Preferred Wiring Methods for Optical Encoders
February 26, 2013
Following these guidelines will reduce the possibility of noise, false counts, ground loops, and other power related problems.
Keep Cable Lengths as Short as Possible
The longer the cable length the more likely it is to act as an antenna and pick up unwanted noise. Keeping the lengths short reduces the possibility of this.
Keep the Number of Terminations to a Minimum
Each time there is an interconnection made, whether it is a butt splice between two wires, or a wire runs to a terminal block, there is an increased chance of a poor connection. Reducing the number of connections in a cable/wire run reduces the chance for connection problems
Don’t Wire in Daisy Chain Fashion
This will most commonly apply to the power supply rails, but is good practice overall. Instead of wiring through a device, wire from the source (Fig 1). This reduces voltage drop across system components, and is a more reliable wiring method, as fewer items will drop out due to a wiring failure.
Do NOT Run Signal Wires Parallel to Power Wires
Avoid running lengths of signal wires, such as those coming from an encoder, along any high voltage or high current carrying wires. Noise from power wires can be coupled into the low signal cable. The deathblow is running signal wires in shielded conduit with power wires. Noise has no choice but to be coupled into the wires. Running the encoder wires outside of the conduit is preferred to grouping it with high current carrying conductors inside conduit. If it is necessary to run signal and power wires together, the preferred method is to run signal wires and power wires perpendicular to each other.
If You are Running into a High Impedance Device, Use a Termination Resistor
A value of 120 Ohms to 1K Ohms is typical for a 5 Volt application. In most instances, noise has a very low current component. This means it will not have as much voltage (which equates to signal) through a low impedance or resistance. Since the line driver in the encoder is made to supply current, any noise loaded with a low resistance will disappear, but the line driver will still be able to deliver signal. In Differential applications, this resistor should go between the desired channel and it’s complement, I.E. A and A’. For single ended applications, this resistor should go between the desired channel and ground.
Use an Encoder with Differential Outputs
Differential outputs greatly increases the ability of a system to handle noise. As opposed to one signal wire and one ground wire, differential outputs provide two signal wires with exactly opposite signals on each wire. Any noise coupled into the system is common mode, or the same on both wires. Since a differential system is set up to look at only signals with exactly opposite voltage potentials, the noise component is rejected. In a single ended system, the noise is indistinguishable from the real signal and cannot be rejected. (Fig 2.)
Make Sure the Power Supply to the Encoder is NOT Creating Noise
Low quality power supplies, can pass enough 60 Hz ripple, or switching noise to cause problems. If this is the case the use of filter capacitors at the power supply may help. The specification for QDI Encoders is 2% ripple.
Ground the Drain Wire
The drain or cable shield wire serves the function of routing any noise coupled to the cable foil shield to ground. Not using this wire means the drain will not carry the noise away. It is particularly important when using a long cable length. It is also important to keep in mind that the drain wire could unintentionally couple noise to the signal wires. Please note that the drain wire is not tied internally to the encoder body.