How to evaluate the deviation in reference voltage of ADC?

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How to evaluate the deviation in reference voltage of ADC?

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vpadmanabhi
Contributor I

I am using MPC5746C. The issue is that ADC reference voltage applied at pin " VDD_HV_ADCx " of micro is not exact 5V. there is slight deviation of  ~50mv (which is quite normal from HW point of view as the SBC that we use to provide VCC has typical tolerance of  2%)

However, this affects the conversion results.

For instance,

With Ideal Vref of 5 V; for input voltage 4.5 V one should get 920 ADC counts (for 10 bit ADC)

With  real Vref of 5.05 V; same input voltage of 4.5 V gives us  911 ADC counts (for 10 bit ADC)

This of difference 9 counts results in a large error in calculation of sensor resistance that we have connected on this ADC channel.

So is there any way to evaluate this deviation in Vref through SW so that we can compensate the ADC count values accordingly.

Off course, we can manually calibrate the ADC in our End of Line by giving some known voltage . However I would like to know if something can be done Automatically, through Software.

Pls advice.

Thanks!!

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vpadmanabhi
Contributor I

Hi Peter,

Thanks for your feedback.

I did the calibration as per the steps given in RM. But there was no change in the output.

I can assure that the calibration was successful. However, I get the same results. Any idea what could be going wrong?

Thanks,

Vijay 

ADC_Calibration_MPC5746C.bmp

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petervlna
NXP Employee
NXP Employee

Hello,

So is there any way to evaluate this deviation in Vref through SW so that we can compensate the ADC count values

Well you can do ADC calibration:

Note:
The ADC must be recalibrated if the operating conditions (particularly VrefH) change significantly.

For details refer to reference manual chapter Calibration (High Accuracy mode)

From SW point of view I do not think there can be done much to increase accuracy when you have voltage deviations on reference.

regards,

Peter

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