Objective : To asses the 4 pole Butterworth low pass Anti-Alaising filter performance and it if is benificial for quadrotor application.

In the experiment, Accelerometer MMA7260Q, the sensor signal characteristics when connected without low pass filter was explored. In this experiment the sensor output after the 4 pole low pass Butterworth filter is sampled and compared with data sampled without low pass filter.

Board                           : Pushpak Quadrotor board.
Processor                     : Atmega644p 

Processor clock            : 20MHz
ADC clock prescalar      : 128
ADC Vref                      : 3V
Final ADC sampling rate : 1.848 KHz (limited by serial port transmission speed)


The figure below is plot of 1000 samples of the sampled data.


From the plot we can see that instantaneous noise, ie variation from one sample to next sample, is one only one LSB unit. The low pass filtered signal has significantly less noise than the signal without low pass filtering. The picture becomes more clear when only 50 samples are plotted as in the figure below. The picture clearly shows only 1 LSB bit toggling, which a significant improvement over signal with no low pass filtering.



For easier comparison the sampled data with and without low pass filter is plotted together in the figure below(1000 samples). Note that the data was sampled in two different experiments. For easier comparison the data is plotted using the same scale for both series. In order to plot the data on the same scale and axis, average of the 1000 samples of the data is subtracted from the each sample and plotted.

 

To form a complete picture here is compairson using histogram plots. In these plots the ADC sample with highest number of occurances is shown as zero on x-axis. The y-axis shows the number of times the value appears in the data set. The same 1000 samples used in all the earlier plots is used in this histogram plot.




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