5962-9755701HPC View Datasheet(PDF) - Avago Technologies
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Description
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5962-9755701HPC Datasheet PDF : 16 Pages
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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-7850, or less than about 5 mV.
To maintain overall circuit bandwidth, 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 component values shown in Figure 24
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 s and a delay to
90% of 8.5 s.
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 25 shows a schematic
for a post amplifier for use in 5 V single supply applica-
tions. One additional resistor is needed and the gain is
decreased to 1 to allow circuit operation over the full input
voltage range. See Application Note 1078, Designing with
Avago Technologies Isolation Amplifiers, for more infor-
mation 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 22 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 char-
acteristic. For the same filter bandwidth, a higher-order
filter can achieve even better attenuation of modula-
tion noise due to the second-order noise shaping of the
sigma-delta modulator. For more information on the noise
characteristics of the HCPL-7850, see Application Note
1078, Designing with Avago Technologies Isolation
Amplifiers.
The HCPL-7850 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 K so that the input
resistance (480 K) 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 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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