Analog input: influence of the VCC changing?

[Alex Co]

Hi I read somewhere that when using internel ref for analog measurements the Vin changing does not influences the analog reading. Is that true?

Please, consider the circuit on this link:

devzone.nordicsemi.com/.../

Suppose we start with a 3.3 coin cell which will be drain more or less fast but nRF51 has a brown out of 1.8v

how can we calculate the influence (if any) in the analog conversion when using internal 1.2 ref (ADC_CONFIG_REFSEL_VBG) (and a brown out of 1.8v!) and a analoginput signal divided by 3 (ADC_CONFIG_INPSEL_AnalogInputOneThirdPrescaling)?

Can we trust in the analog conversion during all time of battery draining (until 1.8v)?

Thanks

Alex

[Turbo J]

Can we trust in the analog conversion during all time of battery draining (until 1.8v)?

Yes, according to the datasheet...

But you also have to check the circuit that produces the analog voltage being measured. That's why I strongly recommend checking the behaviour with a labratory power supply and a resistor simulating the drained coin cell (which would have a quite large internal resistance at this point).

The BTLE TX/RX drains the battery - which then recovers in the low current gaps between transmissions, so the voltage will not be constant unless you use large reservoir caps.

[Alex Co]

By the way i am not planning to read analog value while driving BLE. Should i use even though a large cap in the board vin? Any schematic for that. ?