ADS1115 16bit ADC - Full-Scale Range to Vcc

[jonathanl100]

Hello everyone,

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

My questions:
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

[adafruit_support_bill]

1. You are correct. Since full scale does not correspond to VCC on this ADC, the terms do not cancel out.
2. No. The range is a function of the converter itself, the "1" setting is equivalent to no gain.

Available gain settings are:
2/3 ±6.144V(1)
1 ±4.096V(1)
2 ±2.048V
4 ±1.024V
8 ±0.512V
16 ±0.256V

[jonathanl100]

Thanks for the quick reply.

So this is where a good power supply results in a better accuracy?

[adafruit_support_bill]

That's pretty much true of all ADCs. But it is more of an issue for high resolution converters. No sense having 16 bits of resolution if you have 4-5 bits of noise and power supply ripple.