### Q

Can I achieve a resolution down to +/-0.05deg with ADXL202?

### A

Theoretically this is possible. In practice, however, achieving a resolutiondown to +/-0.05deg will be challenging. You are really asking about the

sensitivity of the accelerometer, that is, the smallest change in angle that

can be reliably measured. The sensitivity is limieted by the noise in the

accelerometer.

The noise in the ADXL202 is specified as 500ug/sqrt(Hz) typically. The total

noise therefore depends on the bandwidth. You as the system designer can

control the bandwidth of the accelerometer using the capacitors Cx and Cy.

Large values of Cx and Cy will reduce the bandwidth and reduce the noise. The

bandwidth is given by:

Noise bandwidth = 1.57 / (2 x PI x C x R)

Where C is the value of Cx or Cy, and R is the value of Rset which is nominally

32kOhm.

For a noise bandwidth of 1Hz, the total noise is 500ug, for a noise bandwidth

of 100Hz, 5mg.

You are measuring the angle of tilt by measuring the component of the

acceleration due to gravity which is along the measurement axis of the

accelerometer. The angle (theta) and the measured acceleration (X) are related

by simple geometry as:

X = Sine (theta)

Assuming you can limit the noise bandwidth to 1Hz, we now convert the noise to

it's equivalent angle:

Theta = sin-1 (X) = 0.029 degrees.

So you can get the resolution you require. However, you'll need to calibrate

the ADXL202 ( as described in the datasheet and app notes), which normally

consists of exposing the accelerometer to +/-1G, recording the maximum

excursion and calculating the calibration co-efficients. You'll also need to

take care with the temperature drift which is typically 2mg/degC. You can

either recalibrate when ever there is a change in ambient temperature or store

different calibration co-efficients for a range of temperatures and measure the

ambient temperature.