ADC_RESULT in nrf51822

[trk]

I was trying to calculate analog voltage in nrf51822 through ADC. I used analog 2 pin ref voltage =1.2v pre scaler = 1/3 so while i connected 0.9v to analog pin. I see analog result as 0xDB. I was trying to calculate voltage through formula input voltage = 1.2v * (1/pre-scalar)*(ADC_RES/256). My voltage doesnt match to ADC_result. Can you please provide me solution for calculating input voltage?

[Stefan Birnir Sverrisson]

Hi trk

When using 1.2V reference and 1/3 prescaler, the range of the ADC is 0V - 3.6V.

If you use 10-bit configuration you should get: 0.9/3.6 * 1023=255
If you use 9-bit configuration you should get: 0.9/3.6 * 511=127
If you use 8-bit configuration you should get: 0.9/3.6 * 255=63

[trk]

Hello stefan, i am checking the result on ADC->RESULT in keil but i didnt get the correct voltafe, when i gave 1 v to pin . Through calculation of ADC->RESULT., i get 1.5v. and one more question Is Result from ADC is in hex format?

[Stefan Birnir Sverrisson]

Hi trk. Yes, the result is in hex format. If you are not getting correct value, it is possible that your voltage source has too high impedance. What is your voltage source? Do you have a voltage divider connected to the input of the ADC by any chance? Generally you should not have more than 1kohm input impedance, in order to avoid noticable error on the ADC output. If you have high impedance input, the general solution is to connect a capacitor between the ADC input pin and ground, as demonstrated on this thread.