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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 

Figure 4 shows the effect of a 6Hz, 2-pole analogue filter, which completely eliminates the spikes. The resulting waveform is 0.4µVrms of noise, which is the internal system noise of the data acquisition system itself when limited to 6Hz of bandwidth.

Many data acquisition systems offer software integration (sometimes referred to as "averaging"), which involves reading the signal multiple times with an A/D converter and calculating an average for each point. For example, if one reads the signal every 6µSec for 16 mSec, then the measured value will be the result of averaging 2600 numbers. If your A/D converter produces 16bit (±32K) integers and you sum 2600 of these to build an average, then your summation will be an integer between -82 M and +82 M, which is 27bits (2600*32 K = 82 M). In other words, you increase the resolution of your 16bit A/D converter when you integrate. This does not necessarily mean your accuracy increases to 27 bits due to multiple sources of error where each is typically in the 10bit to 18bit range.

Figure 4: A 6Hz low-pass filter eliminates spikes from a DC signal.

Figure 5 shows the effect of integrating for 1 mSec (160 A/D values averaged to make each point). Some of the 1 msec integration bins include a spike from our 200Hz square wave, and some don't; therefore we still see some variation, yet at a reduced level.

Figure 5: Integrating a signal can smooth it, reducing the effects of noise.

Note that upward spikes cancel downward spikes when integrating, provided there is an equal number in each integration bin (i.e. N upward cancels N downward). The upward and downward do alternate because they relate to on/off switching. If, however, you get N+1 upward and N downward in one integration bin, then you will get an offset from that last spike not having a partner to cancel with. If one spike is small relative to your other A/D points, the effect is negligible (e.g. if you average 2600 points and one of them is off by 100-fold, the effect of that one point is 100/2600 = 1/26th); however, if you have fewer good points or your bad point is large, then your integration is less effective.

Figure 6 shows the effect of integrating for 16.666 mSec (2600 A/D values per point). This reduces our system noise to 0.2µVrms. Integrating for one power-line cycle (i.e. 1/60th of second = 0.016666 seconds) significantly reduces power-line noise due to the positive part of the power-line sine wave being cancelled out by the negative part.

Figure 6: Integration for 16.6666 mSec reduced noise on a signal.

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