Tolerance/ error in the relative angle between two accelerometer vectors. AIS328DQ

[Prathyusha]

Background:

I want to calculate tilt angle of my resting device (g is the only acceleration present) using the accelerometer(AIS328DQ) outputs. Consider initial accelerometer readiing as Aref and current reading as A. ​

I am using  realtive angle θ = arccos(Aref.A) (dot product & ||A|| = ||Aref|| = 1 g)

The major considerable errors in the Aref and A are offset error and sensitivity error.

Question:

1. I want to ignore the offset error, because it is the same for both the vectors and I am calculating the relative angle. Please ket me know if I am right.
2. I am using the calibration formula/technique mentioned in the AN4508 related sheet (since I couldn`t find related document for AIS328DQ)https://www.st.com/content/ccc/resource/technical/document/application_note/a0/f0/a0/62/3b/69/47/66/DM00119044.pdf/files/DM00119044.pdf/jcr:content/translations/en.DM00119044.pdf

 But when I solved for Δθ mathematically using the calibrated Aref and calibrated A, the offset error is not getting eradicated.

Please let me know if my assumption is wrong. and is there a material related to calibration technique specifically for AIS328DQ

Thank you very much in advance.