Sometimes my 12 bit ADC gives a 16 bit value

RE: Sometimes my 12 bit ADC gives a 16 bit value

Håkon Alseth — Fri, 23 Mar 2018 07:30:45 GMT

You have different types of noise that affects any ADC, like non-linearity, temperature, and offset errors.

I suspect it's the offset error of the ADC that provides you with negative values. If you ground the ADC pin and check the result, it will yield a negative number.

The different offset/linear errors for the SAADC can be found in the el-specs:

http://infocenter.nordicsemi.com/topic/com.nordic.infocenter.nrf52832.ps.v1.1/saadc.html?cp=2_1_0_36_11#unique_397560666

Best regards,

Håkon

RE: Sometimes my 12 bit ADC gives a 16 bit value

Mason — Thu, 22 Mar 2018 17:31:16 GMT

Ok, thank you, I can start doing that.

Question for you though - why is the ADC yielding negative values? Doesn't that not necessarily make sense?

RE: Sometimes my 12 bit ADC gives a 16 bit value

Håkon Alseth — Thu, 22 Mar 2018 07:08:43 GMT

Hi,

Thanks for the detailed graphs and raw-data. The driver will interpret the ADC result as a int16_t type.

ex: 65533 == -3. If you store and treat the data as signed, the graphs should look better.

Cheers,

Håkon

RE: Sometimes my 12 bit ADC gives a 16 bit value

Mason — Wed, 21 Mar 2018 15:46:55 GMT

I've modified the SAADC SDK example to use four channels, sample every 6ms, and write to m_buffer_pool which is configured to have four elements (such that each channel samples once every 6ms). I don't think the situations you linked describe what I'm seeing.

In my SAADC callback, I write the contents of two of the channels to their own dedicated array 2000 elements deep. There will be times (recently had this occur for about 1.1 seconds of samples in the middle of the sample array) There are a few bursts of less than 20 samples within this period in which there are some normal valued samples. Otherwise there is a range of values from 65523 to 65535 being returned from the callback.

I think it is also worthwhile to mention that my ADC values will normally sit at around ~400 at the lowest. Prior to seeing this 16 bit values, the ADC values will start to get lower and lower. Since I am storing my values in an unsigned 16 bit integer, perhaps these are "negative" values that are just rolling over?

This is a graph of the original signal during one of these 16 bit periods:

Here is that same set of values graphed, except I subtracted 65535 from every "16 bit" value such that a negative value was produced:

And just for clarity, here are the raw values captured from the ADC here:

RE: Sometimes my 12 bit ADC gives a 16 bit value

Håkon Alseth — Wed, 21 Mar 2018 08:25:29 GMT

Hi,

Not sure how you have setup your SAADC, but there are four erratums that you can potentially hit:

Started events fires prematurely

Triggering START task after offset calibration may write a sample to RAM

EVENT_STARTED does not fire

END event firing too early

Does your handling of the SAADC resemble any of these scenarios?

If not, could you elaborate on how often you see this, and how you sample your analog input from a fw-perspective?

Best regards,

Håkon