How to add up analogue inputs to average them?

[Stonestreecty]

TL;DR Summary

How read a sensor (acs712) a number of times, add them all up, then divide them by the number of reads?

Hi,
I would like to read a sensor (acs712) a number of times (20), add them all up, then divide them by the number of reads. So I can get an average. to give me a sort of analogue "debounce". Am I guessing the sum function?

I sincerely appreciate any advice you can all give me, I'm incredibly excited about learning all of this! Many thanks.

 

[berkeman]

Summary:: How read a sensor (acs712) a number of times, add them all up, then divide them by the number of reads?

I would like to read a sensor (acs712)

Some of the application circuits at your blog seem strange/wrong. Did you do the schematics or did you get them from somewhere. If the latter, where did you get them? Like this one for example looks wrong to me at first glance:

Summary:: How read a sensor (acs712) a number of times, add them all up, then divide them by the number of reads?

a number of times (20), add them all up, then divide them by the number of reads.

It makes it faster if you add up 16 or 32 of them before doing the division. Can you say why?

 

[Baluncore]

I agree with @berkeman, read it 16 times to make division easier, and to reduce the noise by a factor of √16 = 4.
If the bandwidth is 80 kHz then you must select a slow A-D converter, or wait about 15 μsec between reads. 16 conversions will then take 200 μsec.

 

[anorlunda]

I presume that you're talking about software, not hardware, to do the average. So coding the different ways of doing it could be instructive.

You could do it as a block average: organize into blocks of 16, take the average of each.

You could do it as a running average: with each new sample, compute the average of the most recent 16. Consider using a ring buffer for the running average.

You could do it not an average, but rather as a stream of samples through a low pass filter. How would you implement a filter as opposed to an average?

 

[Tom.G]

Like this one for example looks wrong to me at first glance:

That happens to be Application 3 on pg. 12 of the Allegro datasheet.
https://www.allegromicro.com/-/media/files/datasheets/acs712-datasheet.ashx

 

[berkeman]

That happens to be Application 3 on pg. 12 of the Allegro datasheet.

Thanks Tom, that helps. I still don't understand why they would suggest using an inverting opamp to amplify a signal that runs between 0V and 5V...

I guess you could just invert the readings from your ADC, but why not just use a non-inverting opamp configuration?