Electronics > Metrology

Finding and mitigating EMI in low ppm volt measurements

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lars:
As I follow the threads with calibration clubs – round robins around the world and also the KX and MX boards I see that problems with glitches and unintended offsets are evident. Probably a lot of this can be traced back to EMI in one way or another. Some of the problems are due to other environment problems and some may be due to oscillations due to loading. So a lot of reasons you can´t trust your result. But if we go back to EMI, how can I find it? Glitches are easy to see but an offset (or bias) due to EMI is more difficult.

I have tried to use some simple tests to see if I have an unintended bias.
- Clamp-on ferrite beads on in and output wires. By adding and removing I have tried to see a difference. Probably works best for high frequency EMI
- Adding a parallel capacitor on the output. Maybe mostly helps start an oscillation??
- Adding a series resistor of 1-10k on the output (needs a high impedance DMM). Probably helps if your DMM loads your (inferior) design so your reference oscillates?
- Moving in and output wires including separating the plus and minus on the output. Good to detect 50/60Hz??
- Battery power versus mains power. Breaks loops that can pick up whatever frequencies.
Of course, the effect very much depends on the frequency of the EMI. Is it 50/60Hz or GHz?

I really hope to get more tips and maybe examples.

Lars

Andreas:
Hello Lars,

measures to reduce EMI:
- Ferrites or inductors help only for a relative narrow frequency band.
  So often you do not catch the frequency of interest.

- Adding a capacitor to the output helps usually for a wider frequency range
   But your cirquit has to be designed for capacitive loads.
   (see isolating cirquits for OP-Amps in the application notes).

- A 1-10k series resistor is usually a better idea than a inductor.
  At very high frequencies the ~0.25 pF parasitic capacity dominates.
  In DMMs you often see ladders of serial connected resistors (not only to improve max. working voltage)

- of course a Pi-Filter either with inductors or resistors and (different) capacitor values can be used to create a wideband filter.

Some methods to detect EMI:
- if your cirquit is "hand sensitive" (use your "RF-thumb" on different parts of the cirquit like battery, connection lines etc.)
- you get different results depending on how your measurement cord are placed
- you have changes in reading when you put all your gear on a metal sheet (oops where is that sidewall from my PC)
- track peak-peak noise of your readings. With a mobile phone in 2 m distance of my measurement setup the noise increased to 30uVpp.
  The averaged value changed only by 1uV but the peak-peak noise gave a indicator to discard this measurement.
- To detect the most sensitive pins in your cirquit it may also help to use a steel needle to contact every IC-pin with your "RF-thumb"
   while reading the output.
  (you may want to repeat the experiment under different conditions like battery power / mains power).

with best regards

Andreas

dacman:
Some places use a screen room.  Coaxial cable or (shielded) twisted pair can be used to reduce EMI.  (Twisted pair works because if the same signal is induced into both conductors, it will cancel out due to common mode, in theory.)  Something that may help is to turn off nearby high current devices.  I've seen overhead lighting (non-shielded florescent) and computer monitors (CRTs) cause EMI interference.

floobydust:
EMI is difficult to mitigate and blanket rules "put this there" "don't put that there" doesn't always guarantee quiet.

Mains is very noisy and EMI filters there can make it worse as they all (Y-caps) assume you have a good low impedance ground. This is not the case after many metres of 14/3 electrical cable. I use medical-grade mains EMI filters as they contain no Y-caps. One attenuates a good portion of mains garbage, leaving only the local instrument-generated EMI.

The quietest room I've used (for uV neural preamps) was all steel shielded, DC lighting, electrically isolated from HVAC ducts, power through with isolation transformer and dedicated (lifted neutral) earth-ground. Very quiet but turning on a PC in the room kind of defeated the purpose. A PC makes a fair bit of EMI. CCFL LCD monitors are terrible.

Very tough to track down blips and shifts:

Elevators have VFD's and can make a lot EMI when moving, as the long VFD cables run to the top floor where the motor is located. Took me a year to find that one.
Wi-fi routers, cell phones, tablets, any RF source- these are all polluters.

I connect an audio amp and listen with headphones to the noise. Any periodic stuff, demodulated beeps, clicks, hum - you can hear. With a scope trace it's harder to really know what just happened to cause an offset shift.
My main tool is a junker 900MHz cordless phone, which seems to get into every circuit/cable imaginable. I use that for a quick susceptibility test.
I also take a pocket AM radio and scan a room to find mid-band sources of EMI.

Echo88:
Thanks floobydust for the suggestions. Could you tell us which medical grade EMI-filter u use? Ive also thought about installing LED-DC-lighting, instead of my normal IKEA-LED-lamps. Maybe a 19"-rack which houses the measurement/voltage-reference-stuff would attentuate a good amount of EMI?

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