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ADXL327 Accelerometer Calibration

Question asked by lakshit on Mar 15, 2016
Latest reply on Mar 26, 2016 by NevadaMark

I am using Analog Devices’ ADXL327 three-axis acceleration measurement system in a project that will ultimately be used to find the angle (or change in angle) of the PCB. The supply voltage is 3.3V and the ADC interfaced with the sensor is a 12-bit one with the conversion formula

digital result.png

Since the values of ADREFHI is 3.3 V and ADREFLO is connected to the ground, so the Value of the output voltage from the Accelerometer is

Vout = ((DigitalResult + 0.5)*3.3)/4096

Since output voltage from a particular axis (in this case, the Z axis) is a function of the angle made from a particular plane, I have arrived with the empirical formula:

Vout = Vzgbias + S*g cosϴ

 

Where Vzgbias   = Zero g bias voltage

            S          = Sensitivity (in mV/g)

            ϴ         = Angle from the plane

            g          = acceleration due to gravity

           

 

To calculate the angle for a particular output voltage, we can use the formula

 

ϴ = arccos ((         Vout – Vzgbias_____)/(S*g))

tilt.png

For a logical value of   ϴ, the expression ((         Vout – Vzgbias_____)/(S*g)) must always be numerically less than or equal to 1.

 

I have used the value of Sensitivity by taking the average of sensitivities at 3.6 and 2 volts. As per the device datasheet:

The ADXL327 output is ratiometric; therefore, the output
sensitivity (or scale factor) varies proportionally to the supply
voltage. At VS = 3.6 V, the output sensitivity is typically 500 mV/
g.
At VS = 2 V, the output sensitivity is typically 289 mV/
g.”

 

500 =  k1 * 3.6

289 =  k2  * 2

 

Taking the average of the proportional constant, k = 141.694

Thus sensitivity at supply voltage Vs = 3.3 V is

S = k *3.3 mV/g = 467.591 mV/g

 

And also, since according to the datasheet:

The zero g bias output is also ratiometric; therefore, the zero g output is nominally equal to VS/2 at all supply voltages.”

Thus Vzgbias at VS (= 3.3 V) is 1.65 V (for all three axis)

 

Is there any way to know if my assumptions about the ZERO g BIAS LEVEL and the SENSITIVITY are correct? Since, after measuring the ZERO g BIAS LEVELs of the X and Y axis using an oscilloscope, the values were close to 1.67 V. This may be attributed to a mechanical error or the surface on which

I have kept the sensor might not be completely horizontal as shown below:

horizontal.png

 

Furthermore, is it possible to obtain an application note for the accelerometer that shows how to measure the angle for all three axis and to calibrate the accelerometer?

 

Thank you

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