Hi all, I'm trying to build a portable data acquisition system to gather readings really close to high voltage powerlines. I was hoping to get some advice on how to achieve the best noise immunity. Here's my current plan:
I have a sensor with 4 analog outputs and I was thinking of running 4 shielded twisted pair wires from the sensor to the DAQ running in differential input mode. They all share the same ground so I was wondering if running 4 separate ground wires makes sense or if it's better to just run 1 ground and do everything single-ended and not use twisted pairs? My thinking was that then each pair would pick up the same noise on its ground and reading wires and therefore would cancel the noise. In addition, I will build a Faraday cage around the DAQ. Does this seem like a good plan?
Here's the datasheets for the sensor and DAQ in case:
https://aerospace.honeywell.com/en/~/media/aerospace/files/datasheet/3-axismagneticsensorhybridhmc2003_ds.pdf
http://www.ni.com/pdf/manuals/378030c_02.pdf
Thanks!
Best Answer
There are two sources of noise, capacitive and inductive.
Capacative noise comes from a voltage potential difference between two conductors. In your case the power line would be one conductor and your device would be another, and can be modeled as a capacitor. To stop capacitive coupling, a faraday cage would be the ideal thing to stop any electric field that would produce a voltage on your sensors, cables and DAQ.
The ideal situation would look like the image below example c) The other problem is also grounding as the potential difference between your sensor and DAQ will most likely be at two different potentials. So try and make the whole system like example c) with shielding around the cable, sensor and DAQ. The shielding also needs to be continuous, so use shielded wire.
Source: http://www.7ms.com/enr/online/2010/02/notebook.shtml
Inductive coupling is harder to stop, it comes from magnetic fields and cannot be completely eliminated but can be reduced. Avoid loops as a magnetic field can create a voltage potential around the loop. Twisting wires also helps eliminate loops as it cancels out the magnetic field between loops.
Source: https://www.researchgate.net/post/What_is_the_basic_idea_behind_the_twisted_pair_Why_are_the_two_wires_twisted_How_does_this_arrangement_compensate_undesirable_disturbances