10
Product Overview
Description
The ACPL-796J isolated sigma-delta () modulator
converts an analog input signal into a high-speed (up
to 20 MHz) single-bit data stream by means of a sigma-
delta over-sampling modulator. The time average of the
modulator data is directly proportional to the input signal
voltage. The modulator uses external clock ranges from 5
MHz to 20 MHz that is coupled across the isolation barrier.
This arrangement allows synchronous operation of data
acquisition to any digital controller, and adjustable clock
for speed requirements of the application. The modulator
data are encoded and transmitted across the isolation
boundary where they are recovered and decoded into
high-speed data stream of digital ones and zeros. The
original signal information is represented by the density
of ones in the data output.
The other main function of the modulator (optocoupler)
is to provide galvanic isolation between the analog signal
input and the digital data output. It provides high noise
margins and excellent immunity against isolation-mode
transients that allows direct measurement of low-level
signals in highly noisy environments, for example mea-
surement of moor phase currents in power inverters.
With 0.5 mm minimum DTI, the ACPL-796J provides reliable
double protection and high working insulation voltage,
which is suitable for fail-safe designs. This outstanding
isolation performance is superior to alternatives including
devices based on capacitive- or magnetic-coupling with
DTI in micro-meter range. O ered in an SO-16 package,
the isolated ADC delivers the reliability, small size, superior
isolation and over-temperature performance motor drive
designers need to accurately measure current at much
lower price compared to traditional current transducers.
Analog Input
The di erential analog inputs of the ACPL-796J are im-
plemented with a fully-di erential, switched-capacitor
circuit. The ACPL-796J accepts signal of ±200 mV (full scale
±320 mV), which is ideal for direct connection to shunt
based current sensing or other low-level signal sources
applications such as motor phase current measurement.
An internal voltage reference determines the full-scale
analog input range of the modulator (±320 mV); an input
range of ±200 mV is recommended to achieve optimal
performance. Users are able to use higher input range,
for example ±250 mV, as long as within full-scale range,
for purpose of over-current or overload detection. Figure
13 shows the simpli ed equivalent circuit of the analog
input.
Figure 13. Analog input equivalent circuit.
In the typical application circuit (Figure 17), the ACPL-796J
is connected in a single-ended input mode. Given the
fully di erential input structure, a di erential input con-
nection method (balanced input mode as shown in Figure
14) is recommended to achieve better performance. The
input currents created by the switching actions on both of
the pins are balanced on the lter resistors and cancelled
out each other. Any noise induced on one pin will be
coupled to the other pin by the capacitor C and creates
only common mode noise which is rejected by the device.
Typical value for Ra and Rb is 22 and 10 nF for C.
Figure 14. Simpli ed di erential input connection diagram.
200Ω (TYP)
3 pF (TYP)
3 pF (TYP)
f
SWITCH
= MCLKIN
V
IN
+
V
IN
–
200Ω (TYP)
1.5 pF
1.5 pF
COMMON MODE
VOLTAGE
f
SWITCH
= MCLKIN
ANALOG
GROUND
V
IN
+
V
IN
–
ACPL-796J
V
DD1
GND1
5 V
+Analog Input
–
Ra
Rb
C
–Analog Input
