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AD648

Dual Precision, Low Power BiFET Op Amp

Analog Devices 

INSTRUMENTATION AMPLIFIER

The AD648J’s maximum input current of 20 pA per amplifier

makes it an excellent building block for the high input imped-

ance instrumentation amplifier shown in Figure 29. Total cur-

rent drain for this circuit is under 600 µA. This configuration is

optimal for conditioning differential voltages from high imped-

ance sources.

The overall gain of the circuit is controlled by RG, resulting in

the following transfer function:

VOUT

VIN

=

1

+

(

R3 + R

RG

4

)

AD648

Gains of 1 to 100 can be accommodated with gain nonlinearities

of less than 0.01%. The maximum input current is 30 pA over

the common-mode range, with a common-mode impedance of

over 1 × 1012Ω. The capacitors C1, C2, C3 and C4 compensate

for peaking in the gain over frequency which is caused by input

capacitance.

To calibrate this circuit, first adjust trimmer R1 for common-

mode rejection with +10 volts dc applied to the input pins.

Next, adjust R2 for zero offset at VOUT with both inputs

grounded. Trim the circuit a second time for optimal

performance.

The –3 dB small signal bandwidth for this low power instru-

mentation amplifier is 700 kHz for a gain of 1 and 10 kHz for a

gain of 100. The typical output slew rate is 1.8 V/µs.

Figure 29. Low Power Instrumentation Amplifier

REV. C

–9–

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