AD648

REV. C

–9–

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 following transfer function:

= 1+ (R3 + R4)

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

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.

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