Avago Technologies ACPL-C790-000E - Manual (Page 2)

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AV02-3742EN - August 7, 2012
Figure 3. The Evaluation module schematic
Schematic
The evaluation module circuit schematic is show in Figure
3. The evaluation module is made from two printed circuit
boards (PCBs): PCB1 and PCB2. PCB1 has the shunt resis-
tor and two header connectors to form a current path; a
third header connector is used as an interface. PCB2 has
an ACPL-C79A isolation amplifier (±1% gain accuracy) and
an isolated DC-DC converter. The two PCBs are assembled
with header connectors P4 and P5.
AC or DC current through shunt resistor R1 results in a
voltage that is proportional to current. This voltage is fil-
tered by the anti-aliasing filter formed by R2, R3 and C1
and then sensed by the differential input ACPL-C79A. A
differential output voltage that is proportional to the in-
put voltage is created on the other side of the optical iso-
lation barrier.
Following the isolation amplifier, an OPA237 configured
as a difference amplifier converts the differential signal
to a single-ended output. This stage can be configured to
further amplify the signal, if required, and form a low-pass
filter to limit the bandwidth.
In the evaluation module, the difference amplifier is de-
signed for a gain of 1 with a low-pass filter corner frequen-
cy of 234 kHz. Resistors R6 and R7 can be selected for a
different gain. The bandwidth can be reduced by adding
capacitors to the positions of C6 and C8.
With the ACPL-C79A gain of 8.2, the overall transfer func-
tion is:
Vout = I × R1 × 8.2 + Vref .
The output signal, Vout, is then connected to the next
stage, such as a signal processor, through the P7 header
connector, pin 3.
Shunt Resistor and Current Range
The shunt resistor in this module is fixed at 10 milliohm.
The appropriate current measurement range is about 15
A
RMS
. This is calculated from the nominal input range of
±200 mV of the ACPL-C79A. The ACPL-C79x specifies a
full scale input range of ±300 mV, which allows accurate
overload current detection of up to 21 A
PEAK
. Because the
evaluation module does not provide good heat dissipa-
tion for the shunt resistor due to small PCB size, limit cur-
rent to 10 A
RMS
during evaluation or for a quick functional
check at 15 A
RMS
. For a detailed performance evaluation,
a PCB with proper layout for the shunt resistor and other
components is recommended.
Power Supplies
The evaluation module works on a single 5 V supply,
which can be the same power supply for the signal pro-
cessor and controller device connected through pin 1 and
2 of P7.
The isolated DC-DC converter (U1 in Figure 3) is included
in the evaluation module to power up the signal input
side of the ACPL C79A; this makes the evaluation an easy
plug-and-play operation. However, to make an ACPL C79A
based solution cost effective in mass production, the 5 V
supply would usually be supplied by a floating power sup-
ply, which in many applications could be the same supply
that is used to drive the high-side power transistor. A sim-
ple three-terminal voltage regulator will provide a stable
voltage. Another method is to add an additional winding
to an existing transformer to produce a 5 V supply.
0.47uF
C3
10R,1% R2
0.47uF
C4
GND2 GND1
100nF
C2
GND1
100nF
C5
GND2
22nF
C1
10R,1% R3
GND1
GND2
10K,1%
R4
10K,1% R5
NM C8
68pF C9
10K,1% R7
0.1uF C10 GND2
68pF
C7
10K,1%
R6
NM
C6
Vref
GND2
1K,1%
R9
1K,1%
R8
1uF
C11
Vref
GND2 GND2
VDD2
VDD2
GND2
WSR3R0100F
R1
R01, 3W
-Vin
1
+Vin
4
+Vout
5
-Vout
7
U1
NKE0505DC
1
2
3
P1
Header 3
1
2
3
P2
Header 3
1 2
3 4
P3
Leave Blank
1
2
3
P7
Header 3
1 2
3 4
P4
MHDR2X2
V+
0V
Out
PCB2 PCB1 PCB1
Vinp
Vinn
VDD1
1
VIN+
2
VIN- 3
GND1
4
GND2
5
VOUT- 6
VOUT+
7
VDD2
8
U2
ACPL-C79X
3
2
6
7
4
8
1
5
U3
OPA237UA
VDD1
Vout
1
2
3
4
P6
Leave Blank
1
2
3
4
P5
MHDR2X2
Note *: Dotted lines denote header pin connections.
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