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what you think about ADC backward measuring supply voltage in microcontroller? It looks interesting for battery-powered devices.

The idea is:

  1. Connect constant voltage (some voltage reference IC) to ADC input (not to Vref!)
  2. Set AVcc as reference for microcontroller ADC
  3. Get ADC value
  4. Calculate ADC reference (our AVcc) from ADC result

My questions:

- is this commonly used method?

- can that method be better than measuring Vcc using voltage divider with ADC reference connected "conventionally"?

My question is inspired by this.

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An ADC reads an analogue value via its input but that analogue value is dependent on: -

  1. The analogue value
  2. The ADC's reference voltage

If one of the two above analogue voltages are known the other can be computed or inferred.

I wouldn't have thought it was very common because most folk will want to have a stable voltage reference (from a known voltage reference chip) so that they can make accurate voltage measurements of the various input signals going into their ADC (multiple channels are of course usually "served" by one stable reference).

However, if the inputs are a bunch of RTDs and the RTDs are excited via a resistor from the AVcc rail then ratiometric measurements are the most accurate method of using the ADC and, if AVcc is also needed to be known then a stable voltage reference on a spare ADC input allows you to make this measurement.

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  • \$\begingroup\$ Good answer. Just FYI, some inexpensive microcontrollers do not give the option of connecting a good voltage reference to Vref...it's internally hardwired to the microcontroller supply voltage. \$\endgroup\$ – Joe Hass May 12 '14 at 12:49
  • \$\begingroup\$ I meant projects where there is no need for any ADC measurement. And if it is - in all microcontrollers you can switch reference to some internal precision voltage to get acurrate input measurements. \$\endgroup\$ – Kamil May 12 '14 at 12:52
  • \$\begingroup\$ @Kamil I'm not following what you mean? \$\endgroup\$ – Andy aka May 12 '14 at 12:55
  • \$\begingroup\$ @Andyaka Hmm, sorry, my english is not perfect and I think I missunderstood what you wrote about "most folk will want...". I think now I got it, forget my first comment :) \$\endgroup\$ – Kamil May 12 '14 at 13:39
  • \$\begingroup\$ But what you think about acurracy in circuit like this? I think it should be better than measuring Vcc reduced by some voltage divider... It eliminates drift on resistors etc. \$\endgroup\$ – Kamil May 12 '14 at 13:44

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