HCPL7820 datasheet(14/15 Pages) HP

14 / 15 page

1-246

Figure 24. Pin 2 (VIN+) is con-

nected to the positive terminal of

the shunt resistor, while pin 3

(VIN-) is shorted to pin 4 (GND1),

with the power-supply return path

functioning as the sense line to

the negative terminal of the

current shunt. This allows a single

pair of wires or PC board traces

to connect the isolation amplifier

circuit to the shunt resistor. In

some applications, however,

supply currents flowing through

the power-supply return path may

cause offset or noise problems. In

this case, better performance may

be obtained by connecting pin 3

to the negative terminal of the

shunt resistor separately from the

power supply return path. When

connected this way, both input

pins should be bypassed. Whether

two or three wires are used, it is

recommended that twisted-pair

wire or very close PC board traces

be used to connect the current

shunt to the isolation amplifier

circuit to minimize electro-

magnetic interference to the

sense signal.

The 39

Ω resistor in series with

the input lead forms a low-pass

anti-aliasing filter with the input

bypass capacitor with a 400 kHz

bandwidth. The resistor performs

another important function as

well; it dampens any ringing

which might be present in the

circuit formed by the shunt, the

input bypass capacitor, and the

wires or traces connecting the

two. Undamped ringing of the

input circuit near the input

sampling frequency can alias into

the baseband producing what

might appear to be noise at the

output of the device.

PC Board Layout

In addition to affecting offset, the

layout of the PC board can also

affect the common mode rejection

(CMR) performance of the

isolation amplifier, due primarily

to stray capacitive coupling

between the input and the output

circuits. To obtain optimal CMR

performance, the layout of the

printed circuit board (PCB) should

minimize any stray coupling by

maintaining the maximum pos-

sible distance between the input

and output sides of the circuit and

ensuring that any ground plane on

the PCB does not pass directly

below or extend much wider than

the HCPL-7820/7825. Using

surface-mount components can

help achieve many of the PCB

objectives discussed in the pre-

ceding paragraphs. An example

through-hole PCB layout illustrat-

ing some of the more important

layout recommendations is shown

in Figures 26 and 27. See the

Application Note Designing with

Hewlett-Packard Isolation

Amplifiers

for more information

on PCB layout considerations.

Figure 25. Single-Supply Post-Amplifier Circuit.

Figure 26. Top Layer of Printed Circuit Board Layout.

Figure 27. Bottom Layer of Printed Circuit Board Layout.

TO RSENSE+

TO RSENSE–

TO VDD1

TO VDD2

VOUT+

VOUT–

0.1 µF

+5 V

VOUT

10.0 K

Ω

+5 V

0.1 µF

–

MC34071

10.0 K

Ω

HCPL-7820/7825

R4B

20.0 K

Ω

75 pF

R4A

20.0 K

Ω

+5 V

75 pF

10.0 K

Ω

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