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.

As the use of automation continues to grows, so too does the amount of information that needs to be monitored and processed. More data means more sensors and actuators and correspondingly more PLC I/O modules. However, space inside a factory is also at a premium. Hence, PLC and DCS manufacturers would like to offer more compact modules with higher and higher channel densities as a benefit to their customers. With many high-channel density modules packed close together, the power consumed by all the channels can lead to high module temperature, potentially causing reliability issues. Thermal concerns force each module to be designed for lower power dissipation than before. Also with increased electrification and cabling, the possibility of electromagnetic interference goes up, leading to an increased focus on electromagnetic compatibility (EMC).

These general considerations hold for group isolated and channel-to-channel isolated analog input modules. Group-isolated modules are lower cost, whereas channel-to-channel isolated modules offer robustness and flexibility of use. Careful choice of Isolation solutions is an important decision in each case and is a major factor that decides module size, power and electromagnetic compatibility.

A group-isolated analog module is shown in Figure 3-1. A group-isolated module receives inputs from several sensors such as temperature, pressure and level. with respect to a single reference field ground (ISOGND in Figure 3-1). If the common mode voltage difference between the sensor ground and the ISOGND is expected to be large, the analog inputs may be differential as shown in Figure 3-1. Otherwise, the inputs can be single ended. A differential amplifier (AMP) is used to reject input common mode, and provide a single ended signal referenced to ISOGND. Using an amplifier with a high common mode rejection allows for large common mode difference between the different inputs and with respect to ISOGND. Analog inputs are usually designed to be software configurable as voltage inputs (to receive 0 to 5 V, -5 V to +5 V and -10 V to +10 V inputs) or current input (0-20 mA). For current input mode a burden resistor R_B is switched between the positive and negative terminals of the analog input.

Figure 3-1 Signal and Power Isolation in Group Isolated Analog Input Module.

A multiplexer (MUX) is used to periodically multiplex the different channel inputs to the ADC. The ADC interfaces with the system controller (MCU, FPGA or ASIC) using a single isolated SPI interface. The main advantage of group isolation is that it reduces the ADC and signal and power isolation costs, and overall module size. Additional isolation channels are needed to control the MUX and the burden resistor switches. However, the cost still works out cheaper than having to use a dedicated ADC and isolator for every analog input channel.

A single isolated power supply, for example a push-pull converter, can provide power to all the circuits on the isolated side. Depending on the implementation +15-V and -15-V supplies may be needed in addition to a 5-V supply.