QA350 gets True RMS

The QA350 thus far has been a high-resolution DC volt meter, with a bandwidth of 1.1 Hz (slow sampling) or 441 Hz (fast sampling). Release 1.70 changes that, and adds True RMS. The ADS1256 ADC used in the QA350 is a very low noise ADC, delivering nearly 24 bits of resolution. While that might seem a bit tame these days where everything seems to be 24 bits, keep in mind your 24-bit music player/phone will actually only deliver 18 bits of resolution on a good day. The other bits are lost to noise. Yes, even though it says 24-bits.

The ADS1256 ADC used in the QA350 has a range of sampling rates. For the most accurate readings, you want to rely on the slower sampling rates. At the most detailed resolution, the QA350 delivers a new sample every 400 mS. A few releases back we added in a 1 kSps that can log directly to disk. That higher sampling rate comes at a loss of precision.

This release builds on that and adds in a 15 Ksps sampling rate, and then the QA350 hardware performs the RMS math on the measurements directly. This roughly works as:

1. Grab 24-bit samples at 15 Ksps rate. 

2. For each sample, square it (64-bit int math) and add it to a 64-bit accumulator

3. After accumulating 16384 samples (just under 1 second), compute the mean by dividing the accumulator by 16384 and take the square root of the result. Again, all 64-bit math.

The app on the PC initiates the RMS conversion, and then polls every few hundred milliseconds to see if it's done. When it is, it corrects the result based on the calibration data, displays it, and does it all again.The user sees an update about once per second.

The response of the measurement is shown below:

The ADS1256 uses a digital filter to trade off between sample rate and noise. Of course, as you open the bandwidth, the noise increases. The filter in the ADS1256 is a 5th-order sinc filter. From the above graph, keep in mind on the low-end you are bumping up against the size of the sample buffer. If you want to measure the True RMS of a signal with a very, very low frequency component, you will need to use averaging windows larger than the default 32K used on this release of the QA350. If you want to measure signals with higher frequency components, you need to be aware of the response from the filter. 

Roughly, in this release, understand that the default window isn't long enough to reliably measure True RMS below 10 Hz or so. In that region, you'll be seeing measurements fluctuating around +/- 20 mV. And as your signal of interest exceeds 1-2 KHz, your error will begin to measure low due to the filter response shown above. 

There's a chance to get the sample rate up to 30 kSps with a little more tuning in a future release. An option could also be added to allow for longer integration intervals to reduce measurement uncertainty on the low end.

Note that if you upgrade your QA350 to the new release, you need to change the firmware. After installing the new release, you should see a message that new firmware is required. Just follow the instructions. If you later decide to go back to an old version, you'll need to downgrade your firmware. The firmware that ships with each release is the version that is needed for that release.


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