1pF

200µA

100µA

200kW

Isolation Barrier

1pF

1pF

200µA

100µA

200kW

V

IN

+

–

Osc

Gnd 1 –V

S1

+V

S1

V

OUT

S/H

G = 6

S/H

G = 1

Gnd 2

+V

S2

–V

S2

–

+

A2

Sense

Sense

A1

150pF

150pF

1pF

| |

ISO122

SBOS160A – NOVEMBER 1993 – REVISED JANUARY 2015

www.ti.com

8 Detailed Description

8.1 Overview

The ISO122 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 the 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

ISO122 contains 250 transistors.

8.1.1 Modulator

An input amplifier (A1, Functional Block Diagram) integrates the difference between the input current (VIN/200

k

Ω) and a switched ±100-mA current source. This current source is implemented by a switchable 200-mA 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.

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

current through the 200 kW 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.

8.2 Functional Block Diagram

8.3 Feature Description

8.3.1 Isolation Amplifier

The ISO122 is a precision analog isolation amplifier. The input signal is transmitted digitally across a high-voltage

differential capacitive barrier. With digital modulation the barrier characteristics do affect signal integrity, resulting

in excellent reliability and high-frequency transient immunity.

8

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