MMA7361 Accelerometer: 3.3V and AREF

Using Arduino Sensors
1

Hi everyone,

I am in dire need of understanding the connection between the AREF pin on the Arduino board to the 3V3 pin of the accelerometer.

I am using the MMA7361 accelerometer and understand that it has a voltage operation of 2.2V - 3.6V but can be powered by 5V due to the in built voltage converter. I have been pretty much following these tutorials:

But I have also seen in so many forum threads and other tutorials the connection between the AREF pin on the Arduino board to the 3V3 pin of the MMA7361 accelerometer for greater accuracy and resolution?

So (1) what is the benefits of powering the accelerometer from 5V on the arduino board when couldn't the same effect be achieved by just connecting the 3.3V pin on the arduino board to the accelerometer 3V3 pin for the voltage supply and not connecting the AREF pin at all?

and

(2) does the AREF pin get connected to the 3.3V pin on the arduino board itself or to the 3V3 pin on the accelerometer, as these tutorials seem to say two different things?

Here is the datasheet for the accelerometer I'm using:

Any direction for someone still very new to Arduino would be fantastic.

Thanks!

2

If Aref is connected to 3.3volt, and set to EXTERNAL, then the 10-bit A/D range of 0-1023 is not spread out over 0-5volt (default), but over 0-3.3volt.
Sensitivity increases from 5volt/1024= ~4.9mV per A/D step to 3.3/1024= 3.2mV per A/D step.

You must not forget to set the analogReference to EXTERNAL in setup(), otherwise you will damage Aref.

You could use the 3.3volt pin of the Arduino, but if you use the 3.3volt power pin of the sensor then any tolerance of that sensor supply is compenstated for by the A/D (assuming the sensor is ratiometric).
Leo..

3

Thank you for the reply! That all makes much more sense than everything I have been reading.
I appreciate the help.