Oversampling not working correctly

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ag123
Posts: 1655
Joined: Thu Dec 19, 2019 5:30 am
Answers: 24

Re: Oversampling not working correctly

Post by ag123 »

I'd guess you could be right
bode plot 10uF 10k
bode plot 10uF 10k
bode plot 10uF 10k
bode10uF10k.png (16.66 KiB) Viewed 628 times
-3 db at 1.59 hz

step response
step response 10uF 10k
step response 10uF 10k
step10uF10k.png (16.67 KiB) Viewed 628 times
This assumes the 'input' resistance is 10k, for NTC i'd guess at warmer temperatures it'd be ok.
adc_problems
Posts: 11
Joined: Sat Sep 24, 2022 4:05 pm

Re: Oversampling not working correctly

Post by adc_problems »

The thermal mass is way slower than the RC lowpass with a 1,5hz -3db frequency.

The temperature changes translated to resistance translated to voltage is minimal. There is no way that the temperature is changing that fast.

I will implement a slower RC filter and try again.

But as i tried to explain. That huge RC filter will make the tracking performance really bad.
If you got a load that periodically drops the 3,3V (adc ref voltage) a little, the temperature input voltage will not track that because the filter time constant is too big.
Therefore the temp voltage is stable but the ref voltage isnt which results in unstable measurements.
If both are unstable in the same way (because the supply drop affects both in the same way) it makes more sense to use a smaller filter.
ag123
Posts: 1655
Joined: Thu Dec 19, 2019 5:30 am
Answers: 24

Re: Oversampling not working correctly

Post by ag123 »

Originally, i'm concerned with rapid temperature changes and that too large a RC constant will cause the ADC to read the delayed image rather than the instantaneous temperature. This is a concern when the temperatures is changing fast.
i.e. temperatures will overshoot, by which time it is measured, the actual temperature is (much) higher or lower

However, it seemed with these analysis that it may still work given the bode plots here.
Hence, a larger RC value could be tried if it can approach these bode plots, and graphs, i.e. if the worse delay in response is a fraction of a second
adc_problems
Posts: 11
Joined: Sat Sep 24, 2022 4:05 pm

Re: Oversampling not working correctly

Post by adc_problems »

So looks like no one knows why the oversampling isnt working?
:oops:
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