Linearity calibration? Linearity drift? - Page 1
Forum section: electronics / general

author: jeremy
author_rank: Super Contributor
timestamp: June 02, 2022, 12:25:35 pm
content: Hi,For those of you who deal with calibration of meters from the mid range toward the metrology end of the spectrum (let's say 6.5+ digits), I have a few questions I was wondering if someone could help me with related to linearity (not offset).- Do meters have any appreciable drift (due to anything really) in DNL or INL characteristics ?- If there is some drift, is DNL typically checked during recalibration?- What instruments does one use to measure DNL and INL (say you have a brand new multimeter off the production line and you need to check it)? INL I suppose is just standard references, but DNL seems more tricky. Only thing I can think of is the Fluke 5080A and similar, but this won't work for your higher digit meters. I understand that JJAs are programmable voltage sources, but I can't imagine it is realistic to calibrate every production meter against these.- In general, how does one actually measure DNL for higher resolution ADCs ?

author: Kleinstein
author_rank: Super Contributor
timestamp: June 02, 2022, 02:02:34 pm
content: The standard method to measure the DNL is looking at the histogram, ideally with a relatively low level but good linearity triabgle wave.The problem with slow high resolution ADCs is that this test needs quite a lot samples (e.g. some 20-500 per point to be checked). So this test can not be done with all points. So the DNL test and usually also the INL test is only a partial test, checking some points and hoping that the rest is not much worse.For the INL the standard method is testing against a high grade calibrator or similar test source. In a production test, the same signal may be provided to multiple meters to be tested at the same time. To allow for some drift (e.g. of the meter internal test) this test should be not all in one run, but a few repeats. An INL test is a rather slow thing - every point may take a few seconds for settling and a few seconds averaging for each rund and maybe 5 or 10 repeats. So a reasonable curve (some 100 points) may take a few hours, if the meter is low noise and low drift. The meters reference may well be limiting as a major low frequency noise source and thus the need for multiple runs.Normal calibration does not include much linearity testing. There usually is a quick test of the turn over error: the same voltage is measured with both polarities and the result should no differ too much. Ideally there should be a few more tests that are feasable with the usual modern calibrators. However theusual cal.-procedures do not include this.There are a few exceptions with meters that are a bit prone to develope increasing linearity errors (e.g. the Fluke recirculating remainder ADC). That special Fluke ADC (a kind of SAR type) can show drift that lead to substantial DNL and INL errors. The usual integrating multi slope ADCs usually show litte change in there linearity. A decoupling cap getting bad and maybe a strong drifting resistor networt can also lead to increasing linearity errors, though this would be a rare case and more like a defect. Ideally the critical points of an ADC are known and a test could concentrate at such critical values (e.g. half the range or around 0 V).