9.2.2 Design Requirements

Analog I/O modules are used by programmable logic controllers (PLCs) to interface sensors, actuators, and other field instruments. These modules must meet stringent electrical specifications for both accuracy and robust protection. These outputs are typically current outputs based on the 4-mA to 20-mA range and derivatives or voltage outputs ranging from 0 V to 5 V, 0 V to 10 V, ±5 V, and ±10 V. Common error budgets accommodate 0.1% full-scale range total unadjusted error (% FSR TUE) at room temperature. Designs that desire stronger accuracy over temperature frequently implement calibration. Often the PLC back-plane provides access to a 12- V to 36-V analog supply from which a majority of analog supply voltages are derived.

Analog output modules are frequently multi-channel modules featuring either channel-to-channel isolation between each channel or group isolation where several channels share a common ground connection. As channel count increases it is desirable to maintain small form-factor requiring high levels of integration and reduced power dissipation in order to control heat inside of the PLC enclosure. This design example illustrates a single-channel with isolation, which could be extended for multiple channels to create a channel-to-channel isolated PLC AO module.

Therefore the design requirements are:

• Support of standard industrial automation voltage and current output spans
• Operation with standard industrial automation supply voltages from 12 V to 36 V
• Current and voltage outputs with TUE less than 0.1% at 25°C
• At minimum criteria B IEC61000-4 ESD, EFT, CI, and Surge immunity