Noise is a form of electromagnetic energy picked up by transmission conductors. It is particularly troublesome in long wires that act as antennas.
Noise is usually at very high frequencies (transmission signals) or low power voltage. For sensors with amplifiers noise can travel down the cables and cause the op amp and voltage regulator to malfunction.
Standard methods to reduce noise include:
- Ground shielding of cables
- Avoiding ground loops
- Using conduit to shield signal leads
- Moving wires away from sources of interference
- Avoiding sensors with zero volt output
- Implementing twisted pairs of wires
If cables do have to run through a very noisy environment, it is worth using current-output sensors as they are less sensitive to noise than those with voltage outputs. Many low-cost sensors provide low-voltage outputs and so are highly sensitive to noise.
Improving noise immunity
In addition, there are a number of other methods for improving noise immunity.
Place capacitors from the positive signal to the negative signal and across the power supply. Use two valves in parallel to filter different frequencies.
Filter the output by placing capacitors and resistor networks across the output, taking into consideration that this will affect the frequency response.
Finally, there are a number of sensors available with built-in filters. However, these will not help avoid noise pick-up by the associated electronics, and can at times just be an unnecessary additional cost.
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