Measuring 0 to 5V with <20mV ACCURACY - Page 1
Forum section: electronics / general

author: luky315
author_rank: Regular Contributor
timestamp: November 21, 2021, 11:43:51 am
content: I am considering to design a measurement system which should work 24/7 over at least 10 years without field calibration.Speed (<1sps is ok) and power consumption are not an issue.The temperature range is also modest (10 to 40°C) and there should be no temperature shocks and no mechanical movement of any sort.Which components should I use to have a high confidence in the long term stability of the system?

author: miro123
author_rank: Frequent Contributor
timestamp: November 21, 2021, 02:12:37 pm
content: do you expect higher humidity rate?4000ppm are quate loose requirements. I would rather look at automotive grade components. they provide1. decent performance2. very good long term stability3. low /mid-range price====vref - eg. REF34xx -https://www.ti.com/product/REF3425adc - any type.thin metal resistors for 5V -> 2,5/3.3V volatge divider. In case of higher humidity can you consider vishay dale series.

author: pqass
author_rank: Super Contributor
timestamp: November 21, 2021, 03:24:12 pm
content: Quote from: luky315 on November 21, 2021, 11:43:51 amIt starts with understanding the limitations of your chosen voltage reference. And then your ADC.Your accuracy requirement of 20mV in 5V is 0.4% (or 4000ppm).Many devices meet that (filter by tolerance=±0.2% or less, sort by price):https://www.digikey.com/en/products/filter/pmic-voltage-reference/693Add to that the temperature coefficient over your 10-40°C (Δ30°C) temperature range. For example: a 20ppm/°C tempco could be 600ppm off over your temp range.Add to that again the long term drift (not shown as a selectable column in link above) given in the datasheet.There are other errors but their effects are smaller.In the end, you could be as much as tolerance+temp+drift+other errors off.Some good references are not accurate (out of the box off by 10%) but exhibit very good tempco and drift characteristics. End-user-devices using them would need to be calibrated (with correction value stored in non-volatile memory) before use. If you can't/don't want to calibrate each device then choose a reference with the lowest tolerance you can afford but still keep you under 0.4% given their [likely worse] tempco and drift.With regards to your ADC, how many bits do you need? What's the smallest voltage your application needs to differentiate? For example, 20mV in 5V is 250 steps, so you'll need at least 8 bits. Your ADC can be off by 1 count (LSBit) or 1/256 (0.4%) off for an 8bit ADC. This ADC error adds to the voltage reference errors above.Rather than a discrete voltage reference and ADC, it's likely that many micro controllers with a built-in voltage reference and ADC meet your requirements. Look there first.EDIT: corrected earlier off-by-one order of magnitude error. oops!

author: luky315
author_rank: Regular Contributor
timestamp: November 22, 2021, 12:10:24 pm
content: My problem is not accuracy, I know that the requirements are not that high. And an inital calibration after a burn-in will be done regardless. The problem is the long term reliability.The build-in voltage references of all microcontrollers that I know are not well specified. Only a few have min/max specs, everything else is "TYPICAL" only. Not a good base for having high confidence in the long term stability....

author: fluxgate
author_rank: Contributor
timestamp: November 22, 2021, 06:08:33 pm
content: Long Term Drift is usually specified for high perfomance Voltage References.Take this reference for example:https://www.farnell.com/datasheets/1904455.pdfLong Term Drift -> 30ppm/1000Hr.Now, note that the Long term drift drops logarithmic with time.So my guess is like 300ppm max. after 10 Years for a good reference.However, considering your requirements just search for an ADC with a well specified integrated reference.For better estimates some references have long term drift specified in ppm/(1000h^1/2).Multiply it with sqrt(10 Years in hours) and you get your error intervall.

author: Andreas
author_rank: Super Contributor
timestamp: November 22, 2021, 07:34:17 pm
content: Hello,Quote from: miro123 on November 21, 2021, 02:12:37 pmI would avoid unneccessary voltage dividers. Why not use a 5V reference and a 5V ADC?(or is some kind of "overrange" necessary?)Quote from: pqass on November 21, 2021, 03:24:12 pmAll those values are "typical" (most probably the 1 sigma stray of a large quantity of devices) so individual devices could be much worse.Quote from: fluxgate on November 22, 2021, 06:08:33 pmAgain only a "typical" value.And most probably this is only valid for the best package (hermetically sealed) which is mostly obsolete nowadays.And my 2 samples of MAX6350 in DIP8 package do not have a sqrt function in ageing drift. (attached the first 1500 days)With plastic packages or smd packages on epoxy PCBs you can often measure humidity (as seasonal changes).If money does not play a role my suggestion would be:- use a good hermetically buried zener 5V reference. (E.g. AD586LQ, AD586JQ or LT1236AILS8-5 (on flex pcb or alumina substrate) or similar).- I would use a LTC2400 ADC (has also some (~100 mV) overrange if needed and is self calibrating for offset and full range)If it is mission critical you can use 1-2 additional (different type of) references (with a multiplexer) to check if your main reference has drifted.with best regardsAndreas

author: mendip_discovery
author_rank: Super Contributor
timestamp: November 22, 2021, 08:14:46 pm
content: I would also look at other metrics that you could log, to help account for errors and drift.Ambient Temp/Hum/Pressure that way if the logs show the data is drifting then the other sensors might show why.Maybe to reduce drift would an enclosure designed to maintain a constant temp etc. help reduce the errors.I think a 10year goal is a bit of a stretch especially when you take into account all the things that could have an impact on the equipment. Power supply failures, power fluctuations, electromagnetic interference. What percentage of device failure would happen to a 10yr old bit of kit in constant use.