Broadcom

- 15 -

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

Maximum Power

Dissipation

Maximum Average

Current

Maximum Horsepower

Range

LVR-3.05-1%

50 mΩ

3W

3A

0.8 to 3.0 hp

LVR-3.02-1%

20 mΩ

3W

8A

2.2 to 8.0 hp

LVR-3.01-1%

10 mΩ

3W

15A

4.1 to 15 hp

LVR-5.005-1%

5 mΩ

5W

35A

9.6 to 35 hp