When connecting analog current (such as 4-20 mA) signals to a PLC, data acquisition system or measuring instrument, you can often choose between single-ended or differential inputs. When using transmitters and signal conditioners you must choose between sinking or sourcing I/O. What is the difference between these and which should you use? It is important to keep three things in mind: POWER Which device is providing power to the loop? SIGNAL What is the signal path? It must be connected correctly and be the right type of signal for the circuit to operate. GROUND Where are the ground connections? Is there a potential for a ground loop? An Api signal conditioner can provide solutions to the above issues. It can power a loop, or be passive, convert incompatible signals and provide isolation to break ground loops. Single-Ended Inputs Typically used with a two-wire transmitter, one wire is connected to a power source and the other wires from each signal source are connected to the PLC or receiving device. This assumes the sensor ground and the PLC or measuring device ground have the same value. In reality, earth ground can vary in different locations. These potential differences create current paths or ground loops leading to measurement errors.The errors generally increase as distance between earth grounds increase and with the presence of other electrical equipment in the vicinity. Single ended inputs are also susceptible to radiated electrical noise, since the single wires pick up stray EMI and superimpose it on the signal. Differential Inputs Two signal wires run from each signal source to the PLC or receiving device. One goes to the + input and one to the – input. This allows the PLC or receiving device to measure each of the wires in reference to its own ground, eliminating grounding differential errors. Noise immunity is improved since the pair of wires pick up interference equally. When using differential inputs, the sensor may “float” or have no ground connection. It may be preferable to connect the negative (–) signal wire to the PLC terminal marked 0V, REF or GND.

Sink vs. Source

When connecting various current inputs and outputs it is important to keep in mind what device is powering the circuit. Inputs and outputs can either “sink” current or “source” current. A 2-wire transmitter is a passive device and thus “sinks” current. A 4-wire transmitter operates on an external power source and thus “sources” or provides power to the circuit. Sinking Input The device receiving the signal does not provide power. It acts as a resistive load It must be connected to device that sources its output signal, or a to sinking output with a loop power supply in the circuit. Sourcing Input The device receiving the signal provides power for the input signal. It must be connected to sinking output, such as a 2-wire transmitter which uses the power from the receiving device. Sinking Output The device’s output signal does not provide power. It must be connected to a device that provides power for the output signal, or a sinking input with a loop power supply in the circuit. Sourcing Output The device’s output signal powers the output circuit. It must be connected to a receiving device that provides no power and acts as a resistive load, such as a 2-wire passive transmitter Note that sinking-sourcing and sourcing-sinking pairing is always used, and never sourcing-sourcing or sinking-sinking. EXTSUP (External Supply) Option The Api EXTSUP option provides a sinking or unpowered signal conditioner current output. It is required due to the fact that PLC analog input cards can either be configured to accept differential (individual common) inputs or single-ended (one common) inputs. A PLC often has an input power supply, or one installed in the panel, as the power source for the inputs. Due to differences in ground potential between differential inputs and single-ended inputs, they cannot be intermixed on the same PLC analog input card. Doing so may cause input signal errors or possible PLC shutdown. Use of the EXTSUP option is required to provide a sinking output for the signal conditioner to be compatible with the PLC’s single ended inputs.