Reduce electrical noise in rotary incremental encoders with termination resistors

Cropped-termination-resisto

Since most electronics are set up to have high input impedance, in some situations the addition of a loading or “termination” resistor can help to reduce interference from electrical noise.

This can be of particular help when implementing an incremental encoder where there is a long cable run.  If the input impedance of the receiving device (often a controller or drive) is high, the current through the cable is very small.  The addition of a termination resistor where the encoder is connected to the controller/drive increases the current flowing through the cable.  This increased signal current is less susceptible to interference of any electrical noise that may be coupled into the cable.

The termination resistor is placed in parallel with the output in a single ended set up; this connection is from the encoder channel to signal ground. In a differential setup this connection is between the channel and it’s complement (A and A-, B and B-, etc.).

Values of termination resistors depend on the current capability ability of the encoder. Our standard incremental encoders can deliver up to 20 mA per channel, which at 5VDC means a maximum total termination load of 100 ohms.

When troubleshooting a noise burdened application we tell our customers to start with a value of around 1K ohm and go from there. Many times 1K is more than enough resistance to solve a noise interference issue.

I would also like to point out that this type of termination loading should be done on each channel as each channel is a separate circuit.

It is also good to keep in mind that low value termination resistors may attenuate the amplitude of the incremental encoder output signals and that too low of a value could damage an incremental encoder.

The 100 Ohm value spoke of above works for Quantum Devices rotary incremental encoders. Other  encoders may not have the ability to deliver current into such heavy loads.

Below is a pictorial example of how to add termination resistors to an incremental encoder that is set up  Single ended  (TTL).

SE-Termination

Below is an example of termination resistors wired into an Incremental Encoder that is set up  differentially (RS-422)

Diff-Termination

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About Quantum Devices Inc.
Quantum Devices, Inc. (QDI) Barneveld, WI, was established as a Wisconsin corporation in October 1989, as an outgrowth of activities in the general area of optoelectronics. The main goal of QDI is to provide customers with a complete source for all stages of product design and development, starting from concept to a marketable commodity. These products include application specific Silicon Photodiodes, Optical Encoders and Light Emitting Diodes (LED) for commercial, industrial and medical applications. QDI's photodiodes and LED's are used in instrumentation for photosynthesis and photobiological research, biomedical and medical instrumentation for measuring blood sugar levels (glucometer), oxyhemoglobin and pulse rate (pulse oxymeter). One of the most dramatic products developed at QDI was the application of LED lighting systems for use as the light source for Photodynamic Therapy (PDT). Sales of QDI products include both domestic and international markets. Quality and reliability are very important concepts in maintaining our company wide commitment to overall product performance. Quality simply means continuous process improvement. We are committed to continually increasing our product excellence through increased quality and reliability. Quantum Devices, Inc. is ISO 9001 certified and the ISO Standard will provide the guidance for the vital function of maintaining our commitment to constantly improve our product quality. Quantum Devices is now proud to introduce it's new line of rotary encoders. Incorporating QDI's patented sensor technology, our QPhase™ Family of Encoders feature high resolution, increased frequency response and superior reliability over temperature (0 to 120 degrees C). QDI incremental and absolute encoders set a new standard and are backed by a 2 year factory warranty. We invite you to request an evaluation unit, additional information or price quotations. Mission Statement Quantum Devices is dedicated to the principle that light provides the power for all life on earth. We believe the quality, delivery and control of light is essential to the wellness of man and his advancement into the future.

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