How to improve the accuracy of measured ADC voltage in a circuit connected to Vcc

Question

The question is based on the following setup.

As depicted there is an IC to measure the real value of the 3.3V supply voltage accurately. The measured value of the 3.3V source is transferred via SPI to the microcontroller. This microcontroller is measuring the voltage divider between 3.3V and GND and has a reference voltage for its ADC.

^{simulate this circuit – Schematic created using CircuitLab}

Problem: If the 3.3V supply voltage is changing the ADC measurement from the microcontroller is influenced.

Question: Can I compensate a changing 3.3V source with the measurement of the IC if the accuracy if this chip is within my limits.

Answer 1

If you can get occasional more accurate measurements of the 3.3V you can correct the measurement via the voltage divider, which can be made more frequently. The details, such as filtering, would depend on how the 3.3V changes (how rapidly, and how much). It could be difficult if the changes are relatively large and spikey

Maybe you could even do a one-time self-calibration and store the factor in an EEPROM. What makes sense depends very much on the details of what you are trying to do. For example, for one application we added monitoring of all supply rail voltages and currents because the instrument was going to be very much inaccessible during operation.

I suggest making the maximum allowable correction factor modest, yet large enough to counter changes due to ADC reference tolerance, drift, temperature drift, and resistor tolerance. And consider what has to happen if the accurate measurement is late or is never received, or is clearly out of whack- system-level issues.

Answer 2

^{simulate this circuit – Schematic created using CircuitLab}

You can use an opamp to buffer the internal Vref of your MCU, and it becomes a ratiometric ADC. Example for 12 bit ADC:

$$Code=\frac{4095}{V_{ref}}\cdot V_{ADC}=\frac{4095}{V_{ref}}\cdot V_{ref}\frac{R_2}{R_1+R_2} = 4095\frac{R_2}{R_1+R_2}$$

As you see, the V_ref cancels out.