Accelerometer's Self-Test is not working [ADXL335]

Other Hardware General Electronics
1

Helo, i want to use an accelerometer (ADXL335).
Here is the datasheet of the system : https://www.sparkfun.com/datasheets/Components/SMD/adxl335.pdf

So i'm using a 3,3 V (not from my arduino but directly from a generator)
The code on Matlab is fine, nevertheless, i can't find the right value of tension when gravity is on the same axe for the three axes.

I should find for a 3,3 voltage :

1,65 V for 0g
2,2 V for 1g
1,1 V for -1g

thanks to the equation : acceleration = ((6g/3,3)*Voltage)-3g

However i find :

when gravity is on Y axe (opposite): (using U for voltage)

Ux = 1,22 V Uz = 1,2954 V

when gravity is on Y axe (same direction)

Ux= 1,2142V Uz = 1,283

I did the same experiment for X and Z axes

My conclusion is that :

for Og, we have like 1,28V
for 1g, 1,48V
and -1g, we have 1V

Can't find the issue, so i tried to do a self-test of the system.

I took an output pin from my arduino, then i applied 0,3V to my self-test's pin. This is the max voltage i can give to this pin according to datasheet. Nevertheless i kept finding the same results of my experiment explained above. I checked the pin, and it had 0,3V so i don't understand why i can't activate the self test mode...

Can someone help me please ?

Thank you

2

I think you are misreading the data sheet.

For the Self Test, you connect ST pin to Vs.

When you make the ST connection to Vs the outputs CHANGE about 0.325V

John

3

Hello, ty for your answer. That was my first plan, nevertheless i read that about the self-test on the datasheet p.11

"Never expose the ST pin to voltages greater than VS + 0.3 V.
If this cannot be guaranteed due to the system design (for
instance, if there are multiple supply voltages), then a low
VF clamping diode between ST and VS is recommended"

To make the system work, i need Vs = 3,3V.
So i just took an output and sent 0,3V to Self-test pin while i was sending 3,3V to Vs pin

What would you have done?

Thank you !

Anthony