nRF52840 SAADC real impedance 4,7K ?

[Aylik]

Hi Engineers!

faced with a problem of SAADC impedance too small (4,7K)! We use SINGLE ENDING 8bit mode with following settings:

• NRF_SAADC_REFERENCE_VDD4
• NRF_SAADC_GAIN1_4
• NRF_SAADC_RESISTOR_DISABLED
• NRF_SAADC_ACQTIME_40US ( 40us)
• Vdd = +3.3v

it means, INPUT RANGE = (VDD/4)/(1/4) = VDD = 3.3v , right.

if connect ADC input to Vdd(+3.3v) directly, Digital output = 255 (+3.3V on pin).

if connect ADC input to +3.3v use 1K, Digital output = 212 (+1.934V on pin).

Additionally, if integrated R_PULLUP connected (to VDD) Digital value is 165 (+2.112V on pin).

So, it does mean Calculated ADC imput impedance ~ 4,7K ! Why not suit to Specification data?

We need to know exact input parameners for our purposes.

thx you!

[AmbystomaLabs]

There is no real input impedance associated with an SAADC. Most all utilize the same design of a switched capacitance on the input that they then use as the input voltage sample during the success approximation part.

So your input model will be an exponential model and not a linear resistive model.

Here is a little discussion on the subject: devzone.nordicsemi.com/.../