A/D Averaging

AK · Joined: 20 Apr 2004 Posts: 33

I'm using an averaging filter to help remove some unwanted noise in a 16 bit A/D system that samples a DC voltage that ranges from 0 to 1.024 Volts. The voltage will not change very quickly. Is there any standard number of data points that I should use when calculating the average?
Thanks,
AK

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

The more the better, but the answer only improves by the square root of the number of samples, and that is assuming the noise is gaussian.
What I often use is what I call "Olympic Scoring." I take a number of samples, often 10, and keep the maximun, the minimum, and the sum. Then I subtract out the max and the min and average the rest. The math and RAM consumption is minimal and you eliminate one outlyer either high, low, or both.
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PCM programmer · Joined: 06 Sep 2003 Posts: 21708

If you collect a bunch of data points, you can model your filter in Microsoft
Excel. Just use RS232 output from PIC to send data to a terminal
window, and then save it as a text file and import it into Excel.
Excel has many built-in averaging functions, or can write your own.
You can test different array sizes for your running average and see
which one will work best, based on your expected data. I did this with
a tachometer project and was able to choose a proper Median filter
and averaging filter.

kender · Joined: 09 Aug 2004 Posts: 768 Location: Silicon Valley

Another age-old trick you can use to decrease the amount of processing time is to acumulate 2^n samples and then shift the sum right n bits. This way you avoid time-consuming division. Keep in mind that you'll need a 32-bit variable to accumulate 16-bit measurements (assuming that you are using all of the A/D dynamic range).

AK · Joined: 20 Apr 2004 Posts: 33

SherpaDoug,

I used your "Olympic Scoring" filter idea and it seems to be working pretty well. I take 40 samples and remove the highest 3 values and the lowest 3 values and I keep the rest. Do you think this is overdoing it or that I'll be hurting my results?

Thanks,
AK

MikeValencia · Joined: 04 Aug 2004 Posts: 238 Location: Chicago

I like using PCM Programmer's median filter that he posted in the past. I use it with A/D values measuring motor current.

If I interpret it correctly, it takes the most recent 7 a/d measurements and gives me the middle value.

In my case, a simple average was not good for me because I have a lot of stray values that would also get averaged in.