Reading ADC when the VDD is not fixed

[Dabbot]

Table 14-8 Parameter AD08 has a requirement that the source resistance, as seen by the ADC, be, at most, 10k Ohms.

http://ww1.microchip.com/downloads/en/devicedoc/41203d.pdf

The Comparators have an internal Vref that is ratiometric with Vcc.  I haven't thought through what this means but you might be able to get just a "Good"/"Low" indication.

It's just an internal resistor ladder between VDD and ground. See page 64 of the PDF you linked. It's not an independent reference.

[rstofer]

Table 14-8 Parameter AD08 has a requirement that the source resistance, as seen by the ADC, be, at most, 10k Ohms.

http://ww1.microchip.com/downloads/en/devicedoc/41203d.pdf

The Comparators have an internal Vref that is ratiometric with Vcc.  I haven't thought through what this means but you might be able to get just a "Good"/"Low" indication.

It's just an internal resistor ladder between VDD and ground. See page 64 of the PDF you linked. It's not an independent reference.

And that's why they call it 'ratiometric'.

[wraper]

Table 14-8 Parameter AD08 has a requirement that the source resistance, as seen by the ADC, be, at most, 10k Ohms.

http://ww1.microchip.com/downloads/en/devicedoc/41203d.pdf

The Comparators have an internal Vref that is ratiometric with Vcc.  I haven't thought through what this means but you might be able to get just a "Good"/"Low" indication.

It's just an internal resistor ladder between VDD and ground. See page 64 of the PDF you linked. It's not an independent reference.

And that's why they call it 'ratiometric'.

And that's why it will result in the same reading regardless of voltage if you try to measure Vdd. What's the point of making ratiometric measurement of voltage which is used as ratiometric reference of ADC? Their ratio will always remain the same regardless of Vdd.

[mikeselectricstuff]

The PIC16F153xx for example has an internal reference, programmable to 1.024,2.048 or 4.096V.
Sleep current typ 60nA with everything off, 0.9uA with a wake-up timer running
 

[arslan43711]

The reason for using this micro is availability in the local market.

[IDEngineer]

Be careful relying on the "voltage references" in MCU's. They generally have decent repeatability in the same device, but can have poor absolute accuracy from device to device.

I researched this rather deeply a while back and the final official word from the manufacturer (in this case Microchip, for their PIC series) was +/-7% voltage accuracy. (You get better than that from vanilla voltage regulators, and binned regulators can get under 1%.) There are some more recent units in which they are laser trimming the voltage reference so one might expect better per-device accuracy. Interestingly, they also write the correction they applied into a nonvolatile, readable area on-chip... not sure how I'd use that information unless I was trying to bin "near-process" parts for some reason.

[Yansi]

Digital calibration is easy, so why bothering to implement a reference with good absolute accuraccy?

[wraper]

Digital calibration is easy, so why bothering to implement a reference with good absolute accuraccy?

To avoid another manufacturing step. There are $0.30 MCUs with +/- 1.8% reference in entire operating temperature and voltage range. https://eu.mouser.com/ProductDetail/Silicon-Labs/EFM8BB10F2I-A-QFN20?qs=G1ssh2j%252Bqz4q0w05PnPgQQ%3D%3D