HCPL-7800 Datasheet, PDF (15/17 Pages) Agilent(Hewlett-Packard) – High CMR Isolation Amplifiers

Circuit Information
The recommended application
circuit is shown in Figure 26. A
floating power supply (which in
many applications could be the
same supply that is used to drive
the high-side power transistor) is
regulated to 5 V using a simple
three-terminal voltage regulator.
The input of the HCPL-7800 is
connected directly to the current
sensing resistor. The differential
output of the isolation amplifier is
converted to a ground-referenced
single-ended output voltage with a
simple differential amplifier
circuit. Although the application
circuit is relatively simple, a few
general recommendations should
be followed to ensure optimal
performance.
As shown in Figure 26, 0.1 µF
bypass capacitors should be
located as close as possible to the
input and output power supply
pins of the HCPL-7800. Notice
that pin 2 (V_IN+) is bypassed with
a 0.01 µF capacitor to reduce
input offset voltage that can be
caused by the combination of
long input leads and the switched-
capacitor nature of the input
circuit.
With pin 3 (V_IN-) tied directly to
pin 4 (GND1), the power-supply
return line also functions as the
sense line for the negative side of
the current-sensing resistor; this
allows a single twisted pair of
wire to connect the isolation
amplifier to the sense resistor. In
some applications, however,
better performance may be
obtained by connecting pins 2
and 3 (V_IN+ and V_IN-) directly
across the sense resistor with
twisted pair wire and using a
separate wire for the power
supply return line. Both input
pins should be bypassed with 0.01
µF capacitors close to the
isolation amplifier. In either case,
it is recommended that twisted-
pair wire be used to connect the
isolation amplifier to the current-
sensing resistor to minimize
electro-magnetic interference of
the sense signal.
To obtain optimal CMR perfor-
mance, the layout of the printed
circuit board (PCB) should
minimize any stray coupling by
maintaining the maximum
possible distance between the
input and output sides of the
circuit and ensuring that any
ground plane on the PCB does not
pass directly below the HCPL-
7800. An example single-sided
PCB layout for the recommended
application circuit is shown in
Figure 29. The trace pattern is
shown in “X-ray” view as it would
be seen from the top of the PCB;
a mirror image of this layout can
be used to generate a PCB.
An inexpensive 78L05 three-
terminal regulator is shown in the
recommended application circuit.
Because the performance of the
isolation amplifier can be affected
by changes in the power supply
voltages, using regulators with
tighter output voltage tolerances
will result in better overall circuit
performance. Many different
regulators that provide tighter
output voltage tolerances than the
78L05 can be used, including:
TL780-05 (Texas Instruments),
LM340LAZ-5.0 and LP2950CZ-
5.0 (National Semiconductor).
The op-amp used in the external
post-amplifier circuit should be of
sufficiently high precision so that
it does not contribute a significant
amount of offset or offset drift
relative to the contribution from
the isolation amplifier. Generally,
op-amps with bipolar input stages
exhibit better offset performance
than op-amps with JFET or
MOSFET input stages.
In addition, the op-amp should
also have enough bandwidth and
slew rate so that it does not
adversely affect the response
speed of the overall circuit. 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. Many different op-amps
could be used in the circuit,
including: MC34082A (Motorola),
TL032A, TLO52A, and TLC277
(Texas Instruments), LF412A
(National Semiconductor).
The gain-setting resistors in the
post-amp should have a tolerance
of 1% or better to ensure
adequate CMRR and adequate
gain tolerance for the overall
circuit. Resistor networks can be
used that have much better ratio
tolerances than can be achieved
using discrete resistors. A resistor
network also reduces the total
number of components for the
circuit as well as the required
board space.
The current-sensing resistor
should have a relatively low value
of resistance to minimize power
dissipation, a fairly low
inductance to accurately reflect
high-frequency signal compo-
nents, and a reasonably tight
tolerance to maintain overall
circuit accuracy. Although
decreasing the value of the sense
resistor decreases power
dissipation, it also decreases the
full-scale input voltage making
iso-amp offset voltage effects
more significant. These two
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