Question about SAADC measurement result

[SeanOY]

Hi engineers,

I want to use saadc to measure the battery's voltage, but the result I get is incorrect. 

So I use the saadc demo of the nrf5_sdk17, then I measure GND(negative result) and VDD(less than 853). Why VDD's adc result is not 1023?

Best Regards,

Sean

[SeanOY]

Hi Karl,

Sorry for the inadequate description.

1. Yes, I run the default demo of saadc and connect P0.02(AIN0) with gnd/vdd. The reason i test them is i want to know the upper and lower limit value of adc. My nRF5 SDK version is 17.

2. I use DK PCA10100(nRF52833).

3. The power source is USB of computer.

4. Result of measuring GND's voltage.

Result of measuring VDD's voltage.

Thanks!

Best regards,

Sean

[Karl Ylvisaker]

Hello Sean,

SeanOY said:

Sorry for the inadequate description.

No problem at all.
Thank you for elaborating, this is very helpful!

SeanOY said:

4. Result of measuring GND's voltage.

I think these measurements of GND look reasonable.
You must expect a few LSB of noise on the SAADC's output, you can read more about that in this reply by my colleague Carsten.

SeanOY said:

Result of measuring VDD's voltage.

The nRF52833 DK regulates the 5 V supply down to 3 V, please see the DK's power section for more information about this. The unmodified SAADC peripheral example is configured to support a voltage range of 0 - 3.6 V, so your measurements of 855 of the 1024 resolution translates to 3.01 V, which is as expected. The SAADC's input range is determined by the formulas from the SAADC peripheral documentation, you may configure this to match the requirements of your application.

Please do not hesitate to ask if any part of my answer should be unclear, or if you should have any other questions!

Best regards,
Karl

[SeanOY]

Hi Karl,

Thanks!

I use multimeter to measure the voltage of VDD, it's about 2.98V. I get it now :)

Best regards,

Sean