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Filter topology and cuttof guess

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Hi audioguru,

Thanks for mentioning that. I do have to wonder however, why you
want to choose that filter response when it sounds like he wants to
filter the dc from a low frequency noisy input mixed with dc.
I could have misunderstood the application, but it seems to be that
we would want to reject all frequencies above dc as
well as possible and not attenuate any if possible. Forcing the
response to be -3db at the cutoff doesnt seem to have any useful
purpose for this app, again unless i misunderstood something.
He also didnt mention what kind of time profile he needed so i
didnt think that was important either.

BTW, there is another approach we have not talked about yet and
that is going totally digital (well almost). The implementation of
such would reduce the hardware requirement to one resistor and
one capacitor, regardless of the type of characteristic response
required.
This kind of implementation might require a bit more program code
memory but probably not a huge amount. A few calculations have
to be performed, usually done as part of the micro controller program.

Hi MrAL , thanks for the suggestion and also the previous one its useful . Actually I was unsure what to do to get a pure signal (DC) . The sensor I mentioned was Piezo (it not a raw sensor but a part of pressure sensor) . I measure fluid level with this sensor , which is outputting a proportional DC level at its outout. And when the vehicle moves the slosh imposes a DC output to flicker . Though I am doing other averaging technique whith my uC but still I am looking for how to make the output DC as smooth as possible. Also there is 5mV ripple imposed by the Pressure sensor.

I am having in my mind should I go as down as 2Hz-3Hz to correct this DC modulation due to sloshy environment.

audioguru , yes I require a low pass filter to smooth the DC as much as possible .

Steve, I will check what wrong I am doing as per simulation as you are getting it right!!

Regards
-Adi
 
If the cutoff frequency of the lowpass filter is too low then its output will be delayed and will change slowly.
 
Hi again,


Oh ok, so i guess the actual sensor here puts out dc. That changes the game
a bit and so you want a smooth output hopefully even in the event of some
noise.

We then have to know what is the fastest change in actual signal you have to
measure? In other words, do you have to measure every 1 second or every
10 seconds or every 100 seconds etc.?
This changes the way the filter can be set up.

Also, what kind of control do you have over the actual program that the
(presumably) micro controller will have programmed into it?
 
Hi again,


Oh ok, so i guess the actual sensor here puts out dc. That changes the game a bit and so you want a smooth output hopefully even in the event of some noise.

We then have to know what is the fastest change in actual signal you have to measure? In other words, do you have to measure every 1 second or every 10 seconds or every 100 seconds etc.?
This changes the way the filter can be set up.

Also, what kind of control do you have over the actual program that the
(presumably) micro controller will have programmed into it?

Hi MrAL , Refresh rate isnt critical 5seconds would be fine . I implement MAV topology . But the concern is also the internal noise and the slosh generated.

Regards
 
Hi again dark,


Well ok then, with the values i gave previously of 230k and 0.1uf
you should get some decent filtering and a response time of about
300ms to 99 percent accuracy. That 300ms is directly scalable too,
so if you change the RC time constant of all the R's and C's by a
factor of A then you multiply 300ms times A to get the new response
time. If you change all the C's to 0.2uf for example that will mean
a 600ms response time. For your app then if you change all the C's
to 1uf you will end up with a 3 second response time to 99 percent.

The cutoff is again f=1/(2*pi*R*C) in case you want to calculate that.
The response to about 99 percent of input is t=R*C*13 .

From those two we can see that when we lower the cutoff we increase
the response time and since when we lower the cutoff we increase the
dc filtering action there is a bit of a trade off between response time and
filtering action.

In the final circuit you probably want to change the last section to R=R/10
and C=C*10 to keep R a bit lower to work better with the AD input.
 
Last edited:
Thanks MrAL , and all others for the help.

Regards/
 
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