14
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, a 0.1
μ
F
bypass capacitor (C2, C4) should
be located as close as possible to
the input and output power
supply pins of the HCPL-7850.
The bypass capacitors are
required because of the high-
speed digital nature of the signals
inside the isolation amplifier. A
0.01
μ
F bypass capacitor (C3) is
also recommended 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.
Inductive coupling between the
input power-supply capacitor and
the input circuit, including the
input bypass capacitor and the
input leads of the HCPL-7850,
can 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 capacitors 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
this effect, see Application Note
1078,
Designing with Agilent
Technologies Isolation
Amplifiers
.
Shunt Resistor Selections
The current-sensing shunt
resistor 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-
7850. Agilent Technologies
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
(Isabellenhuette). When sensing
currents large enough to cause
significant heating of the shunt,
the temperature 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,
Designing with
Agilent
Technologies 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 24. Pin 2 (V
IN+
) is
connected 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 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
electromagnetic interference to
the sense signal.
The 68
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. Undampened 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