HCPL7840 datasheet(11/12 Pages) HP

Part #	HCPL7840

Description	Analog Isolation Amplifier

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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

1K

Ω so that the input resistance

(480 K

Ω ) 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

Maximum

Shunt

Power

Average

Horsepower

Shunt Resistor Part Number

Resistance

Dissipation

Current

Range

LVR-3.05-1%

50 m

Ω

3 W

3 A

0.8-3.0 hp

LVR-3.02-1%

20 m

Ω

3 W

8 A

2.2-8.0 hp

LVR-3.01-1%

10 m

Ω

3 W

15 A

4.1-15 hp

LVR-5.005-1%

5 m

Ω

5 W

35 A

9.6-35 hp

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