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
this effect, see Application Note
1078,
Designing with Hewlett-
Packard Isolation Amplifiers.
Shunt Resistor Selection
The current-sensing shunt resis-
tor should have low resistance (to
minimize power dissipation), low
inductance (to minimize di/dt
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
smaller and improving circuit
accuracy by making it larger and
utilizing the full input range of
the HCPL-7840. Hewlett-Packard
recommends four 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
signal 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
Application Note 1078,
Design-
ing with Hewlett-Packard
Isolation Amplifiers,
contains
additional information on
designing with current shunts.
The recommended method for
connecting the isolation amplifier
to the shunt resistor is shown in
Figure 23. Pin 2 (V
IN+
) is con-
nected to the positive terminal of
the shunt resistor, while pin 3
(V
IN-
) 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
separate 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 recom-
mended that twisted-pair wire or
very close PC board traces be
used to connect the current shunt
to the isolation amplifier circuit
to minimize electromagnetic
interference to the sense signal.
The 68
Ω
resistor in series with
the input lead forms a low-pass
anti-aliasing filter with the input
bypass capacitor with a 200 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. To be
effective, the damping resistor
should be at least 39
Ω.
PC Board Layout
In addition to affecting offset, the
layout of the PC board can also
affect the common mode rejec-
tion (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 coup-
ling 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-7840. Using surface mount
components can help achieve
many of the PCB objectives
discussed in the preceding para-
graphs. An example through-hole
PCB layout illustrating some of
the more important layout
recommendations is shown in
Figures 25 and 26. See Applica-
tion Note 1078,
Designing with
Hewlett-Packard Isolation
Amplifiers,
for more information
on PCB layout considerations.
Post-Amplifier Circuit
The recommended application
circuit (Figure 23) 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
1-257
AGILENT [ AGILENT TECHNOLOGIES, LTD. ]
