ADC accuracy

stabilo — Tue, 20 Feb 2018 17:21:39 GMT

Dear support team,

I use the ADC0 of the S32K144 to measure voltage (0 to 36V).

The ADC channel is configured to measure continuously at a sample rate of 5µs. DMA transfer are triggered to transfer 32 ADC measurement into the buffer. Then, filtering and conversions are performed.

I have two problems:

The raw values into the buffer vary a lot (constant noise of ~ 15 digits)
The final voltage measured (filtered) is not accurate enough:

This picture shows the absolute error in function of the input voltage. Each point on the graph is a mean of 10 measurements (on these 10 measurement, the noise on measured values never exceed 5mV). As you can see, the error is very random (e.g. +40mV at 25.0V, -30mV at 26.0V).

I tried to enable the ADC self calibration, and the results are even worse.

Do you have any idea on how to improve my measurement accuracy ? (measurement rate ? reference voltage ? analog input capacitor ?)

I hope you can help,

Thank you very much.

Re: ADC accuracy

lukaszadrapa — Wed, 21 Feb 2018 12:05:31 GMT

Hi,

you wrote the voltage is 0-36V. I guess there’s resistor voltage divider, right? If so, what is the resistance? If it is too high, the error can be caused by input leakage offset error. Take a look at this application note, page 14:

https://www.nxp.com/docs/en/application-note/AN2438.pdf

Regards,

Lukas

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ADC accuracy

Re: ADC accuracy