ADC sampling rate with oversampling issue

[jawadk]

Hi,

I am using the nRF52840 to sample some channel.

I am using the nRF5SDK160098a08e2 for that and based on the saadc example.

These are the results for my tests.:

1. 4000sps with 64 oversampling, I got sampling rate of 1370Sps, not expecting to get 4000Sps cause 4000*64 > 200K
2. 3000sps with 64 oversampling, I got sampling rate of 1015Sps, need to fix this one
3. 2000sps with 64 oversampling, I got sampling rate of 1015Sps, need to fix this one
4. 4000sps with 32oversampling, I got sampling rate of  2028Sps, need to fix this one
5. 3000sps with 32 oversampling, I got sampling rate of 1522Sps, need to fix this one
6. 2000sps with 32 oversampling, I got sampling rate of 2008Sps
7. 4000sps with 16oversampling, I got sampling rate of  4000Sps
8. 3000sps with 16 oversampling, I got sampling rate of 3003Sps
9. 2000sps with 16 oversamplingadc, I got sampling rate of 2001Sps
10. 4000sps with 8 oversampling, I got sampling rate of   4000Sps
11. 3000sps with 8 oversampling, I got sampling rate of   3000Sps
12. 2000sps with 8 oversampling, I got sampling rate of   2000Sps

The problem is that the sampling rate from test number 2 to test number 5 is not as expected!

How can I fix this please?

Best Regards

JK

[Karl Ylvisaker]

Hello JK,

I am using the nRF5SDK160098a08e2 for that and based on the saadc example.

What changes did you make to the example? Did you modify the channel / SAADC configurations? - if so, how have you modified them?

The SAADC example uses the default SAADC and channel configurations for Single ended inputs - which is configured to 10 µs Acquisition time. Did you change this?

Looking forward to resolving this issue together!

Best regards,
Karl

[jawadk]

Hi,

Thank you for your reply.

The Samples that the ADC sample is 2^oversampling number ? and not just the oversampling numer.

For example if the oversampling is 2, so the actual samples number the ADC sample is 2^2 = 4 yes ?

BR

JK 

[Karl Ylvisaker]

Hello JK,

jawadk said:

The Samples that the ADC sample is 2^oversampling number ? and not just the oversampling numer.

For example if the oversampling is 2, so the actual samples number the ADC sample is 2^2 = 4 yes ?

Yes, this is correct - you can read more about this in the SAADC peripheral documentation, specifically looking at the Oversampling section, and the Oversampling register.

Did you check which Acquisition time you used during your testing?

Best regards,
Karl 

[jawadk]

Hi,

Thanks for your reply.

The Acquisition time is NRF_SAADC_ACQTIME_10US.

Best Reagrds

JK

[Karl Ylvisaker]

Hello again JK.

jawadk said:

The Acquisition time is NRF_SAADC_ACQTIME_10US.

That explains why you saw the sampling results that you posted about originally then.
To be able to achieve 200 kHz sampling, you will need to use the 3 US ACQTIME. With 10 Us, you can expect to see exactly the results you posted in your original ticket.

Best regards,
Karl

[jawadk]

Hi,

Last question:

the calculation isn't seem right for me

When I sampled in 2000Sps with 32 oversampling it succeeded !, but the calculation is (2^32)*2000 = 8589934592000 Sps. this is way bigger than 200KSps.

What I am missing here !?

Thank you for your help

Best Regards

JK

[Karl Ylvisaker]

Hello again JK,

jawadk said:

the calculation isn't seem right for me

When I sampled in 2000Sps with 32 oversampling it succeeded !, but the calculation is (2^32)*2000 = 8589934592000 Sps. this is way bigger than 200KSps.

What I am missing here !?

Please see the overview of the Oversampling register and register that I referenced you in my previous comment - there is no way for you to configure 2^32 oversampling - that would indeed be way too big, and frankly not very useful.
Instead, you will be configuring 2^5, to achieve 32 oversampling - as detailed in the documentation.

Please do not hesitate to ask if anything still should be unclear! :) 

Best regards,
Karl

[jawadk]

Hi,

Ok thank you for your help

Best Regards

JK