What is BL702 ADC reference voltage

makc

Hello,

I'm using BL702 with VDDCORE, AVDD33_AON, AVDD33_PA powered from 2.5V power source.
Reference manual states, that there is only two choices for ADC reference voltage: 3.2V and 2.0V (no external reference is possible):

Using current bl_mcu_sdk HAL library for ADC interaction and ACH_CH7 connected to 2.5V through pull-up resistor I'm getting accurate 2521 mV value while reading ADC data. How could it happen?
I supposed, that AVDD input is used as ADC reference voltage. So in described conditions I supposed, that I shall get about 0xFFF (full scale saturated value) for 12bit conversion mode (2.5 V input with 3.2V external reference, having actual 2.5V reference value). But it seems, that BL702 has internal charge-pump for ADC reference generation or something like this.

Could somebody explain my case? What is actual ADC reference value with 2.5V MCU power voltage?

asaki

In mcu sdk, adc_parse_result function will caculate sample value to millivolt.

makc

asaki, yes, I see. But used in adc_parse_result voltage constant is equal to the selected ADC reference voltage. My question is about ADC internals and actual Vref value/generation method. Could it be greater than MCU analog power source (2.5 V in my case)?

asaki

makc But you use 3.2V reference right? The parse result will not be overflow adc vref.

makc

Yes, parse result won't overflow reference. I can't understand how it could be 3.2V internal reference with 2.5V external power source. Could your make clear this? Is there internal step-up (boost) converter for ADC reference voltage to get 3.2V regardless external power voltage level?

asaki

makc No boost, you should use 2.0V reference.

makc

I can't use 2.0V reference, cause hightest input voltage is 2.5V and ADC properly shows about 2.5 (after parsing raw value). Datasheet states, that allowed ADC input range is up to 3.2 V (my 2.5V is much, much lower):

So, how could it be? What I'm missing about ADC reverence voltage and allowed input range?