AD648AQ AD [Analog Devices], AD648AQ Datasheet - Page 9

• Manufacturer Part Number: AD648AQ
• Description: Dual Precision, Low Power BiFET Op Amp
• Manufacturer: AD [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 A. This configuration is
optimal for conditioning differential voltages from high imped-
ance sources.
The overall gain of the circuit is controlled by R
the following transfer function:
REV. C
V
V
OUT
IN
1
(R3 R4)
R
G
Figure 29. Low Power Instrumentation Amplifier
G
, resulting in
–9–
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.
Next, adjust R2 for zero offset at V
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/ s.
10
12
. The capacitors C1, C2, C3 and C4 compensate
OUT
with both inputs
AD648