5962-9755701HYC データシートの表示（PDF） - Broadcom Corporation
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5962-9755701HYC Datasheet PDF : 16 Pages
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ACPL-785E, HCPL-7850, HCPL-7851, 5962-97557
Data Sheet
Post-Amplifier Circuit
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-785x, 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 μs and a 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 25 shows a schematic for a post
amplifier for use in 5V single supply applications. 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 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 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 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-785x, see Application
Note 1078, Designing with Avago Technologies Isolation
Amplifiers.
The HCPL-785x 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 μA) do not affect the accuracy of the
measurement. An input bypass capacitor is still required,
although the 68Ω 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 Resistor
Part Number
Shunt Resistance
LVR-3.05-1%
50 mΩ
LVR-3.02-1%
20 mΩ
LVR-3.01-1%
10 mΩ
LVR-5.005-1%
5 mΩ
Maximum Power
Dissipation
3W
3W
3W
5W
Maximum Average
Current
3A
8A
15A
35A
Maximum Horsepower
Range
0.8 to 3.0 hp
2.2 to 8.0 hp
4.1 to 15 hp
9.6 to 35 hp
Broadcom
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