3 Group-Isolated Analog Input Modules

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.