nrf52 SAADC noise

Question

Hi,
I would like to know how much noise I have to expect with the SAADC. 
 I tried the example "saadc_pca10036" and attached the pin P0.02 (AIN0) to a ground pin on the pca10036 board and I get values between -2 and -5, that is a noise of about 3.
Is this correct? Moreover why I get negative values?
I tried also with a custom board and I get similar results. 
 From the electrical I read that the "differential offset error" is +-15, what exactly means this value? 
 I noticed also that that there is a "tasks_calibrateoffse", what is the purpose of this?

Carsten Wulff · Accepted Answer

The noise levels of the SAADC depends on the GAIN setting. But with GAIN=1/6 and 12-bit mode you should expect a standard deviation of a few LSB. 
 If you average the output over a long time with a zero input you get what is called the "differential offset error", this is the static deviation from the ideal output code (0). 
 Single ended mode is actually differential mode with negative input to ground. Since there is offset and noise in the ADC there can be negative outputs. 
 The TASKS_CALIBRATEOFFSET can be used to calibrate the offset of the SAADC.

nrf52 SAADC noise

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