EQADC_B Module accuracy

[amey_murkute]

Hi all,

             I am using MPC5777C for my application in which I am using EQADC_A & EQADC_B module which is a 12 bit ADC. When I read the EQADC_A channel with 5V input I get the max count as 4054 which is correct but when I read the EQADC_B with same input I get count as 3070. So why this difference is seeing as the configuration for both the adc's are same.

Thank You.

[davidtosenovjan]

Hi,
if you want 0-5V range on ADC inputs ANB8-23 & ANB40-44, you must power VDDEH7 with 5V.
I guess you have VDDEH7 connected to 3.3V, am I right?

[amey_murkute]

Hi , David Tosenovjan

Thanks for reply,

Yes we have connected VDDEH7 to 3.3 V. We are feeding 4.8 V to ADC input pin, Vref for ADC is also 5V.

Would this clip the voltage reaching to ADC pin input to the max range of 3.6V?  

Can you please let us know what will be the impact of this on ADC reading?

I would like to understand the internal structure/block diagram of EQADC_B  specifying how VDDEH7, EQADC_B input pins , ADC reference and internal cut-off diodes (if exists for clipping) are operating.

Thank You.

[davidtosenovjan · Accepted solution]

Yes, mentioned pads ANB8-23 & ANB40-44 are clamped to VDDEH7 as you can see in the excel sheet embedded in the reference manual.

Possibly these channels may also suffer for reduced accuracy, pay attention to e10453:

https://www.nxp.com/docs/en/errata/MPC5777C_3N45H.pdf