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Low pass filter

Discussion in 'Electronic Design' started by tiger66, Mar 11, 2007.

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  1. tiger66

    tiger66 Guest

    Hi All
    I need to connect a low pass filter to the output of a fully
    differential amplifier.

    I read a book and the first order low pass filter seem to only have
    one input. Does that mean that I need to create two LPF, one for Vout+
    and one for Vout-?

    Thanks
     
  2. You need to be more descriptive of the power levels and frequencies
    involved. If you're talking about line level audio, then download TI's
    FilterPro software. It does differential input filters too.
    http://focus.ti.com/docs/toolsw/folders/print/filterpro.html
     
  3. Jamie

    Jamie Guest

    I assume that you must not have access to the internals of this
    amp or you need to remove noise from the other end of the leads?
    And, if you don't have access to the input to where this is going?
    then you must make an inline filter.
    you didn't specify what spectrum you're trying to filter out?
    if this is spike noise ? a common mode choke will work, you simply
    pass both outputs through it..
     
  4. Tam/WB2TT

    Tam/WB2TT Guest

    No, you need a balanced filter. That assumes you are going to feed a
    balanced load. However, common mode problems might make the two filter
    approach better.

    Tam
     
  5. tiger66

    tiger66 Guest

    Thanks for the response

    I am trying to design a chopper stabilized amplifier and the last
    stage is a LPF. The stage before LPF is a modulator and its output is
    a modulated signal. So I try to use a LPF to keep the first harmonic
    and filter the other harmonics of the signal coming out of the
    modulator.

    My input is a 1khz, 5V DC signal, so I think I should have a LPF with
    cutoff frequency at 1kHz.

    Can you example what do you mean by pass both outputs through it.
    capacitor. The -ve pin of the opamp connects to one output of my
    modulator and the +ve pin of the opamp connects to ground.

    So, do you suggest that I should connect the other output from
    modulator to the +ve pin of the opamp instead of the ground?

    Thanks
     
  6. tiger66

    tiger66 Guest

    Hi Tam
    Can you example how I can implement a two filter approach?

    Do I need to make additional connection beside connecting one LPF to
    each modulator output?

    Thanks
     
  7. Eeyore

    Eeyore Guest

    How about a differential filter ?

    Graham
     
  8. Tam/WB2TT

    Tam/WB2TT Guest

    I have always used a balanced filter, but if your amplifier can drive the
    signals with respect to ground you might be able to use two filters . Can't
    think of any other connections you would have to make. Unless the two
    filters are absolutely identical, the phase shift and amplitude will be
    slightly different for the + and - signals, and things that should cancel
    out won't.

    Can't you use another op amp, and convert to a single ended signal? This
    would take care of the common mode, and give you a clean signal with respect
    to ground. From your original posting I couldn't tell if this was at
    microwave , or what.

    You really should simulate this in SWCad, or the like. For two filters, see
    what happens if the components are slightly different.

    Tam
     
  9. Ban

    Ban Guest

    You can simply take two equal resistors from the outputs into a common
    capacitor with half the computed (f=RC*2pi) value. An Instrumentation Amp
    would then give you the single ended output.
    IMHO if you do the switching right, you actually don't need a filter.
     
  10. Jamie

    Jamie Guest

    Hmm, i just replaced a Chopper Op-amp on a voltage sensing board
    at work, it just used a RC network to smooth off the edges.
    In any case, i would try a simple common mode filter and check it
    on the scope ..
    I'm assuming that you must be using a very square wave signal that
    you're trying to remove the harmonics that are getting generated from
    it ?
    Most likely, any filter you use is going to curve the left and right
    shades.. if that is fine with you, i think maybe just having a non
    inductive type resistor on each output would fix your problem.
    a little by pass capacitance should be applied on the output side of
    the R's
    I'm thinking 100 Ohm R should do it with like a .001 uf by pass, but
    that is just off the top of my head.

    filters like Active types may tend to take your nice square wave and
    truncate it. That has been my experience on that subject, it only makes
    sense that it would do that. But then again, the idea i gave you above
    may also give you some deforms thus giving you what i call a hook tooth
    wave, in which case an inductor added to the scene can handle that.
    I think you need to experiment a little.
     
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