microcontrollervoltage measurement
When using an Analog to Digital Converter (ADC) I want to convert to millivolts (mV) afterwards. My microcontroller is running on 3.3V, but I want to measure more accurately to get better result.
$$mV = \frac{adcData}{2^{12}} * V_{cc} * \frac{1}{1000}$$
Vcc is ideally 3.3V, but I want to measure it.
Is there any way to do this, except measuring the voltage with a multimeter on the board directly?
A few things to make sure of:
Let us know how it goes, it should be a reasonably easy issue to track down, but no doubt Murphy will have something to say about that ;-)
A proper schematic with all parts present would be helpful (i.e. I'm assuming the opamps have decoupling caps but you did not include them on the LTSpice schematic)
The correct formula is \$V_{IN} = \$\$D \cdot V_{REF}\over 2^{NBITS}\$ where D is the digital value. NBITS is 10, so the denominator is 1024. That's assuming Vref(-) is 0V.
If the reference is 3.3V/0V you'd expect a 1V input to produce a digital word of 310 base 10, so something is seriously wrong. I suggest you carefully read the reference manual chapter on the 10-bit ADC as linked here. The data sheet is generally sketchy in the operation of many things.
As @Ignatio Vasquez-Abrams points out, adequate \$T_{AD}\$ is very important.
Best Answer
Your equation is a bit confusing. I would write it as:
$$V_{unknown} = \frac{ADCreading}{2^{12}} V_{ref}$$
where Vref is your microcontroller's supply voltage.
In other words, you have to know one voltage in order to measure the other. You either know a priori what Vref is, and calculate Vunknown from that, or you can connect Vunknown to a calibrated standard and solve the equation for Vref:
$$V_{ref} = \frac{2^{12}}{ADCreading} V_{unknown}$$