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 perfor-
mance. The offset of the op-amp
should be low relative to the out-
put offset of the HCPL-7840, 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
200 kHz. To obtain a bandwidth
of 200 kHz with a gain of 5, the
op-amp should have a gain-
bandwidth greater than 1 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 bandwidth). The compo-
nent values shown in Figure 23
form a differential amplifier with
a gain of 5 and a cutoff frequency
of approximately 100 kHz and
were chosen as a compromise
between low noise and fast
response times. The overall
recommended application circuit
has a bandwidth of 66 kHz, a rise
time of 5.2
µs
and delay to 90%
of 8.5
µs.
The gain-setting resistors in the
post-amp should have a tolerance
of 1% or better to ensure adequate
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
24 shows a schematic for a post
amplifier for use in 5 V single
supply applications. One addi-
tional resistor is needed and the
gain is decreased to 1 to allow
circuit operation over the full
input voltage range. See Applica-
tion Note 1078,
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-7840, see Application Note
1078,
Designing with Hewlett-
Packard Isolation Amplifiers.
The HCPL-7840 can also be used
to isolate signals with amplitudes
larger than its recommended
input range through 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 KΩ so that the input resistance
(480 KΩ ) and input bias current
(0.6 A) do not affect the accuracy
of the measurement. An input
bypass capacitor is still required,
although the 68
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.
Table 1. Current Shunt Summary
Shunt
Resistance
50 mΩ
20 mΩ
10 mΩ
5 mΩ
Maximum
Power
Dissipation
3W
3W
3W
5W
Maximum
Average
Current
3A
8A
15 A
35 A
Maximum
Horsepower
Range
0.8-3.0 hp
2.2-8.0 hp
4.1-15 hp
9.6-35 hp
Shunt Resistor Part Number
LVR-3.05-1%
LVR-3.02-1%
LVR-3.01-1%
LVR-5.005-1%
1-258
AGILENT [ AGILENT TECHNOLOGIES, LTD. ]
