Digital techniques for optical isolation

DIGITAL TECHNIQUES FOR

OPTICAL ISOLATION

Isolation amplifiers provide electrical isolation and an electrical safety barrier.

They protect the patients from leakage currents.

They break the ohmic continuity of electrical signals between input and output.

Isolated power supplies are provided for both the input and output stages.

Used to amplify low level signals.

GENERAL PIN CONFIGURATION OF ISOLATION AMPLIFIER

TYPES

Three types of methods are used to design isolation amplifier

Transformer isolation Optical isolation Capacitive isolation

TRANSFORMER COUPLED ISOLATION AMPLIFIER

It uses either frequency modulated or a pulse width modulated carrier signal.

Internally it has 20KHz oscillator, transformer, rectifier and filter to provide supplies for each isolated stages.

Rectifier- input to primary OPAMP. Transformer- couples the supply. Oscillator – input to secondary OPAMP. A low pass filter is used to remove the

other frequency components.

Advantages

High common mode rejection ratio High linearity High accuracy

applications

MEDICAL

Patient Monitoring and Diagnostic

Instrumentation INDUSTRIAL

Ground Loop Elimination and

Off-ground Signal Measurement NUCLEAR

Input/output/Power Isolation

Optically isolated isolation amplifier

The biological signal is converted into a light signal by LED for further process.

It has patient circuit which is the input circuit and a phototransistor which forms the output circuit.

Each circuit is battery driven. The input circuit converts the signal to

light and the output circuit converts the light back to signal.

advantages

Original frequency and amplitude is obtained.

High linearity. No modulator or demodulator is needed as

it couples optically. Improves patient safety

Application

INDUSTRIAL PROCESS CONTROL DATA ACQUISITION INTERFACE ELEMENT BIOMEDICAL MEASUREMENTS PATIENT MONITORING TEST EQUIPMENT CURRENT SHUNT MEASUREMENT GROUND-LOOP ELIMINATION SCR CONTROLS

Capacitively coupled isolation amplifier

It uses digital encoding of the input voltage and frequency modulation.

The input voltage is converted to proportional charge on the switched capacitor.

It has modulator and demodulator circuits. The signals are sent across a differential

capacitive barrier. Separate supplies are given for both sides.

advantages

Ripple noises are removed. It avoids device noise, radiated noise and

conducted noise. High immunity to magnetic noises. Useful for analog systems. Has high gain stability and linearity.

applications

DATA ACQUISITION INTERFACE ELEMENT PATIENT MONITORING ECG EEG

Comparative study

Commonly used

Transformer isolation amplifier Cost

Optical- low cost due to less components

Transformer

Capacitor coupled- high cost Isolation voltage

Optical- low (800V)

Transformer- medium (1200V)

Capacitance- high (2200V)

Isolation resistance

Optical- 10^12

Transformer- 10^10

Capacitance- 10^12 Gain stability and Linearity

Optical- 0.02%

Transformer- between 0.005% and 0.02%

Capacitance- 0.005% (best)

VECTOR CARDIOGRAPHY (VCG)

ANGULAR VIEWS

VCG WAVEFORM