Protection: How much is enough for an analog output?

Analog outputs drive motors and actuators in factories and plants. They are the final control step in process logic controllers (PLC). The output amplifiers typically drive 0 to 10V and 4 to 20mA circuits. They need protection from high-voltage motor faults, electrostatic discharge (ESD), electromagnetic interference (EMI), electromagnetic susceptibility (EMS), and radio frequency interference (RFI). Many circuits are suggested to meet the application requirements.