strange Signal at ADC input

[edgarmichel]

Hello all,

I see a strange signal at ADC input which might cause the issue I am faced with.

I have attached the osci picture.

This signal looks like a sawtooth wave.

When I change the paramter SMPLTS then the area marked on the picture changes accordingly.

So I assume that when the ADC does "sample" the signal is forced to input level.

When the ADC does "hold" for internal sampling the signal follows a sawtooth wave.

I do not see this signal analysing the sample values. However the sampled signal does not match what I expect. Also I have

issue processing it.

The ADC input in connected with 1nF capacitor to ground.

Does anybody has n idea why this signal is so strange? What do I need to do here?

Thanks for your help

[edgarmichel]

Hello Daniel,
I found an Application Note

https://www.nxp.com/docs/en/application-note/AN4373.pdf 

It seems that is the root cause ie the input impadance does not match.
I will check that

BR Edgar

[danielmartynek]

Hello,
Could you please send the schematic and describe the ADC operation in more details?

Thanks,
BR, Daniel