10 years, 1 month ago.

F401RE Analog In oddness

I've had good success with an F401RE interfacing several kinds of displays, an RFID reader, and some other odds and ends. I'm trying to read voltage with analog in and now I'm running into problems.

I'm using two resistors as a voltage divider exactly as described here: http://startingelectronics.com/articles/arduino/measuring-voltage-with-arduino/

I'm using 1M and 100k resisters, again, exactly as shown in the above page.

Here is the code that converts the value from the AnalogIn to a voltage:

#define R1 990000.0
#define R2 98100.0
#define VCC 5.030
#define V(x) (((VCC*x))/(R2/(R1+R2)))

AnalogIn analog_value(A0);

pc.printf("Value: %f\n", V(analog_value.read()));

R1 and R2 are the actual values of the resistors I'm using. VCC is the measured reference voltage of the board.

So here are the problems I'm running into: I have to subtract 6.5 volts from the calculated value for it to read correctly. That makes no sense to me whatsoever why it reads 6.5 volts higher than what the math should result in.

When measuring VCC (5.030) and 3V3 I get really good values (again, after subtracting 6.5 volts from the value). However when I measure ground I get around 0.3 volts.

If I switch to a different analog in, like A1, then suddenly that 6.5 volt difference above drops to around a 2 volt difference (thus it is reading 2 volts higher than it should with A1, and 6.5 volts higher than it should with A0). I don't understand why switching from one analog input to another causes totally different results.

Any ideas on what is going on here? Something is definitely out of whack somewhere. Thanks!