1 Introduction

Programmable Logic Controllers (PLC) and Distributed Control Systems (DCS) are used for automation and control in diverse industries such as oil refineries, paper and pulp, chemicals, cement, and food and beverage. PLC and DCS systems use analog input (AI) modules to receive inputs from temperature, pressure, flow, level, gas and other sensors and detectors. Analog input modules digitize the analog information received from the field and transfer it to the CPU module of the PLC/DCS. Then, the CPU module can take any required action, for example, control a valve or turn a relay on or off.

Isolation is invariably used in the analog input modules and for two reasons:

1. Isolation breaks the ground loop between the sensor and the CPU module - which can be hundreds of meters away from each other - for reliable communication.
2. Isolation offers robustness to electrostatic discharge (ESD), electrical fast transients (EFT), surge and other disturbances present in an industrial environment.

In this article, I discuss the key considerations for choosing signal and power isolation solutions in group isolated and channel-to-channel isolated analog input modules.