Question asked by lakshit on Mar 15, 2016

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

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))

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:

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