ISOLATION AMPLIFIERS ( PAGE 725 ) BY : Muhammad Afif bin Mohd Azuan Ahmad Syahmi bin Berahim

After completing this section, you should be able to : Explain and analyze the operation of an isolation amplifier Describe a basic capacitor-coupled isolation amplifier 1. Show the block diagram 2. Define modulation 3. Discuss the modulation process 4. Describe the ISO124 as an example of an isolation amplifier Describe a transformer-coupled isolation amplifier 1. Discuss the 3656KG 2. Establish the voltage gain 3. Describe a medical application

A Basic Capacitor-Coupled Isolation Amplifier Isolation amplifier is a device that consists of two electrically isolated stages: i.Input stage ii.Output stage There are separated by an isolation barrier so that a signal must be processed in order to be coupled across the isolation barrier. Some isolation amplifiers use optical coupling or transformer coupling to provide isolation between the stages. Modern isolation amplifiers use capacitive coupling for isolation. In each stages, supply voltages and grounds are separated so that there are no common electrical paths between them. Figure below show a simplified block diagram for a typical isolation amplifier Notice that there are two different ground symbols are used to reinforce the concept of stage separation.

Input stages Consist of an amplifier, an oscillator and a modulator Modulation allowing a signal containing information to modify a characteristics of another signal, such as amplitude, frequency, or pulse width, so that the information in the first signal is also contained in the second. Modulator uses high-frequency square-wave oscillator to modify the original signal. Without modulation, prohibitively high-value capacitors would be necessary with a resulting degradation in the isolation between the stages.

Output stage Consist of a demodulator that extracts the original input signal from the modulated signal so that the original signal from the input stage is back to its original form. When separate dc supply voltages and an input signal are applied, an amplified output signal is the result.

Example 14-3 The ISO124 is an integrated circuit isolation amplifier. It has a voltage gain of 1 and operates on positive and negative dc supply voltages for both stages. This device uses pulse-width modulation ( or duty cycle modulation ) with a frequency of 500kHz. It is recommended that the supply voltages be coupled with external capacitors to reduce noise. Show the appropriate connections. Solution : The manufacturer recommends a 1 mF tantalum capacitor ( for low leakage ) from each dc power supply pin to ground. This is shown in Figure below where the supply voltages are +-15V. Basic signal and power connections for an ISO124 isolation amplifier.

A Transformer-Coupled Isolation Amplifier 3656KG is an example of an isolation amplifier that uses transformer coupling to isolate the two stages. Unlike the ISO124, which has a fixed unity gain, 3656KG provides for external gain adjustment of both stages. Below the 3656KG isolation amplifier.