How accurate is ST VREFINT Calibration?

mp035 · ‎2021-02-22

I am using STM32F030F4P6.

VREFINT is supposed to be calibrated at 3.3v +/- 10mV and 30 degrees C. In my application the VREFINT calibration value is 1543 meaning the reference voltage is 1.243V (3.3/4096*1543) this is on the high end of the spectrum, but within limits.

When using the calbirated VREFINT, I found that my VDDA calculations were stable, but off by a bit over 1%. I have a VDDA/VDD voltage of 3.3013V measured with a calibrated multimeter (I have a DC standard to check calibration) and ambient temperature is 24.3 deg C.

Taking the average of 4000 samples from VREFINT with this setup yields a reading of 1527 or 1.230V which is bang-on specification. It is also 1.01% different to the calibrated value. How accurate should I expect the VREFINT calibration value to be? I ask because if factory calibration should be better than this, I need to find out where my design is inaccurate.

Uwe Bonnes · ‎2021-03-02

****** tricky error sequencing. Thanks for sharing! However I do not see a hint in the errate sheet. Did I miss the hint?

Antoine Odonne · ‎2021-03-02

Dear Uwe,

I agree it lacks clarity in tracking down this issue.

It is I think linked to the fact that in parallel, we realized that the DS data for Vrefint itself were incorrect in DS, It has been associated to a F030 DS update in January 2017 and a PCI "product change information" with below descritpion (not referencing calibration correction explicitly but the VREFINT testing improvement) it was previously stated that Vrefint typ value was 1.20V:

Extract of PCI

"No change on product function nor performance.

VREFINT documentation is updated thanks to 100% production testing:

- Minimum value is improved.

- Maximum value, that was previously based on characterization, is now aligned with

production testing criteria.

- Typical value is updated accordingly."

I hope it helps explain a bit the situation.

Best regards,

Antoine

mp035 · ‎2021-03-02

Hi Antoine,

Thank you for your confirmation. It is good to have an answer to the issue. I was still a little concerned that the real problem may lie elsewhere in my design, but your answer dispels any doubt. Have a good day.