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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


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    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,
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