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DAQ Wiring

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!

Alex Colbourn
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  • Close to a high voltage power line is a good place to find strong magnetic fields that might interfere with what ever it is you really want to measure. – JRE Oct 08 '18 at 18:04
  • The magnetic fields are actually what we are measuring which unfortunately puts us in the worst case scenario for noise! – Alex Colbourn Oct 08 '18 at 18:26
  • The easiest way to deal with noise in connecting wires is... get rid of the wires. i.e. make them as short as possible. I did not see any distance requirement in your question. To me it means you can place sensor and DAQ really close to each other. Couple centimeters **shielded** twisted pairs with shields connected to DAQ shield should not have much noise, IMHO. – Maple Oct 08 '18 at 18:30
  • Also, it would be nice if you define "extreme noise". What frequency? It is a white noise or just a mains hum? And if it is magnetic field you are measuring then any magnetic field fluctuations are not a noise, are they? – Maple Oct 08 '18 at 18:38
  • @Maple Alex mentioned power "high voltage power lines". The largest noise in that case is 50Hz or 60Hz hum. The lousy news is that there may be both E-field coupling (high voltage) and H-field coupling. There may be powerline communication signals too (100kHz to 200kHz in the US). Alex, please confirm or challenge. – Nick Alexeev Oct 08 '18 at 19:37
  • @NickAlexeev Yes, that was what I meant. If the "noise" is from those power lines, and the device purpose is to measure the radiation from those same voltage lines, then it is not a noise at all, but measured parameter. And at 50/60 Hz it is easily measured/filtered even with slow acquisition rates – Maple Oct 08 '18 at 19:43
  • @Maple Yup. Twisted pair and instrumentation amplifier are effective against 60Hz hum. Hopefully, there are InAmps in the DAQ front end. The common mode needs to stay within the rails of the rails of the InAmp (minus the margins). Shielding helps with that. – Nick Alexeev Oct 08 '18 at 19:50
  • Keeping wiring distance is a good suggestion but unfortunately in our case we will eventually be measuring both the magnetic and electric fields and need to keep our DAQ a few feet away so we don't pickup it's electrical noise. We are trying to strike a balance between keeping cable length short to pickup less cable noise and distance to DAQ far enough to not sense it's fields. – Alex Colbourn Oct 08 '18 at 20:32
  • Also, you are correct that the frequencies are 50/60Hz and the harmonics up to around 300Hz are what we are interested in and will be the largest fields present. Our electric/magnetic field sensors are extremely sensitive so in addition we are expecting white noise from many other unknown nearby sources. Ok, so sticking with the shielded twisted pair wiring and building a faraday cage will likely be sufficient as long as our DAQ is designed well? Any idea if its ok for the DAQ chassis to be grounded/touching the Faraday cage? I'm worried about accidentally making an antennae instead of cage – Alex Colbourn Oct 08 '18 at 20:41

1 Answers1

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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.

enter image description here
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.

enter image description here

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

Voltage Spike
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