Lowpass active filter failing at the lower range

Discussion in 'General Electronics Chat' started by atferrari, Jul 18, 2007.

  1. atferrari

    Thread Starter AAC Fanatic!

    Jan 6, 2004
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    Please let me ask about this without showing any circuit, YET.

    There is an active 4-poles Sallen Key lowpass filter. Cut off freq (-3 db) = 740 Hz. Both LM 358 opamps fed with +/- 9 V and duly filtered.

    At the input, the signal from a network of 12 resistors driven by CMOS shif registers generating sine sequence of 24 voltage steps. Their output is referred to ground and so input to filter properly centered on 0V.

    To my surprise, from 120 Hz to 500 Hz (upper limit of this design), filter output is remarkably smooth and looks OK, but going down from 120 to 10 Hz, the filter output is more and more jagged (one peak for every step at the input).

    My question= is it any typical reason for this filter to behave like this? What would you check first in a case like this?

    Just in case, please note that I tried two designs already: one with equal value resistors and the current one designed with Filterlab from Microchip.

    I am far from my bench so posting values/circuits is not feasible by now.

    My intention was to generate those steps in LT Spice to see what could be the reason but (see my post somewhere in this forum) could not find how yet.

    Do lowpass filter behave like this?
     
  2. hgmjr

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    Jan 28, 2005
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    I think I will wait until you can publish the circuit before trying to answer this one.

    hgmjr
     
  3. atferrari

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    Please give me some 48 hrs.

    Gracias
     
  4. techroomt

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    May 19, 2004
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    i am suspecting that a critical circuit impedance is changing on you, esp when you say "more and more". please post ckt.
     
  5. atferrari

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  6. hgmjr

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    The problem with your low frequency response is the presence of C5.

    C5 introduces a "zero" into the transfer function. This means that the circuit cannot pass DC. At the low frequency governed by the value of the capacitor and the resistor to ground, the response of the circuit will begin to drop off. This is causing the distortion in the output that appears at the low frequency.

    hgmjr
     
  7. atferrari

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  8. hgmjr

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    If loading of the network is a problem then I would suggest you introduce a unity-gain opamp buffer stage between the output of the network and the input to the first stage of the filter.

    hgmjr
     
  9. atferrari

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    I am slowly improving (my job precludes going faster).

    As you say, the network was loaded by the low impedance of the filter. To avoid redesigning it at this stage I inserted a unity-gain buffer. Steps are flat as in my first picture.

    Will come back when I get more good news.

    Gracias.
     
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