Use AREF1(pin0.06) as analog reference voltage for ADC instead of internal VBG

[yunlong.wang]

Dear Sir: Could I use the external reference AREF1(pin0.06) instead of internal VBG 1.2V for ADC operation ? I do the following settings to config using external reference

   nrf_adc_config.resolution = NRF_ADC_CONFIG_RES_10BIT;
   nrf_adc_config.scaling = NRF_ADC_CONFIG_SCALING_INPUT_FULL_SCALE;
   nrf_adc_config.reference = NRF_ADC_CONFIG_REF_EXT_REF1;

But it doesn't work, is there any other settings before using external reference for ADC.

Thanks a lot

[yunlong.wang]

Hi： I will try to add a capacitor to the AREF port, but how to define the capacitor value? Is there a formula to caculate the value ? Thanks a lot

[Øyvind Karlsen]

It has to be "large enough" so that if the power lines are pulled down the capacitor will still provide stable reference, somewhere in the µF range probably around 5-10 should be fine.

[yunlong.wang]

Hi:

One more question, I search for the relation between ADC and capacitor yesterday , I found some suggestion that insert an RC circuit before ADC input. So, is it neccessary to put a RC circuit before ADC input pin, or a reservoir capacitor is just enough ?

Thanks a lot

[Øyvind Karlsen]

I imagine the RC circuit was some kind of filter? It depends on your usecase, if you know you have noisy power lines then it might be a good idea. If you do have noisy power lines it might not be the best idea to use an external reference to start with.