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Using Single And Differential Signals


Objectives

This article explains the basics of single-ended and differential signals, how to wire them and the effects of noise on both signaling methods.

Introduction
As the name implies; an amplifier amplifies an input signal that is provided as an output signal. The output signal lies between +Vsupply and -Vsupply. Please refer to the illustration below for generic schematic overview of an amplifier. 
 

 
An amplifier has two variants of receiving an input signal; single-ended and differential input.
 
Single Ended Input
 
A single-ended input, as the name implies, uses a single input signal. The signal is referenced by ground (-). A single-ended input is more sensitive for EMI (Electromagnetic Interference) than his counterpart; the differential input. Another difference between a single-ended and differential is the cable length. The cable length of a single-ended input is considerable shorter compared to a differential input (up to 100 meter) due to the sensitivity of EMI. Below is the schematic overview of a single-ended input.
 
 

 
 
Differential Input
 
The differential input is slightly different compared to the single-ended input. The differential amplifier uses two input signals instead of a single input. Secondly, the signal is not referenced with ground although it is possible to add ground however this is not common. The advantage by adding ground would be to improve the noise immunity even further.
Instead of a single input the differential input uses two inputs. The differential amplifier amplifies the difference between the two inputs and provides the amplified signal as an output. Below is the schematic overview of a differential input.
 
 

 
 
Content
 
 
 
Wiring of single and differential type signals varies for different applications, but in general the following guidelines apply.
  
 
Single-ended sensor to a single-ended input

 
Single-ended sensor to a differential input

 
Differential sensor to a differential input

 
Differential sensor to a single-ended input
 
NOT POSSIBLE. You would need to convert the differential signal first. 
 
 
NX-AD2203 single-ended current input unit and ZX Series laser measurement sensor.

 

NX-AD2204 differential current input unit and ZX-Series laser measurement sensor.

 

NX-AD2203 single-ended current input unit and ES1C-A40 infrared thermo sensor.
 
NOT POSSIBLE. You would need to convert the differential signal first.
  
NX-AD2204 differential current input unit and ES1C-A40 infrared thermo sensor.

 
 
In many applications where a signal must be transmitted from one device to another over some distance, external electrical interference (noise) must be considered. Noise can be introduced from a variety of sources that generate electrostatic, electromagnetic or radio frequency energy. Common sources of noise producing devices are:
  • - Large electric motors
  • - Fluorescent lighting
  • - Variable frequency drives
  • - Lightning or other high-voltage surges
  • - Welding equipment
Differential signals provide superior common-mode noise rejection in most cases. The external noise tends to affect both signal wires equally, therefore the difference (potential) between them is normally not disturbed. 
A single-ended signal is simpler and less expensive to implement, but also more vulnerable to external noise. There are two sources of erroneous signals with this type of scheme; differences in ground voltage level between transmitting - receiving circuits and the signal wire acts as an antenna to pick up induced noise. Additionally, when there are several signal wires sharing the same "return" ground, this can sometimes cause interference (crosstalk) between the signals.  
Summary
Unexpected behavior of a control system can be difficult to rectify when the cause is external noise. Avoiding and mitigating noise in a control circuit ensures stable and consistent operation for the life of the machine. This article aims to provide fundamental details of single and differential signals to aid in the proper selection and design of control systems.




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Created 2014-02-27
Modified 2017-12-06
Views 126

 

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