Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

ADXL calculations

  1. Jun 9, 2007 #1
    Dear experts,
    I'm trying to get an equation for tilt angle into a MPU. This is no problem if you can find the equation.
    The ADXL103 data sheet is indeed mysterious in that it is about a chip that measures acceleration. and yet gives no formula for the relation between input signal and the output signal. i.e. Input g and output delta mV

    I can see that when the chip is flat on the earths surface the output is zero , referenced to half of the supply voltage .

    This is the information, the useful starting information:

    So the DC output is 2.500 V at 0g
    The next part of the specification gives SENSITIVITY "typical" as 1000mV/g

    PAGES 6 to 8
    Great load of graphs on statistical information which shows sources of error. Great, but this is later when the beast is functioning.

    The theory of operation, but still no equation o/p = function(i/p)

    All good basic theory on filtering and responses, fine if we know the o/p we are filtering.
    Self test mentions that 750mg change in output produces a delta of 750mV, (typical I suppose )


    There is a slight whiff of an equation. Just looking at the x-axis response.

    "When the accelerometer is perpendicular to gravity, its output changes
    nearly 17.5 mg per degree of tilt. At 45°, its output changes at
    only 12.2 mg per degree, and resolution declines. "

    Going back to PAGE3
    'SENSITIVITY "typical" as 1000mV/g'
    Taking a mean for my small range of 0º to 15º ; (17.5+12.2)/2 =15mg per degree
    So if for example my tilt is 5º then the acceleration, Ax = 75mg

    Wait a minute, this is putting the cart before the horse, because
    I need to calculate Ax and then I can get the degrees from....

    PITCH = ASIN(Ax/1 g)

    If any one can see the relation between tilt_angle and the DC output of this chip, I would be most grateful.

    Other wise I'll just have to put it on a wooden table. Tweak the orientation for exactly 2.5000V
    Then carefully change the angle with respect to the table in 3 steps of 5º Measure the change in DC output
    and interpolate to get the 1degree steps from 0 to 15º

  2. jcsd
  3. Jun 10, 2007 #2


    User Avatar
    Science Advisor
    Homework Helper

    You posted the info you seem to be asking for from page 3. :confused:
    So if you turn the chip upside down you should get a reading of 0.5v.
  4. Jun 11, 2007 #3
    Dear Notime , I hadn't thought of that!
    The equation for output V I think I can deduce would have the form
    V = k2 *2500 + k1 *500*sinA for positive angles ;
    V = k2 *2500 - k1 * 500*sinA for negative angles,
    But I also note the ADXL202 is 1999, betteer to use current offering 2003, and it also looks simpler, the ADXL103

    Thanks anyway,
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: ADXL calculations
  1. MTBF calculation (Replies: 1)

  2. Calculating force? (Replies: 7)

  3. Beam Calculator (Replies: 2)