Recently I recieved my ads1115. It's a replacement for the mcp3008 because it comunicates over I2C.
I want to hook up 4 10K thermistors (single-ended connections).
I followed the discription on http://learn.adafruit.com/thermistor/using-a-thermistor
It seems to me that it's not possible to cancel out Vcc when combining the 2 equations.
(1) Vo = R / (R + 10K) * Vcc
Where Vcc is the power supply voltage (3.3V in my case for the RPi)
(2) ADC-value = Vi * 65535 / Vcc
for the ads1115 with PGA SETTING 1 (+-4.096V) Vcc would be 8.192
1. Is it possible to cancel out Vcc (when combining those 2 equations) ór
2. Is it possible not to use the PGA so the Full-Scale Range ends at Vcc (Vcc = Vref)?
Now if I want to calculate the resistance of the thermistor I have to use the following equation:
R = 10K / (65535/ADC - 1)
with ADC = ADC-value * 8.192/3.3