuracy and gain drift.

Op Amp A2 is a more conventional op amp, with standard

inverting and noninverting inputs and an output.

INSTRUMENTATION AMPLIFIER

Gain Selection

The transfer function of the AD8295 is

VOUT = G × (VIN+ − VIN−) + VREF

where placing a resistor across the RG terminals sets the gain of

the AD8295 according to the following equation:

kΩ

1 +

Resistor values can be obtained by referring to Table 9 or by

using the following gain equation:

kΩ

−

Table 9. Gains Achieved Using 1% Resistors

1% Standard Table Value of RG

Calculated Gain

49.9 kΩ

1.990

12.4 kΩ

4.984

5.49 kΩ

9.998

2.61 kΩ

19.93

1.00 kΩ

50.40

499 Ω

100

249 Ω

199.4

100 Ω

495

49.9 Ω

991

The AD8295 defaults to G = 1 when no gain resistor is used.

Gain accuracy is a combination of both the RG accuracy and

the accuracy listed in the specifications in Table 2, including

accuracy over temperature. Gain error and gain drift are kept

to a minimum when the gain resistor is not used.

Common-Mode Input Voltage Range

The AD8295 in-amp architecture applies gain internally and

then removes the common-mode voltage. Therefore, internal

nodes in the AD8295 experience a combination of both the

gained signal and the common-mode signal. This combined

signal can be limited by the voltage supplies even when the

individual input and output signals are not. Figure 7 through

Figure 10 show the allowable common-mode input voltage

ranges for various output voltages and supply voltages.

If Figure 7 through Figure 10 indicate that internal voltage

limiting may be an issue, the common-mode range can be

improved by lowering the gain in the instrumentation amplifier

by one half and applying a second G = 2 stage. Figure 53 shows

how to do this amplification with the internal circuitry of the

AD8295, requiring no additional external components.

REF

+IN

–IN

20kΩ

A1 OUT

A1 TOTAL GAIN = IN-AMP × 2

–

IN-AMP

–

20kΩ

Figure 53. Applying Gain in a Later Stage Allows Wider Input

Common-Mode Range

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