Which is the optimum position for low passfilter?

Discussion in 'General Electronics Chat' started by picstudent, Feb 13, 2009.

  1. picstudent

    Thread Starter Active Member

    Feb 3, 2009
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    0
    My current measuring circuit goes like this .

    Shunt resistor - first opamp amplifier - second opamp for offsetting to my ADC range - low pass filter with Fc at 20Hz - then multiplexer - then ADC.

    I have 12 channels which goes into this multiplexer. should I use low pass filter between multiplexer and ADC or before the multiplexer as I lined out above?
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
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    Might as well get rid of the noise and unnecessary signals right away. Place the filter between the first amp and the offset amp.
     
  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
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    To eliminate as much noise as possible, the filter should be as close to the A/D (circuit-wise) as possible. This means putting it right in front of the MUX, unless the noise is so large as to overdrive the following stage (not likely). If you put it between the MUX and the A/D, you can save a lot of filters, but the filter will have to settle after each channel change before the conversion can be made.
    Keep in mind that a 20Hz LPF has a long settling time, no matter where you put it. Can you afford this when you are trying to sense a short circuit?.
     
  4. picstudent

    Thread Starter Active Member

    Feb 3, 2009
    76
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    Initially I was planning to do the short circuit protection in software only.Now I plan to use a opamp comparator based hardware short circuit protection also. The signal I tap from the output of the first signal amplifier. So in short circuit hardware will cut off the MOSFET and wait for sometime with a integrator before trying again. So the settling time wont interfere here. Is it ok?

    I will post a schematic soon.
    Thanks for the help
     
  5. picstudent

    Thread Starter Active Member

    Feb 3, 2009
    76
    0
    Here are my schematics and proteus simulation states upto filter.

    I have to draw filter and multiplexor later, after getting feed back.

    Thanks for the help
     
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