HCPL7820 datasheet(15/15 Pages) HP

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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-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 k

Ω) so that the

input resistance (280 k

Ω) 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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