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

8

ISO124

SBOS074D – SEPTEMBER 1997 – REVISED JULY 2016

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Copyright © 1997–2016, Texas Instruments Incorporated

8 Detailed Description

8.1 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.

8.1.1 Module

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

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

8.1.2 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

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

demodulation process.

8.2 Functional Block Diagram