Power Supplies and Bypassing
As shown in Figure 22, 1±± nF bypass capacitors (C2, C3)
should be located as close as possible to the pins of the
isolation amplifier0 The bypass capacitors are required
because of the high-speed digital nature of the signals
inside the isolation amplifier0 A 1±± pF bypass capacitor
(Cin) is also recommended at the input pins due to the
switched-capacitor nature of the input circuit0 The input
bypass capacitor Cin also forms part of the anti-aliasing
filter, which is recommended to prevent high-frequency
noise from aliasing down to lower frequencies and inter-
fering with the input signal0 When R1 is far greater than R2,
the low-pass anti-aliasing filter corner frequency can be
calculated by 1/(2R2Cin)0 The input filter also performs
an important reliability function – it reduces transient
spikes from ESD events flowing through the high voltage
rails0
A power supply of . V is required to power the ACPL-C87x
input side VDD10 In many motor drive DC bus voltage
sensing applications, this .V supply is most often obtained
from the same supply used to power the power transistor
gate drive circuit using an inexpensive 78L±. three-ter-
minal regulator0 To help attenuate high frequency power
supply noise or ripple, a resistor or inductor can be used
in series with the input of the regulator to form a low-pass
filter with the regulator’s input bypass capacitor0
In some other applications a dedicated supply might be
required to supply the VDD10 These applications include
photovoltaic (PV) inverter voltage tracking and measure-
ment, temperature sensor signal isolation0 In these cases
it is possible to add an additional winding on an existing
transformer0 Otherwise, some sort of simple isolated
supply can be used, such as a line powered transformer or
a high-frequency DC-DC converter module0
HV+
R1
Floating
PositiveSupply
OUT
IN
78L05
5V
C2
0.1μF
C1
0.1μF
VDD2
VDD1
VIN
VOUT+
GateDrive
Circuit
C3
ACPL-C87A
0.1μF
Cin
0.1nF
R2
VOUT-
SHDN
GND1
GND2
HV-
Figure 22. Recommended Power Supply and Bypassing
13
AVAGO [ AVAGO TECHNOLOGIES LIMITED ]
