Electronic controllers are implemented for the error detection function.
The detection of the error signal is accomplished in electronic controllers by taking the difference between the voltages. The one voltage generated will be the process signal current passed through a resistor. The second voltage represents the setpoint, usually generated by a voltage divider using constant voltage as the source.
We assume a two-wire system is in use so that the current drawn from the floating power supply is the 4-20mA signal current. The signal current is used to produced a voltage IR across the resistor R. This is placed series opposition to a voltage Vsp tapped from variable resistance Rsp connected to constant positive source V0.
The result is an error voltage Ve = Vsp - IR. This is then used in the process controller to calculate controller output.
An error detector also can be made from a differential amplifier. such system can be used only if the current from the transducer is referred to ground. The sensor signals current pass to ground through Rl, providing signal voltage, Vm= IRL. The differential amplifier then subtracts this from the setpoint voltage.