the HCPL-7820/7825 through an
RC anti-aliasing filter (R5, C3).
And finally, the differential output
of the isolation amplifier is con-
verted to a ground-referenced
single-ended output voltage with a
simple differential amplifier
circuit (U3 and associated
components). Although the
application circuit is relatively
simple, a few recommendations
should be followed to ensure
optimal performance.
Inductive coupling between the
input power-supply bypass
capacitor and the input circuit,
which includes the input bypass
capacitor and the input leads of
the HCPL-7820/7825, can
induced voltage spikes which
could adversely affect operation),
and reasonable tolerance (to
maintain overall circuit accuracy).
The value of the shunt should be
chosen as a compromise between
minimizing power dissipation by
making the shunt resistance
introduce additional DC offset in
the circuit. Several steps can be
taken to minimize the mutual
coupling between the two parts of
the circuit, thereby improving the
offset performance of the design.
Separate the two bypass capaci-
tors C2 and C3 as much as
possible (even putting them on
opposite sides of the PC board),
while keeping the total lead
lengths, including traces, of each
bypass capacitor less than 20
mm. PC board traces should be
made as short as possible and
placed close together or over
ground plane to minimize loop
area and pickup of stray magnetic
fields. Avoid using sockets, as
they will typically increase both
loop area and inductance. And
finally, using capacitors with
small body size and orienting
them perpendicular to each other
on the PC board can also help.
For more information concerning
inductive coupling, see the
smaller and improving circuit
accuracy by making it larger and
using more of the input range of
the HCPL-7820/7825. Hewlett-
Packard recommends 4 different
shunts which can be used to sense
average currents in motor drives
up to 35 A and 35 hp. Table 1
shows the maximum current and
horsepower range for each of the
LVR-series shunts from Dale.
Even higher currents can be
sensed with lower value shunts
available from vendors such as
Dale, IRC, and Isotek (Isabellen-
huette). When sensing currents
large enough to cause significant
heating of the shunt, the tempera-
ture coefficient of the shunt can
introduce nonlinearity due to the
amplitude dependent temperature
rise of the shunt. Using a heat
sink for the shunt or using a shunt
with a lower tempco can help
minimize this effect. The
Supplies and Bypassing
As mentioned above, an inexpen-
sive 78L05 three-terminal regula-
tor can be used to reduce the
gate-drive power supply voltage
to 5 V. To help attenuate high-
frequency power supply noise or
ripple, a resistor or inductor can
be used in series with the input of
the regulator to form a low-pass
filter with the regulator’s input
bypass capacitor.
As shown in Figure 24, 0.1 µF
bypass capacitors (C2, C4) should
be located as close as possible to
the input and output power supply
pins of the HCPL-7820/7825. The
bypass capacitors are required
because of the high-speed digital
nature of the signals inside the
isolation amplifier. A 0.01 µF
bypass capacitor (C3) is also rec-
ommended at the input pin(s) due
to the switched-capacitor nature
of the input circuit. The input
bypass capacitor should be at
least 1000 pF to maintain gain
accuracy of the isolation amplifier.
Application Note Designing with
Hewlett-Packard Isolation
Amplifiers.
Application Note Designing with
Hewlett-Packard Isolation
Amplifiers contains additional
information on designing with
current shunts.
Shunt Resistor Selection
The current-sensing shunt resistor
should have low resistance (to
minimize power dissipation), low
inductance (to minimize di/dt
The recommended method for
connecting the isolation amplifier
to the shunt resistor is shown in
Table 1. Current Shunt Summary
Shunt Resistor
Part Number
Shunt
Resistance
Maximum
Power Dissipation
Maximum
RMS Current
Maximum
Horsepower Range
LVR-3.05-1%
LVR-3.02-1%
LVR-3.01-1%
LVR-5.005-1%
50 mΩ
20 mΩ
10 mΩ
5 mΩ
3 W
3 W
3 W
5 W
3 A
8 A
15 A
35 A
0.8-3.0 hp
2.2-8.0 hp
4.1-15 hp
9.6-35 hp
1-245
HP [ HEWLETT-PACKARD ]
