Improve resolution of accelerometers with lower ODR and oversampling

Q 

How to improve resolution of accelerometers ?

 

A 

The digital output filter is for averaging the signal and is set with output
data rate ODR. The lower ODR the more accurate is your signal. This is shown in
so called Root Allan Deviation graphs. The averaging period is reciprocal to
ODR. There you see the lower ODR the lower the error.

For instance in ADXL345 datasheet revC page 33 in figure 52 Root Allan
Deviation  is shown.

Not only averaging the signal, also oversampling improves the error. The offset
error goes down with √oversampling. For explaining this, I prepared two
examples for you, each on one slide. At both ODR is 100Hz.

Slide 1: On slide 1 the offset error is 2.9mg/LSB. Sensor data rate is 100Hz.
No oversampling. ODR = 100Hz.

Slide 2: Now sensor data rate is 3200Hz, at ODR = 100Hz. We oversample 32x. The
offset error is reduced with factor √32.  

When you compare both graphs, you see the output signal is still noisy (=same
ODR, BW) but the resolution is greatly improved.

We offer a document with explanation about oversampling technique to improve
ADXL345 output resolution. Please go to www.analog.com and search for AN-1063