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