16
Also, multiple layers of the PC board can be used to
increase current carrying capacity. Numerous plated-
through vias should surround each non-Kelvin terminal of
the sense resistor to help distribute the current between
the layers of the PC board. The PC board should use 2 or
4 oz. copper for the layers, resulting in a current carrying
capacity in excess of 20 A.
Note: Please refer to Avago Technologies Application Note 1078 for
additional information on using Isolation Ampliers.
Sense Resistor Connections
The recommended method for connecting the ACPL-782T
to the current sensing resistor is shown in Figure 18. V
IN+
(pin 2 of the APCL-782T) is connected to the positive
terminal of the sense resistor, while VIN- (pin 3) is shorted
to GND1 (pin 4), with the power-supply return path func-
tioning as the sense line to the negative terminal of the
current sense resistor. This allows a single pair of wires
or PC board traces to connect the ACPL-782T circuit to
the sense resistor. By referencing the input circuit to
the negative side of the sense resistor, any load current
induced noise transients on the resistor are seen as a
common-mode signal and will not interfere with the cur-
rent-sense signal. This is important because the large load
currents owing through the motor drive, along with the
parasitic inductances inherent in the wiring of the circuit,
can generate both noise spikes and osets that are rela-
tively large compared to the small voltages that are being
measured across the current sensing resistor.
If the same power supply is used both for the gate
drive circuit and for the current sensing circuit, it is
very important that the connection from GND1 of the
ACPL-782T to the sense resistor be the only return path for
supply current to the gate drive power supply in order to
eliminate potential ground loop problems. The only direct
connection between the ACPL-782T circuit and the gate
drive circuit should be the positive power supply line.
Output Side
The op-amp used in the external post-amplier circuit
should be of suciently high precision so that it does not
contribute a signicant amount of oset or oset drift
relative to the contribution from the isolation amplier.
Generally, op-amps with bipolar input stages exhibit
better oset 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
aect the response speed of the overall circuit. The post-
amplier circuit includes a pair of capacitors (C5 and C6)
that form a single-pole low-pass lter; 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 amplier. Many
dierent op-amps could be used in the circuit, including:
TL032A, TL052A, 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.
Figure 21. Recommended circuit for voltage sensing application.
Line 1
1
2
3
4
8
7
6
5
ACPL-782T
+5 V
+15 V
0.01
PF
0.1 PF
0.1 PF
0.1 PF
10.0 k:
150 pF
2.0 k:
2.00 k:
-15 V
TL032A
+
-
V
OUT
0.1 PF
+ SUPPLY
150 pF
78L05
IN OUT
0.1 PF
Line 2
6
5
8
4
7
10.0 k:
39 :
+
Ra
Rb
Note for the Voltage Divider:
V (Line) x [ Rb / (Ra+Rb) ] <= 200 mV
