Applications of analogue converters and galvanic separation

They convert electrical signals generated by sensors which take physical measure-ments such as temperature (thermocouples and PT100 resistance thermometers), frequency (proximity, contacts, photocells), current (TA, Hall sensors), resistance (potentiometers), voltage, pressure, level, etc. into standardised electrical signals, adapting them to PLC, DCS and industrial PC (control) outputs, or they convert a given analogue signal into a different one, adapting it to control inputs/outputs or allowing for long-distance signal transmission without interference by means of galvanic separation (fig. 1).

Adaptation between sensor output signal and control input signal

Long-distance signal transmission
Voltage signals can reach a max. distance of 10-20 m, beyond which they lose reliability and become highly sensitive to induced and ground-derived interference, therefore in order to transmit to distances beyond 20 m a voltage signal must be converted into a current signal and galvanically separated (fig. 2).
Current signals can surpass a transmission distance of 300 m and are less sensitive to induced interference. The long-distance transmission of a current signal requires galvanic separation.

Galvanic signal separation (signal isolation):

• isolates and electrically separates the sensor circuit from the control circuit and from the power supply circuit; each circuit therefore operates in relation to its own zero potential which, being isolated from other circuits, cannot be altered by ever-present potential differences between different ground references (fig.3)

• isolates and separates different ground potentials between power supply, control and sensors/actuators
• allows for signal transmission without errors or interference and with greater reliability
• the higher the isolation (in kV), the greater the security of the transmission in the presence of ground potentials, electromagnetic or temporary interference (light-ning, discharge, etc.). (fig 4)

Galvanic separation is necessary when:

• the distance between control and sensor/actuator is greater than 20 m
• ground or mass references are different
• ground potentials are high, or may become high in case of discharges or currents leaked to ground
• electromagnetic interference is present
• signal cables are wired in ducts with power cables (fig. 5)

Connection of analogue converters in series and in parallel

• To obtain signal redundancy or to simply duplicate it, multiple converter inputs can be connected to a single sensor.
• In case of current signals, the converter input will be connected in series (fig. 6)

• In case of voltage signals, the converter input will be connected in parallel (fig. 7