ACPL-790B-300E Datasheet, PDF (14/14 Pages) AVAGO TECHNOLOGIES LIMITED – Precision Isolation Amplifiers

Ra
+Input
Rb
–Input
5V
VDD1
VIN+ _ACPL-790B/
C
ACPL-790A/
ACPL-7900
VIN–
GND1
Figure 24. Simplified differential input connection diagram.
Output Side
The op-amp used in the external post-amplifier circuit
should be of sufficiently high precision so that it does not
contribute a significant amount of offset or offset drift
relative to the contribution from the isolation amplifier.
Generally, op-amps with bipolar input stages exhibit
better offset performance than op-amps with JFET or
MOSFET input stages.
In addition, the op-amp should also have enough
bandwidth and slew rate so that it does not adversely
affect the response speed of the overall circuit. The post-
amplifier circuit includes a pair of capacitors (C5 and C6)
that form a single-pole low-pass filter; these capacitors
allow the bandwidth of the post-amp to be adjusted in-
dependently of the gain and are useful for reducing the
output noise from the isolation amplifier.
The gain-setting resistors in the post-amp should have a
tolerance of 1% or better to ensure adequate CMRR and
adequate gain tolerance for the overall circuit. Resistor
networks can be used that have much better ratio toler-
ances than can be achieved using discrete resistors. A
resistor network also reduces the total number of compo-
nents for the circuit as well as the required board space.
Voltage Sensing
The ACPL-790B/790A/7900 can also be used to isolate
signals with amplitudes larger than its recommended
input range with the use of a resistive voltage divider at
its input. The only restrictions are that the impedance of
the divider be relatively small (less than 1 k:) so that the
input resistance (22 k:) and input bias current (0.1 PA)
do not affect the accuracy of the measurement. An input
bypass capacitor is still required, although the 10 : series
damping resistor is not (the resistance of the voltage
divider provides the same function). The low-pass filter
formed by the divider resistance and the input bypass
capacitor may limit the achievable bandwidth.
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AV02-2733EN - December 30, 2010