AD8278ARMZ-R7 datasheet(19/24 Pages) AD

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AD8278

Rev. 0 | Page 19 of 24

As with the other inputs, the reference must be driven with a

low impedance source to maintain the internal resistor ratio. An

example using the low power, low noise OP1177 as a reference

is shown in Figure 55.

INCORRECT

CORRECT

AD8278

OP1177

–

REF

AD8278

REF

Figure 55. Driving the Reference Pin

INSTRUMENTATION AMPLIFIER

The AD8278 can be used as a building block for a low power,

low cost instrumentation amplifier. An instrumentation amplifier

provides high impedance inputs and delivers high common-

mode rejection. Combining the AD8278 with an Analog Devices,

Inc., low power amplifier (see Table 10) creates a precise, power

efficient voltage measurement solution suitable for power

critical systems.

–IN

+IN

AD8278

40kΩ

20kΩ

40kΩ

REF

VOUT

VOUT = (1 + 2RF/RG) (VIN+ – VIN–) × 2

Figure 56. Low Power Precision Instrumentation Amplifier

Table 10. Low Power Op Amps

Op Amp (A1, A2)

Features

AD8506

Dual micropower op amp

AD8607

Precision dual micropower op amp

AD8617

Low cost CMOS micropower op amp

AD8667

Dual precision CMOS micropower op amp

It is preferable to use dual op amps for the high impedance inputs,

because they have better matched performance and track each

other over temperature. The AD8278 difference amplifier can-

cels out common-mode errors from the input op amps, if they

track each other. The differential gain accuracy of the in-amp

is proportional to how well the input feedback resistors (RF)

match each other. The CMRR of the in-amp increases as the

differential gain is increased (1 + 2RF/RG), but a higher gain

also reduces the common-mode voltage range. Note that dual

supplies must be used for proper operation of this configuration.

Refer to A Designer’s Guide to Instrumentation Amplifiers for

more design ideas and considerations.

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