S32K344 ADC: Non Linearity Correction at Calibration

[Samuel_DCosta]

Hi,

I would like to understand how to correct non-linearity in the output during the calibration of the ADC.

Figure 10, Document Number: AN13413, Rev 0

As the image shows, how do I perform the calibration by giving different inputs across the range of the ADC during calibration? I am using low-level drivers and I have the below function:

Adc_Sar_Ip_StatusType Adc_Sar_Ip_DoCalibration(const uint32 u32Instance)

S32K3XX RM, Rev 8

What is the 'known reference voltage' sampled during calibration? Is it the bandgap voltage? What is the exact value used? (The data sheet says 1.2V but when I read the bandgap voltage it is 1.19V which is alright but then there is an error of 0.01V) Is this usually acceptable?

In the values that I read during functional conversions, I see that there is an error of 0.2 V (calibration was successful as a return from the above function) from what is set. I am sure the power supply is not in error as it gives the output voltage set corresponding to the input voltage requested. We have also confirmed this via a DMM.

How do I ensure that the calibration is done correctly? 

Is there something I need to additionally consider during calibration?

[davidtosenovjan]

Bandgap reference (trimmed) is specified as 1.18-1.22V. So what you read is possibly error of bandgap. I think bandgap voltage is not used for calibration process, but it is used during ADC selftest.

Calibration influence offset, gain, and capacitor mismatch.

Here you may see some description, but take it with a grain of salt as this is a slightly different module, but similar:

https://community.nxp.com/t5/Kinetis-Microcontrollers/16-bit-SAR-ADC-calibration/ta-p/1102292

Basically you can understand the principle and section ‘Recommended Calibration Procedure’ is more or less applicable.

What is quite important for precise ADC conversion is

- to have input impedance adequately low

- to have noise free ADC reference voltage

[Samuel_DCosta]

Hi @davidtosenovjan ,

Thank you for your reply but I do not get my answer yet.

"I think bandgap voltage is not used for calibration process, but it is used during ADC selftest"

1. What signal ('known reference voltage') value is sampled during calibration?
2. How do I perform the calibration by giving different inputs across the range of the ADC during calibration?

[davidtosenovjan]

1) ADC is capable to connect VRH and VRL to every capacitor, there are no other internal inputs. It is not explicitly described how calibration mechanism work internally.

2) Voltage on ADC input does not affect calibration. It works internally and it can be only influenced by analog reference voltages.