SAADC oversampling sampling interval?

[Tajpeer]

Hello All,

Method 1:   I am writing a test application to read the  ADC data at a specific intervals(10 samples for every 5ms interval for every 50sec). I am using blocking mode here with oversampling disabled and using delays between samples. It works well. Managing averaging manually.

Method 2:

In addition, I have seen the concept of oversampling and tried to understand it's usage. After going through the ticket below, I have got all my queries answered except one.

https://devzone.nordicsemi.com/f/nordic-q-a/14583/nrf52832-saadc-sampling#comment-84340

It's all clearly mentioned that, oversampling can be used to get the average of multiple samples by using oversampling of *x. Suppose,

• With oversampling 8x and buffer size of 1, the SAADC will collect 8 samples, take the averaged value and output that averaged value to the RAM buffer. The application will get a notification(NRF_DRV_SAADC_EVT_DONE to callback function) as the buffer is full with only one averaged sample.

But I am not clear with the sampling rate at which the SAADC driver/ sample task operates to collect the samples? Suppose, if we set oversampling as 8x, what will be the time taken to get a notification?

If in case, SAADC driver uses some inbuilt sampling, can it be modified as per our application? I want to read average of 10 samples for every 50sec and each sample should have time difference of 5ms.

Can someone help me to understand the concept in details as per above requirement.  Please comment on power and performance stand point difference as well between 2 methods.

[Jørgen Holmefjord]

Hi,

I would recommend you to enable BURST when oversampling is enabled, as described in the Oversampling chapter in the SAADC peripheral documentation. Then you only need to trigger the SAMPLE task once to get all samples required by oversampling to generate one result.

The timing will depend on the configured acquisition time, <(t_ACQ+t_CONV)×2^OVERSAMPLE . If you want to sample manually, the time between each SAMPLE needs to be at least t_ACQ+t_CONV.

Burst is enabled in the channel config structure if you are using the SAADC driver.

Best regards,
Jørgen

[Tajpeer]

Hi Jorgen,

Thanks for the information.

But still I am not clear how to configure the sample time for each sample manually.

My requirement is to sample 6 samples in a period of 30ms using oversampling.

[Jørgen Holmefjord]

The sample time depends on the configured acquisition time, which can be set between 3 and 40 us. The minimal delay between two samples needs to be t_ACQ+t_CONV. t_CONV is given in the product specification, and is maximum 2 us.

How to setup the sampling depends on your configuration and what source you use to trigger the sampling. If you want to capture 6 samples in 30 ms with 8x oversampling, the simplest solution would be to enable BURST and trigger the sample task every 5ms. Even with maximum acquisition time, you will not get near 5ms: (40us + 2us) * 2^3 = 42us * 8 = 336us.