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Sallen key problem stability

Discussion in 'General Electronics Discussion' started by luke67, Nov 5, 2012.

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  1. luke67

    luke67

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    Mar 18, 2012
    Hello! I am developping a filtering board for analog signals (bandwidth of interest 300-5khz). I used for this purpose a lowpass Sallen key, a high pass sallen key (all butterworth configuration) and a passive RC low pass for adding a pole. I tried various Gain (3,5 , 5 ,8) on a breadboard and they all seemed ok but when I tried Gain = 10(10 for the first SK and 10 for the second. So the total is 100) I noticed a strange behaviour. (see the attachment - the comparison is with the Ltspice simulation). The strange thing is that if separated the two sallen key don't show any problem...
    Could be this due to sensitivity of circuit parameters to tolerance components ?
    Is maybe the impedance of the passivr RC too low ? It's only 143 ohm cause after that should be linked an ADC with impedance of 10khm
    thank you
     

    Attached Files:

    Last edited: Nov 5, 2012
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    11,517
    2,652
    Nov 17, 2011
    Looks like you have some resonances here due to the breadboarding.
    Try to arrang ethe filters in the reverse order: low-pass first, then high-pass. That way the low pass will filter out any unwanted high frequency noise. This may improve stability.

    You might also try to use a direct Sallen-Key band-pass configuration.
     
  3. luke67

    luke67

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    Mar 18, 2012
    yes! Now it seems to work! Do you know the exact reason why this happened ? Do you think that this problem could affect also lower gain (3, 5, 7 ... ) ?
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    11,517
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    Nov 17, 2011
    I can't tell you exactly what happened. Just that it is a good idea to first filter noise (low-pass) and then cut off the unwanted lower band (high-pass) because a high pass tends to magnify noise.
     
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