Isolating the source of electrical noise on analog inputs.
Where electrical noise can get in:
- Directly into the RMC input circuit (unlikely, but significant noise on the Cmn pin could couple onto the signal inside the RMC).
- On the wires. Likely suspect if shielded/twisted pair are not used or used incorrectly.
- From the transducer itself. Likely suspect. Noise from the transducer can have several sources:
- Wires
- Power supply
- Radiated through the housing (least likely, but possible if the transducer has inadequate shielding or is in an extremely noisy environment).
Process of elimination for finding the source of noise:
-
Test the RMC Inputs:
Disconnect the transducer from the RMC and apply a clean input. For voltage inputs simply short the + and – inputs together. For 4-20 mA inputs apply 1.5V battery across the RMC inputs. Noise will probably go away. If it doesn’t, then the noise could be coming from a Cmn wire, so disconnect all of those. If there is still noise, there is an issue with the RMC inputs. -
Test the transducer cable:
Reconnect the transducer cable and disconnect the cable at the transducer end if possible. (If the cable is integral to the transducer, then it cannot be independently evaluated.) Either short or apply a battery to the + and – input wires out by the transducer. If there is noise, then the wiring needs to be improved. Verify the correct wires are used (don’t mix pairs). Use better quality cable. Put the cable in metal conduit. Move cable away from noise sources. -
Test the transducer:
Reconnect the cable to the transducer. Check if noise is getting into the transducer from the power supply by using a separate power supply that does not power anything else. If possible, move the transducer to away from suspected noise sources. If possible, add shielding around the transducer (aluminum foil makes for great temporary shielding for testing.)
Note that sometimes what appears to be noise is actually a mechanical issue such as a loose coupling or a loose sensor. Sometimes the transducer itself or some part of it could be mounted in such a way that it can vibrate.