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 

[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