So you cannot claim 0.05 if the datasheet said 0.1. And INL errors will not integrate out, because ADC will make the same bad measurement every time the signal is there.
Of course you can do better than the datasheet claims ! it's even the whole subject of this topic ! "Operating Chips Outside Their Spec"And there are many ways to do it !
Doing it properly involves a risk evaluation.
One method how I would do it in this case, if the manufacturer gives no indication:
1) get a few chips with different date codes from different suppliers, samples, etc...
2) characterize them to see the lot variation on the parameter of the Datasheet. You only need to measure the parameter you want to be outside spec, in our case the INL
3) characterize the temperature variation (or voltage if variable, or any other variable external parameter for your application)
4) Calculate or evaluate which is the limit of the chip INL which will make you fail your end calculation. Let's say you could demonstrate that 0.08% INL on the chip gives 0.05% accuracy in your application, due to the sample spread when measuring AC RMS. So our limit is set to 0.08%
5) Evaluate the spread, evaluate statistically how much percent of chips will probably fail in your application (over 0.08% at the ADC).
6) Set a strategy. In our case, we decide:
- Alternative uC with better specced adc costs 10 times the price
- we produce 1000 parts a Year.
- we measured the deviation of INL from the chips in the same batch to be under 0.01%
- typical INL is around 0.04%
- we buy a reel of EFM8BB10F2G-A-QFN20R (1500 parts) every 1,5 Years, for about 500 Euros, we could resell a roll of non useable 1499 parts for 200 Euros, but administrative effort reduces this to 100 Euros.
- we have a 2% chance to have an entire reel outside our spec
- we have an 3% chance of a marginal reel, where some parts would be outside our spec
- Product lifecycle is estimated at 8 Years -> 5 rolls needed max (count ramp up and down)
- Acuracy testing is part of our normal EOL QA testing, so characterising one chip from a new roll costs us only about 300 Euro effort, and two weeks waiting.
So we decide that a meaningful strategy is:
- we plan a reel ahead
- Every time we buy a roll, we get one chip of it, solder it manually onto a board, and pass the final test
- if ever we get a reel outside or marginal to our criteria ( 0.07% ADC, which corresponds to 0,04375% final calc), we reject the reel and resell it, re-order one, retest.
7) Talk to your boss about the added risk, and the strategy. The strategy and following decision of management have to be written down in the project minutes (very important)
go on and implement the strategy.
As I said already, we didn't do all that, because we didn't need better than 0.5% anyway.
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