Datasheet

ISO124
6
SBOS074C
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THEORY OF OPERATION
The ISO124 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 ISO124 is 250 transistors.
MODULATOR
An input amplifier (A1, as shown in 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
FIGURE 1. Block Diagram.
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 volt-
ages inherent in the demodulation process.
BASIC OPERATION
SIGNAL AND SUPPLY CONNECTIONS
Each power-supply pin should be bypassed with 1µF tantalum
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).
The ISO124 output has a 500kHz ripple of 20mV, which can
be removed with a simple 2-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.
See Figure 9 for an example of this application.
CARRIER FREQUENCY CONSIDERATIONS
The ISO124 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
200kΩ
1pF
1pF
1pF
1pF
150pF
Osc
200µA
100µA
Sense
200µA
100µA
200kΩ
150pF
S/H
G = 1
S/H
G = 6
Sense
A2
A1
+V
S2
Gnd 2 –V
S2
+V
S1
Gnd 1 –V
S1
V
IN
V
OUT
Isolation Barrier