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Detect errors in low-voltage measurements

06 Aug 2015  | Glenn Weinreb

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Software integration often reduces the maximum sample rate because the A/D needs to focus on reading one channel multiple times to get one point. In the above illustration for example, we integrate for 16 mSec yet we also reduce the maximum sample rate from 166 ksamples/sec aggregate (all channels) to 60 samples/Sec.

If too much filtering rejects the signal of interest; consider better shielding, electrically isolating the sensor from the device under test, shorter sensor cable, and/or identify source of noise and reduce it.

AC mains coupling
A typical problem with low-level measurements is the coupling of 60Hz (or 50Hz) power into the sensor signal. To detect this form of interference, digitise one channel as fast as possible with analogue filters off, digital filters off, and integration off. Look for a sine wave that is 16 mSec long (or 20 ms with 50Hz power) while plotting 20 mSec to 50 mSec per full screen. Below is an example where we digitise 5k points at 166 ksamples/sec. Notice the slight 60Hz sine wave. If our cable was longer, this problem would be more severe (figure 7).

Figure 7: AC mains frequencies can couple onto your signals.

Figure 8 shows the effect of a 6Hz, 2-pole analogue filter. This reduces 60Hz noise by a factor of 100. Two poles reject frequencies beyond the cut-off frequency by 40 dB (100-fold) per decade (60Hz / 6Hz = 1 decade). We end up with 0.4µVrms of system noise below 6Hz, which is mostly from the A/D converter.

Figure 8: Applying a 6Hz, 2-pole low-pass filter will reduce AC mains interference from a signal.

Alternatively, we turn off analogue filters and instead integrate for 16.666 mSec to reduce system noise to 0.2µVrms, as figure 9 shows.

Figure 9: Integration reduces system noise as well as reducing noise for external sources.

If your sensor signals are fast (i.e. > 60Hz) and you don't want to filter out your signal of interest, then consider a digital band-stop filter. In Figure 10, we still see 1.2µVrms of data acquisition system internal system noise, yet the 60Hz sine is gone.

Figure 10: A band-stop filter can reduce AC mains noise, but will leave other frequencies intact.

Internal system noise
All data acquisition systems have internal noise that is added to the digitized signal. This is due to a property of amplifiers—they all add a little noise to their received signal. To see this, ground the input (wire from IN+ to IN-, and wire from IN+ to measurement system GND), digitise one channel as fast as possible with filter/integration off, and view 100µSec to 25 mSec per full screen. Figure 10 is an example of what you might see.

This noise decreases with decreased bandwidth. For example, we measured the instruNet i423 system noise on the ±10 mV measurement range at 6µVrms with 125kHz full bandwidth, 1.2µVrms with 4kHz bandwidth and 0.4µVrms with 6Hz bandwidth. So, turning on analogue filters can help.

When you integrate (average multiple A/D values), noise decreases with the square root of the number of values that you average. For example, we measured the i423 digitizer noise at 6µVrms with no integration and 0.2µVrms with 16 mSec integration (2600 A/D values). In this example, we see a 30-fold reduction in noise (6 /0.2 = 30), which is similar to SQRT(2600) = 51.

Thermal drift and sensor instability
If your sensor is supposedly not changing and you see it change slightly over several minutes or several hours, then the question becomes, "What is the cause and how do we measure it?" The cause is typically due to one or more of the following: data-acquisition system thermal drift, sensor thermal drift, or sensor instability.

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