THEORY OF OPERATION
The ISO122 isolation amplifier uses an input and an output
section galvanically isolated by matched 1pF isolating ca-
pacitors built into the plastic package. The input is duty-
cycle modulated and transmitted digitally across the barrier.
The output section receives the modulated signal, converts it
back to an analog voltage and removes the ripple component
inherent in the demodulation. Input and output sections are
fabricated, then laser trimmed for exceptional circuitry match-
ing common to both input and output sections. The sections
are then mounted on opposite ends of the package with the
isolating capacitors mounted between the two sections. The
transistor count of the ISO122 is 250 transistors.
MODULATOR
An input amplifier (A1, Figure 1) integrates the difference
between the input current (V
IN
/200kΩ) and a switched
±100µA
current source. This current source is implemented
by a switchable 200µA source and a fixed 100µA current
sink. To understand the basic operation of the modulator,
assume that V
IN
= 0.0V. The integrator will ramp in one
direction until the comparator threshold is exceeded. The
comparator and sense amp will force the current source to
switch; the resultant signal is a triangular waveform with a
50% duty cycle. The internal oscillator forces the current
source to switch at 500kHz. The resultant capacitor drive is
a complementary duty-cycle modulation square wave.
DEMODULATOR
The sense amplifier detects the signal transitions across the
capacitive barrier and drives a switched current source into
integrator A2. The output stage balances the duty-cycle
modulated current against the feedback current through the
200kΩ feedback resistor, resulting in an average value at the
V
OUT
pin equal to V
IN
. The sample and hold amplifiers in the
output feedback loop serve to remove undesired ripple
voltages inherent in the demodulation process.
BASIC OPERATION
SIGNAL AND SUPPLY CONNECTIONS
Each power supply pin should be bypassed with 1µF tanta-
lum capacitors located as close to the amplifier as possible.
The internal frequency of the modulator/demodulator is set
at 500kHz by an internal oscillator. Therefore, if it is desired
to minimize any feedthrough noise (beat frequencies) from
a DC/DC converter, use a
π
filter on the supplies (see Figure
4). ISO122 output has a 500kHz ripple of 20mV, which can
be removed with a simple two pole low-pass filter with a
100kHz cutoff using a low cost op amp. See Figure 4.
The input to the modulator is a current (set by the 200kΩ
integrator input resistor) that makes it possible to have an
input voltage greater than the input supplies, as long as the
output supply is at least
±15V.
It is therefore possible when
using an unregulated DC/DC converter to minimize PSR
related output errors with
±5V
voltage regulators on the
isolated side and still get the full
±10V
input and output
swing. An example of this application is shown in Figure
10.
CARRIER FREQUENCY CONSIDERATIONS
The ISO122 amplifier transmits the signal across the isola-
tion barrier by a 500kHz duty cycle modulation technique.
For input signals having frequencies below 250kHz, this
system works like any linear amplifier. But for frequencies
above 250kHz, the behavior is similar to that of a sampling
amplifier. The signal response to inputs greater than 250kHz
Isolation Barrier
200µA
1pF
1pF
200µA
Sense
200kΩ
V
IN
–
+
A1
||
150pF
100µA
1pF
1pF
Sense
150pF
100µA
V
OUT
–
A2
S/H
G=1
+
200kΩ
Osc
S/H
G=6
+V
S1
Gnd 1 –V
S1
+V
S2
Gnd 2 –V
S2
FIGURE 1. Block Diagram.
®
ISO122
6
TI [ TEXAS INSTRUMENTS ]
