ADXL345 accelorometer: what kind of numbers should i expect tapping sensor?

Hooked up the 345 to some example code. just flicking the sensor, im getting around 66 G's. does this sound right? it is a pretty small mass, but sometimes i can get it to go a lot higher also.

then again, im not sure if this thing is outputting in milli G or just G's. The code is pretty vague to me.

any info on this topic will help.


So one thing i noticed in the ADXL345 example code,

this is changing the range to 4G on the Data_Format register:
//Put the ADXL345 into +/- 4G range by writing the value 0x01 to the DATA_FORMAT register.
writeRegister(DATA_FORMAT, 0x01);

BUT this code here is using the equation for 8 G. so i changed the values to 0.0039 for 4G instead, this seems to be giving me more realistic results.

//Convert the accelerometer value to G’s.
//With 10 bits measuring over a +/-4g range we can find how to convert by using the equation:
// Gs = Measurement Value * (G-range/(2^10)) or Gs = Measurement Value * (8/1024)
xg = x * 0.0078;
yg = y * 0.0078;
zg = z * 0.0078;

arduinoADXL345 register.PNG

arduino ADXL345 range.PNG

im still getting data that im not sure is accurate or not. I know some of you other guys have played with these, could anyone post some excel values they are getting?

im going to order and hook up another one. i think this one may have some problems. it will read 0 G and then when i touch some of the VIAs on the pcb where my wires are connected, it will read consistent .11 G , even if the accelerometer did not move at all (still held down to the table).

I hooked up the ADXL326 Analog Sensor, here is some of the data i got while flicking it pretty hard, looks like you can spike 20G’s while flicking it. My sample rate is pretty decent too, the code is negligible so it just comes down to the serial print functions being the limiting factor.

It doesn’t seem like there is much help on the serial sensors, and a lot of people having similar issues with no resolution. A shame really.

Here’s some more sample data just tapping on the chip, the PCB is sitting flat on a desk and I am tapping right on top of the 16 pin LQFN chip, trying not to touch the analog pins with my fingers. looks like you can spike around 2 G’s

I also held it in the air by the wires attached, and tapped it, it got considerable more G’s this way, so i imagine if this is mounted cantilever on some stand off’s by the screw holes on the PCB, you would get maybe closer to 10 G’s with some hard tapping, the type you would do on your mouse if you were mad at the computer.