Achieving isolation in an isolation amplifier

Isolation amplifiers create a galvanic isolation barrier between input and output circuits to protect sensitive equipment and personnel from high voltages. Such isolation can be optical, capacitive, or inductive in an isolation amplifier. This FAQ will help us understand how each isolation type works and how they differ.

How do optical isolation amplifiers work?

Optical isolation amplifiers convert the input electrical signal into light, transmit it across a gap, and then reconvert it back into an electrical signal on the secondary side. The transmission across the gap happens through a combination of LED on the primary side and a photodetector on the secondary side, as illustrated in Figure 1.

Figure 1. Block diagram of an optical isolation amplifier. (Image: Rakesh Kumar, Ph.D.)

This process isolates the input and output circuits and prevents direct electrical connection between them. Because light transfers energy, optical isolation amplifiers are immune to electromagnetic interference (EMI). It is also worth noting that the intensity of the LED is directly proportional to the measured signal.

How is isolation achieved in a capacitive isolation amplifier?

A capacitive isolation amplifier transmits signals across an electrical isolation barrier using small high-voltage capacitors. Therefore, an electrical field is used as energy to transfer the signals.

The electrical field at the primary side charges the capacitor to the necessary level depending on its intensity. On the secondary side, the charge level is detected and converted to a signal at the output. The phenomenon is illustrated in Figure 2.

Figure 2. Block diagram of a capacitive isolation amplifier. (Image: Rakesh Kumar)

How do inductive isolation amplifiers function to achieve isolation?

Inductive isolation amplifiers (Figure 3) use transformers to generate an electromagnetic field as energy to transmit signals. The input signal is modulated into a high-frequency carrier signal using pulse width modulation or frequency modulation. The signal is demodulated on the secondary side to recover the input signal.

Figure 3. Block diagram of an inductive isolation amplifier. (Image: Rakesh Kumar)

How do the three types of isolation amplifiers perform for different performance variables?

The three isolation amplifier types find applications depending on their performance. Let us compare their performance to get a deeper understanding.

Temperature sensitivity: Optical isolation amplifiers are more sensitive to temperature changes. This phenomenon is attributed to the use of LED and photodetector, which are generally known to be sensitive to temperature variations.

The capacitive and inductive isolation amplifiers are relatively less sensitive to temperature changes and offer stable operation over a wide temperature range.

Aging: Due to the presence of LEDs, optical isolation amplifiers are more susceptible to aging effects. Temperature changes have a pronounced effect on optical isolation amplifiers, which can accelerate their aging process.

Capacitive isolation amplifiers show minimal deterioration over time. Modern capacitive isolation techniques using SiO2 capacitors offer excellent long-term stability. Inductive isolation amplifiers are similar to capacitive isolation amplifiers in aging, as the core materials and winding insulation generally degrade at a much lesser pace.

Isolation voltage: All three amplifier types have different isolation voltage ranges. The optical isolation provides up to 5-7 kV RMS isolation voltages. The capacitive isolation amplifier can isolate 1-5 kV RMS, while inductive isolation amplifiers offer a higher isolation voltage of up to 10 kV or more.

Frequency bandwidth: Optical isolation amplifiers typically provide a bandwidth of up to 60 kHz. Capacitive isolation amplifiers can provide a bandwidth of 100 kHz or more. The inductive isolation amplifier offers a lower bandwidth of 2 kHz to 20 kHz.

Power transfer: While the isolation amplifiers are generally meant to operate at signal levels, the inductive isolation amplifiers are an exception. The inductive isolation amplifiers can transfer power across the isolation barrier and are capable of providing isolated power for external signal processing devices on the isolated side of the system.

Summary

The isolation amplifiers find applications depending on the features of each type. The optical isolators are immune to EMI, making them suitable for medical equipment and industrial control systems with high EMI environments. The capacitive isolation amplifiers are well suited for high-frequency operation and applications that require very high common-mode transient immunity. An inductive isolation amplifier can transfer signal and power across the isolation barrier and is therefore helpful for audio equipment, power supply monitoring, and industrial process control requiring isolated power.