AD628 datasheet(19/24 Pages) AD

19 / 24 page

AD8278/AD8279

Rev. C | Page 19 of 24

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 58.

INCORRECT

CORRECT

AD8278

OP1177

–

REF

AD8278

REF

Figure 58. Driving the Reference Pin

DIFFERENTIAL OUTPUT

The two difference amplifiers of the AD8279 can be configured

to provide a differential output, as shown in Figure 59. This

differential output configuration is suitable for various applications,

such as strain gage excitation and single-ended-to-differential

conversion. The differential output voltage has a gain twice that

of a single AD8279 channel, as shown in the following equation:

VDIFF_OUT

= V+OUT − V−OUT = 2 × GAD8279 × (VIN+ – VIN−)

If the AD8279 amplifiers are each configured for G = ½, the

differential gain is 1×; if the AD8279 amplifiers are each

configured for G = 2, the differential gain is 4×.

20kΩ

40kΩ

20kΩ

+VS

–IN

+IN

+OUT

40kΩ

AD8279

20kΩ

40kΩ

20kΩ

–VS

40kΩ

–OUT

Figure 59. AD8279 Differential Output G = 4 Configuration

INSTRUMENTATION AMPLIFIER

The AD8278 and AD8279 can be used as building blocks 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 11) creates a precise,

power efficient voltage measurement solution suitable for power

critical systems.

–IN

+IN

AD8278/

AD8279

40kΩ

20kΩ

40kΩ

REF

VOUT

VOUT = (1 + 2RF/RG) (VIN+ – VIN–) × 2

Figure 60. Low Power Precision Instrumentation Amplifier

Table 11. 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 and AD8279 difference

amplifiers cancel 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.

Refer to A Designer’s Guide to Instrumentation Amplifiers for

more design ideas and considerations at www.analog.com,

under Technical Documentation.

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