8

ISO124

SBOS074E – SEPTEMBER 1997 – REVISED JUNE 2018

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Copyright © 1997–2018, Texas Instruments Incorporated

7 Detailed Description

7.1 Overview

The ISO124 isolation amplifier uses an input and an output section galvanically isolated by matched 1-pF

isolating capacitors 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 matching common to 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 ISO124

contains 250 transistors.

7.1.1 Modulator

An input amplifier (A1, as shown in Functional Block Diagram) integrates the difference between the input current

(VIN/200 kΩ) and a switched ±100-µA current source. This current source is implemented by a switchable 200-µA

source and a fixed 100-µA current sink. To understand the basic operation of the modulator, assume that VIN = 0

V. 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 500 kHz. The resultant capacitor drive is a

complementary duty-cycle modulation square wave

7.1.2 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 200-kΩ feedback resistor, resulting in an average value at the VOUT pin equal to VIN. The

sample-and-hold amplifiers in the output feedback loop serve to remove undesired ripple voltages inherent in the

demodulation process.