Single Ended vs Differential Inputs

One of the most common questions asked is the difference between single-ended and differential signals inputs, and what applications they should be considered in.

It is important to know the difference between single-ended and differential inputs as it can greatly affect the quality of your data

Differential signaling is a method for electrically transmitting information using two complementary signals. The technique sends the same electrical signal as a differential pair of signals, each in its own conductor. The pair of conductors can be wires (typically twisted together) or traces on a circuit board. The receiving circuit responds to the electrical difference between the two signals, rather than the difference between a single wire and ground. The opposite technique is called single-ended signaling.

A single-ended input measures the voltage difference between a wire and the ground. This difference is then amplified to provide the output.

Single-ended inputs can suffer from noise as the wire that carries the signal picks up the electrical background noise. The signal on a single-ended input could also be subject to ground loops.

For example, a transducer wire that is near a power cable (carrying 110V at 60Hz) could be subject to the EMF (Electric and Magnetic Field) noise from that power cable.

Here an accelerometer wire is connected to the Device 1. A power wire is lying on top of the transducer wire.

The graph below shows what is happening on the wire. On the left, we have the wire (grey box) and the EMF coming in from the side (red arrow). Inside the wire, we see the desired signal on the wire (purple) and the EMF (red, 60Hz) on the wire. On the right, we see the output of the wire.


Left: The grey box represents the wire connecting the transducer to the PLC front end. The purple is the desired signal, the red is the EMF. 
Right: The PLC output is composed of the addition of the desired signal and the noise. 
Bottom Right: An FFT of the output: it is composed of the signal and noise (60Hz) components. So, single-ended inputs are susceptible to noise.

Remember, that noise is relative to the signal level. When using transducers that have a high output voltage (accelerometers) the EMF noise may not impact the output. However, if the transducer has a relatively low voltage output (strain gauges), then the noise will have a great impact on the output.

A differential input has no reference to the ground. Rather, the differential input carries the signal on two wires, a (+) signal wire, and a (-) signal wire. Look at the figure below. The (+) and (-) signal wires are carried along the cable and the EMF is introduced along the side of the cable.


 

The grey box on the left represents the wire with the (+) and (-) signal wires. The purple waves on the left are the desired signals on the wires. The red waves are EMF noise. The red arrow represents the EMF from a power cord. The purple wave on the right is the result after subtracting the signals on the two wires from one another.

The output value is the difference between the signals on the two wires (common mode rejection). Notice the output eliminates the noise and the desired signal doubles. This is because the signal on the (+) wire will be the opposite of the signal on the (-) wire (because the signal on the (-) wire is being multiplied by -1). Therefore, by subtracting the (-) signal wire from the (+) signal wire, the desired signal doubles. The EMF induced noise is the same on each wire (because it comes in from the side). By taking the difference between the two signal wires, much of the EMF noise is rejected.

When to use single-ended or Differential signals?

Differential inputs provide a more stable reading when EMI or RFI is present, and therefore, it is recommended to use them whenever noise is generally a problem. This is especially true when measuring thermocouple, strain gage, and bridge type pressure sensor inputs since they produce very small signals that are very susceptible to noise. Single-ended inputs are lower in cost and provide twice the number of inputs for the same size wiring connector since they require only one analog HIGH (+) input per channel and one LLGND (-) shared by all inputs. Differential signals require signal HIGH and LOW inputs for each channel and one common shared LLGND. Single-ended inputs save connector space, cost, and are easier to install.



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