HCPL-7820 Datasheet, PDF (15/15 Pages) Agilent(Hewlett-Packard) – High CMR Analog Isolation Amplifiers

Post-Amplifier Circuit
The recommended application
circuit (Figure 24) includes a
post-amplifier circuit that serves
three functions: to reference the
output signal to the desired level
(usually ground), to amplify the
signal to appropriate levels, and
to help filter output noise. The
particular op-amp used in the
post-amp is not critical; however,
it should have low enough offset
and high enough bandwidth and
slew rate so that it does not
adversely affect circuit
performance. The offset of the op-
amp should be low relative to the
output offset of the HCPL-7820/
7825, or less than about 5 mV.
To maintain overall circuit band-
width, the post-amplifier circuit
should have a bandwidth at least
twice the minimum bandwidth of
the isolation amplifier, or about
400 kHz. To obtain a bandwidth
of 400 kHz with a gain of 5, the
op-amp should have a gain-
bandwidth greater than 2 MHz.
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 independently of the gain
and are useful for reducing the
output noise from the isolation
amplifier (doubling the capacitor
values halves the circuit band-
width). The component values
shown in Figure 24 form a
differential amplifier with a gain
of 5 and a cutoff frequency of
approximately 200 kHz and were
chosen as a compromise between
low noise and fast response times.
The overall recommended
application circuit has a band-
width of 130 kHz, a rise time of
2.6 µs and delay to 90% of
4.2 µs.
The gain-setting resistors in the
post-amp should have a tolerance
of 1% or better to ensure ade-
quate CMRR and gain tolerance
for the overall circuit. Resistor
networks with even better ratio
tolerances can be used which
offer better performance, as well
as reducing the total component
count and board space.
The post-amplifier circuit can be
easily modified to allow for single-
supply operation. Figure 25 shows
a schematic for a post-amplifier
for use in 5 V single-supply appli-
cations. One additional resistor is
needed and the gain is decreased
to allow circuit operation over the
full input voltage range. See the
Application Note Designing with
Hewlett-Packard Isolation
Amplifiers for more information
on the post-amplifier circuit.
Other Information
As mentioned above, reducing the
bandwidth of the post amplifier
circuit reduces the amount of
output noise. Figure 21 shows
how the output noise changes as a
function of the post-amplifier
bandwidth. The post-amplifier
circuit exhibits a first-order low-
pass filter characteristic. For the
same filter bandwidth, a higher-
order filter can achieve even
better attenuation of modulation
noise due to the second-order
noise shaping of the sigma-delta
modulator. For more information
on the noise characteristics of the
HCPL-7820/7825, see the
Application Note Designing with
Hewlett-Packard Isolation
Amplifiers.
The HCPL-7820/7825 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 imped-
ance of the divider be relatively
small (less than 1 kΩ) so that the
input resistance (280 kΩ) and
input bias current (1 µA) do not
affect the accuracy of the
measurement. An input bypass
capacitor is still required,
although the 39 Ω 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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