I’m trying to use analogRead() to read voltage in a project (eventually will read the battery supply voltage to a 328p) and am using the usual voltage divider and 1.1v internal reference setup, however my readings seem a bit low, although they are consistent at least.
If I use R2=4.7k and R1=10k, to drop the 3.3v supply (actually ~3.28v) to under the 1.1v reference, i get 3.20v from the sketch and VOUT (which is fed into analog0) is measured as 1.04v according to the meter, raw analog reading of 992 or 993.
if i use 470k/1M i get 3.17v reported (smoothed with a 100nF cap), VOUT=0.76v according to the meter (odd?!) and raw analog reading of 982 or 983.
i tried replacing my breadboard powersupply with a coincell that had about 3v left in it, and got 2.85v measured.
I wondered if I should divide the analog reading by 1023 or 1024, doesn’t seem to make a difference, I’d assume 1023 but some sites quote 1024. should i be multiplying the reading by the 3.3v maximum supply or the ~3.12 of r2+r1/r1 perhaps?
I’m using a mega2560, so added some code to use the correct reference voltage, grounds on the mega and breadboard are connected.
am i missing some maths in my code (i thought of something like the ratio of 1.1v aref to VOUT?) or are the inaccurate readings due to using 5% resistors or something, adding 5% to the readings makes them too high.
// vout = rb/(ra+rb)*vin
// 1.055 = 4.7k/(10k+4.7k)*3.3
// 1.055 = 470k/(1M+470k)*3.3 with 100nF cap
void setup()
{
// init serial monitor
Serial.begin(9600);
// chose 1.1v aref based on chip
#if defined(__AVR_ATmega2560__)
analogReference(INTERNAL1V1);
#else
analogReference(INTERNAL);
#endif
}
void loop()
{
// read from voltage divider
int reading = analogRead(A0);
Serial.print("raw: ");
Serial.println(reading);
// convert 0-1023 reading to 0-3.3v input voltage
float voltage = reading * (3.3/1023.0);
Serial.print("calc: ");
Serial.println(voltage);
delay(2000);
}