Resolution and accuracy

Hi,

I converted the radian values into degrees given in the following paper page 4

http://www.nxp.com/files/sensors/doc/app_note/AN3107.pdf

( 12 bit ADC) :

for zero degree -----> 0.067 radians ------> 3.8 degrees

for 90 degrees -----> 1.63 radians ------> 93 degrees

The paper says that the 3.8 degrees is the resolution at the highest sensitivity point and 93 degrees is the resolution at the lowest sensitivity point.

What does it mean?

For 12 bit ADC, the 1LSB = 0.8mV. Does it mean that 3.8 degrees is equal to 0.8mV and 93 degrees is equal to ?? I am confused. Please provide some direction.

atan2() is much more precise and preferred for tilt sensing. See this application note. The important result is to use either equations 25 and 26, or 27 and 28.

It is very important that you calibrate the accelerometer for highest accuracy. Here is one approach.

Thanks for your reply! Can you help me understand the paper from nxp in my original post.

Here is what that paper says for the inferior method, which does not agree with your summary.

A 12-bit ADC cuts 3.3V supply into 4095 steps of 0.8mV for each step. Therefore, by taking one ADC reading of the MMA6260Q again at 0g (0° of tilt for an x-axis device), would now result in the following: 0°: 1650mV + 0.8mV = 1650.8mV 90°: 2450mV + 0.8mV = 2450.8mV This results in a 0.057 degree resolution at the highest sensitivity point (0°) and 1.63 degree resolution at the lowest sensitivity point (90°).

Work through the calculation for the specified 0.8 mV step in ADC input voltage (one unit) to see the change in output angle.

My inquiry is what is the resolution and accuracy of an accelerometer. ADXL345 , what is its accuracy and resolution if used with 10 bit ADC. I already mathematically calculated it using the paper I mentioned. But still does not understand it

With 10 bit ADC for one accelerometer axis only and ignoring noise that paper at top of page 4 says:

This results in a 0.229 degree resolution at the highest sensitivity point (0°) and a 3.26 degree resolution at the lowest sensitivity point (90°).

walraven92: Hi,

I converted the radian values into degrees given in the following paper page 4

http://www.nxp.com/files/sensors/doc/app_note/AN3107.pdf

( 12 bit ADC) :

for zero degree -----> 0.067 radians ------> 3.8 degrees

for 90 degrees -----> 1.63 radians ------> 93 degrees

The paper says that the 3.8 degrees is the resolution at the highest sensitivity point and 93 degrees is the resolution at the lowest sensitivity point.

What does it mean?

For 12 bit ADC, the 1LSB = 0.8mV. Does it mean that 3.8 degrees is equal to 0.8mV and 93 degrees is equal to ?? I am confused. Please provide some direction.

Also, you mis-read / mis-quoted the paper. Your first post says "zero degrees---->0.067 [u]radians[/u]..." and "90 degrees -----> 1.63 [u]radians[/u]" for a 12-bit ADC.

That's not right.

The paper says "This results in a 0.057 [u]degree[/u] resolution at the highest sensitivity point (0 deg) and 1.63 [u]degree[/u] resolution at the lowest sensitivity point (90 deg)"

Makes a bit more sense now, eh?

.