Help with calibrating my 24bit ADC - Page 1
Forum section: electronics / general

author: drakejest
author_rank: Regular Contributor
timestamp: March 27, 2021, 08:58:30 pm
content: Hello i just have finished creating my very own 24bit sigma delta differential adc and i would like to calibrate it. When i tried measuring a voltage from a battery, the voltage difference i got from the reading and using my multimeter is 0.1v. My multimeter is a midrange sanwa brand so i dont think it would be off by that much. And the adc reading is stable, thus making me conclude the internal voltage reference might off by a bit from what the data sheet said.So i scheduled to borrow equipment from the university lab close to me. Which would be better to use to calibrate ? A bench power supply or a signal generator ? From what i remember both of them were capable of voltage adjustment upto the fourth decimal place.How to do the actual calibration ? Is it really just as straight forward as placing a source voltage and just subtract the reading from the actual to get the offset ? Although im planning to keep the internal gain to 1 which that might work but if i set the gain other than 1, will it still work?

author: Kleinstein
author_rank: Super Contributor
timestamp: March 27, 2021, 09:24:23 pm
content: A lab supply or function generator is usually a bit unstable for an easy calibration. A carefully handled battery may be the better option. Testing a ADC involves usually more than just measure the scale factor at 1 or 2 points. Usually one would first do some tests, to see of the ADC is working reasonable before caring about an accurate scale factor. Usually references dirft a bit more in the early days. It gets more accurate after operation time. The initial drift may still be interesting, but could be tricky with a reference only to borrow.Depending on the speed of the ADC there are different methods to test linearity. If relatively fast, one could try a 2 tone test: apply the summ of 2 sine waves and look for the mixing products.The range and reference also determine which accurcy is expected / needed.

author: Anders Petersson
author_rank: Regular Contributor
timestamp: March 29, 2021, 10:39:39 am
content: You need to do at least two measurements; reading zero voltage (by shorting the inputs) and reading a value towards the upper range limit of your ADC. Ideally, your ADC is perfectly linear. That is, all values in-between can be linearly interpolated between these two reference points. In reality, ADC's are not perfectly linear and in particular with a new design we can't be sure. Hence, the need to test the linearity by measuring at intermediate voltage levels. The more points the better. If your ADC supports multiple gain levels, you need to repeat all readings for each gain level. Also, at 24 bits resolution you will probably find quite a few bits of noise from reading to reading, so sample enough data points at each voltage level to see the peak-to-peak or standard deviation statistics.A professional evaluation would repeat this many times, looking at the temperature dependence, influence of noise, stability over time etc. For that an automated test sequence would be necessary. It can be illuminating to study the datasheet details of commercial ADCs, where they already did all this work.Once you have enough numbers, you need to apply a correction to report the actual voltage. The more tests you did, the better you will understand the limit of how accurate your correction can be.As for calibrator / reference voltmeter, you need to look at the datasheets to find the accuracy. The uncertainty will be a sum of a fraction of the reading and a fraction of the range. The uncertainty also increases with time elapsed since the last calibration. The higher accuracy, the more expensive the instrument.You can buy a cheap voltage reference on Ebay, with 4-5 different voltage levels. I wouldn't trust the cheapest ones to completely fulfill their stated accuracy, but I guess they beat basic multimeters.

author: Andreas
author_rank: Super Contributor
timestamp: March 29, 2021, 06:52:06 pm
content: Quote from: drakejest on March 27, 2021, 08:58:30 pmWhat you need is a low noise voltage source.So either a reference voltage or at best a calibrator.e.g. something like this:https://www.eevblog.com/forum/metrology/ian-johnston-pdvs2/with best regardsAndreas