On Fri, Aug 12, 2011 at 5:52 PM, Jonathan Lin <[email protected]> wrote: > > 1) I was reading though the Shimmer_AccelCalibration document, and the > document cites the datasheet for the MMA7260Q, which is different than the > ones on the SHIMMER2R. Does the scaling values cited in this document, > particularly 744 (for sensitivity) and 372 (conversion to g) still apply to > the MMA7361? >
MMA7361 doesn't have a 4g mode (used in Shimmer_AccelCalibration), so you need to adjust the calculations accordingly using either 1.5g or 6g. 744 comes from the following: 4g sensitivity in the datasheet @ 3.3V is 300mV/g. So 2g = 600mV Scale to 3.0V, gives 545mV. Converting to bits based on a 3V 12bit ADC gives: (545mv/3000)*4095 = 744 > 2) The equation provided in Shimmer_AccelCalibration, to convert ADC units > to units of g, is: > > [1] sensitivity_error * (measured_value - calculated_midpoint) / 372 > > In an ideal case, this equation would be (measured_value - 2048)/372 > > However, looking at Section 9.1 of Shimmer User Manual 2Rc, for 1.5g, my > ideal equation is > > [2] (measured_value - 2048) / 993 > > Evidently, these two equations would return different values. Which equation > should I be following? 372 * 4g / 1.5g = 992... difference is g setting for calibration. > Can we assume that the SHIMMER2R is able to provide a constant 3.0V? For Accel, this doesn't matter as the Shimmer ADC and accel share the same voltage. There will be some droop at the very end of battery charge which results in a bit of extra power supply noise. If it matters to your application, just charge more frequently. _______________________________________________ Shimmer-users mailing list [email protected] https://lists.eecs.harvard.edu/mailman/listinfo/shimmer-users
