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Minimising errors in low-voltage measurements

Posted: 07 Aug 2015     Print Version  Bookmark and Share

Keywords:data-acquisition systems  sensors  EMI  RFI  AC voltage 

On/Off switching RFI: When an offending signal in the vicinity of a sensor wire makes a low-to-high transition, an upward spike couples into the wire, and when it makes a high-to-low transition, a downward spike couples (or vice-versa if RFI flux is in opposite direction). This is why we sometimes see spikes on a digitized waveform—they relate to an offending digital signal or device turning on or off.

Sinewave RFI: Alternatively, a sinewave can travel through air and couple another sinewave of the same frequency onto one's signal. AM radio is approximately 1MHz and FM radio is approximately 100MHz, and both are notorious for entering the laboratory or factory.

How to detect RFI
Set up your data-acquisition system to digitise from one channel as fast as possible with all analogue and digital low-pass filters off and integration (averaging) off. Then view the resulting wave at different horizontal scales (e.g. 100µSec to 50 mSec per full screen). Do this even if your ultimate experiment is to digitise multiple channels at a different sample rate with integration/filtering on. You might feel compelled to turn on filtering to make your signal look good. Yet for now, resist this temptation, and focus on learning more about your signal. The trick to understanding measurement error is to let go of your ultimate goal for a moment, and do some simple experiments. Figure 2 shows 600µV spikes from a 200Hz square wave where we digitise 8 ksamples at 166 ksamples/sec from our 600 Ω load cell.

Figure 2: High-frequency components from a square wave can couple into your signals, creating unwanted interference.

Find the source
While repeatedly digitizing oscilloscope traces, turn devices in the vicinity on and off (e.g. machine, pump, power supply), and view the effect on your digitized waveform. If you turn off a nearby power supply, and see spikes disappear, then that power supply is coupling into your sensor.

Is the offending signal traveling through air and coupling into your sensor cable, or does it travel through your ground wire? Try moving your sensor cable and look at the effect on the digitized wave. Does the position of the cable effect the plot? If so, RFI in the air is passing through a loop of wire (your cable) with a different physical geometry (different flux). Change in the radiated field due to moving cable is the telltale sign of through-the-air RFI. Added cable shielding might help, in addition to several other techniques discussed below.

Is your cable/sensor ground attached to external metal (e.g. device under test)? If so, physically disconnect and look at the effect on your signal. If your signal changes, then you know current is flowing along your ground wire due to an AC signal between your data-acquisition ground and device under test ground. This is called a "ground loop" and is often fixed by electrically isolating the sensor. The AC voltage difference between grounds is often caused by changing power, which involves changing current on the ground return path and its associated voltage drop on that ground wire. A typical difference between grounds is 15 mVac on top of 50 mVdc. To measure this with a sensitive data acquisition system, attach IN+ to ground #1, attach IN- to ground #2, digitise one channel as fast as possible and view 100µSec to 25 mSec per full screen.

Will Differential Amplifier Common Mode Rejection Save Me? Data acquisition systems have differential inputs that measure the voltage difference between two inputs. All differential amplifiers have a specification for how much common signal on both inputs is rejected. A typical specification is 80 dB rejection at 60Hz. This means that 1/10000th of 60Hz on both pins is seen as a differential signal. For example, connect IN+ to IN- with a bare wire, apply 60Hz, 1 Vrms between IN+ and GND, digitize, and you will see 60Hz 100µVrms between IN+ and IN-. The dirty little secret of data acquisition is this rejection gets worse by 20 dB per decade, which means you get 1/1000 rejection at 600Hz, 1/100th at 6kHz, 1/10th at 60kHz and nothing beyond. Digital switching (e.g. spikes) often involve frequencies in excess of 60kHz. Therefore, in many cases, amplifier common-mode rejection will not save you, especially with digital switching RFI.

Will an analogue low-pass filter save me?

Will an analogue low-pass filter save me? Figure 3 shows the effect of a 4kHz-2pole analogue filter, which caused our 600µV spikes to decrease in amplitude to 15µV. We increased the plotted vertical resolution from 150µV/div to 12µV/div to better see the smaller spikes.

Figure 3: Low-pass filters can reduce high-frequency noise on your signals, provided the filter doesn't remove wanted frequencies.

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