Accel mV signal conversion to Gs

Projects General Guidance
1

Hello all,

I am having some trouble understanding how to convert from mV to Gs using the method described by AWOL in the forum linked:http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1272149318/3

I am using an ADXL322, with a Vs=3.3v. Is the analog reference to the arduino analog pins 3.3v or 5v?
This other question may be related to my confusion with the first question:
Assuming my ref is 3.3v, then the increment should be 3.3/1024 = 3.22mV / increment. The accel sensitivity is 420mV/g. given that info: 420mV / 3.22 mV/increment = 130.4 mV / increment.
Ok, so with this information I know that 1G of acceleration gives me 420mV at the pin. AWOL then says you can subtract 420mV - 130.4mV/increment. however, they are different units. If I do that I get 290 increments as my zero which is not what I read at the accel. Reading the accel value I get something closer to 334.
But... If I do this using an analog reference of 5v, then I get 5/1024 = 4.88mV. Then 420mV/4.88 = 86. 420-86 = 334 and voila! it makes sense. My problem is, I cant get over the subtraction of the different units and the analog reference being 5v vs 3.3.

Any and all help would be greatly appreciated.

Thanks!!

2

The analog reference is by default the Arduino supply voltage, 5V for most Arduino boards. The fact the sensor is 3V3 doesn't change the Arduino's supply.

The accel sensitivity is 420mv/g (property of the sensor powered on 3V3), ADC resolution is 4.88mV (property of Arduino on 5V), thus the ADC outputs 86 counts per g when you connect them.

3

Ah, ok. So the accel, sends a 420mV signal to the adc, and it in turn converts that voltage into a value between 0 and 1024, using the conversion factor I provided the program. So when the adc reads 86, I get a voltage of 248 and 1g displayed. Am I reading this ok?