ISO124 datasheet(6/19 Pages) TI1

6 / 19 page

ISO124

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 (VIN/200kΩ) and a

switched

±100µA 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 = 0.0V. 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

200k

Ω 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 volt-

ages inherent in the demodulation process.

BASIC OPERATION

SIGNAL AND SUPPLY CONNECTIONS

Each power-supply pin should be bypassed with 1

µF 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 200k

Ω

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

200k

Ω

1pF

150pF

Osc

200

µA

100

µA

Sense

200

µA

100

µA

200k

Ω

150pF

S/H

G = 1

S/H

G = 6

Sense

Gnd 2

–V

Gnd 1

–V

OUT

Isolation Barrier

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