Reachable ADC tolerance using ADC calibration with Flash IFR values of bandgap voltage

[curious]

Hello Community,

using the standard VDDX/VDDA voltage as ADC reference has a tolerance of +/-3% with the S12ZVML31 device. What ADC reference tolerance can be reached using the ADC Calibration described in Chapter 1.13.1 of the RM?

Thank you very much.

[curious]

Hello Stano,

thanks again for your feedback. Just for my understanding I have a few more questions:

- The Reference Manual states in Table A.4 regarding the ADC Calibration that 5V is used as ADC reference voltage during measurement and IFR storage of VBG in production. Furthermore it is described that Tj =-40°C and +150°C is used. How does this align with your stated 5.12V and 25°C? How accurate is that 5V/5.12V?

- The remaining accuracy of the ADC reference voltage after VBG calibration is about 0.5%? 

Thanks again!

[StanoA]

Hello Curious,

The calibration values stored into IFR location during manufacturing process are referenced to +25C temperature. The internal bandgap accuracy is in Table B-1in defined temperature range.

The ADC conversion performance for defined temperature range is in Table C-3 and C-4 for higher temperature range.

You can measure the chip temperature by internal temperature sensor and recalculate the bandgap reference deviation yet.

The next improvement of the bandgap reference accuracy could be usage of average of several bandgap reference values to eliminate ADC measurement errors.

I hope it helps you reach the best ADC reference accuracy.

Best Regards,

Stano.

[curious]

Hello Stano,

thanks for the fast reply.

Ok, so the use of IFR values does not help to reduce the initial ADC reference tolerances. I have to calculate with VDDX=VDDA=V-ADC-ref = 4.85V ... 5.15V in worst case.

Reduction of this tolerance can probably only be achieved with external calibration of the ADC reference voltage during board production, right?

Best regards.

[StanoA]

Hello Curious,

The IFR values are programmed during chip manufacturing. It is value measured by VDDA = VREFH = 5.12V and 25C. It is accurate 12-bit value.

The IFR values have to be used in real application for the accurate ADC measurements recalculation. So the equation in Section 1.13.1 have to be implemented in application for each ADC measurement series to make corrections to actual VDDA = VREFH voltage and actual temperature. The temperature can be measured by internal temperature sensor. This recalculation in application eliminates VDDA and chip temperature variations. The calibration for each board in production is not needed.

In case you need better accuracy (~0.5%) you must use the external accurate and stabile reference, but the correction to the actual temperature has to be implemented too. Then you can reach the max accuracy of measurements.

Btw – the output impedance of the measured signal and sampling time have not negligible influence to final results.

Best Regards,

Stano.