Programmable logic controllers (PLCs) process data from sensors or transmitters by transmitting through an isolator to a microcontroller (MCU). Since the field-side voltages are typically 24 V, basic isolation usually suffices for breaking the ground loops. Working voltages of 100 V_RMS to 500 V_RMS and isolation voltages of 2.5 kV_RMS are good enough for most low-voltage PLC applications. Packages with small creepage and clearance distances are preferable in these space-constrained applications.

The dual-channel 24-V to 60-V ISO1212 digital input receiver is designed to help simplify and improve PLC digital input designs by combining accurate current limiting, protection circuitry and isolation in one package to help reduce component count and improve performance. It also increases reliability by integrating a long-lifetime SiO₂ isolation barrier and reducing system board temperatures.

The Sub 1-W, 16-Channel, Isolated Digital Input Module Reference Design is designed to withstand electrostatic discharge, electrical fast transient and surge events according to IEC 6100-4-2, while using less than 1-W input power combined. Each channel can withstand input voltages as high as ±60 V.

In the PLC digital input module shown in Figure 13, the serializer and field side of the isolator need power from a 5-V or 3.3-V supply. An isolated power supply or digital isolator with an integrated power supply providing the required bias from the MCU side eliminates the need for a separate power supply on the field side. PLC analog input modules process signals through an analog input front end, through a digital isolator such as the ISO7741, and then into the MCU.

Another challenge in factory automation involves the isolation of field transmitters. Since the entire system is powered from 4 mA to 20 mA, the zero-scale value sets the system’s maximum budget, usually less than 3.3 mA.

Historical isolation solutions can consume anywhere from 500 µA up to 1 mA per channel, forcing designers to either minimize the number of communication lines brought across the isolation barrier or slow data transfer speeds. The ISO7041 family integrates two to four ultra-low-power digital isolator channels in small packages while providing power consumption as low as 3.5 μA per channel, data rates up to 4 Mbps, a temperature range from –55°C to 125°C, and the stability and reliability advantages of TI’s SiO₂ insulation dielectric. The Isolated Power and Data Interface for Low-Power Applications Reference Design shows the ISO7041 in a 4-mA to 20-mA transmitter application.