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How to make a sharp anti-alias LP-filter?

Discussion in 'Electronic Design' started by Elektro, Mar 14, 2006.

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  1. Elektro

    Elektro Guest

    I need to make an anti-alias LP-filter. The filters cut of frequency
    should be 110 kHz and it should ideally attenuate 98 dB at 250 kHz.

    The A/D-converter is a 12-bit SAR converter with a sampling frequency
    of 500 kHz.

    How is it possible to make such a sharp LP-filter? Should I use a
    passive L/C filter?
     
  2. Tim Wescott

    Tim Wescott Guest

    If I read your post right your highest frequency of interest is 110kHz,
    and you are trying to get 16 bits worth of rejection at Nyquist. But if
    your highest signal frequency is 110kHz then then you shouldn't really
    have to worry about signals between that frequency and the first alias
    frequency, or 390kHz. Is there something going on in your environment
    outside of sampling that makes 250kHz special?

    A passive LC filter won't gain you any attenuation over an active filter
    with a like number of poles, and at those frequencies it probably won't
    be smaller. Even if you can put your 98dB point at 390kHz you're still
    talking about 10 poles for a Butterworth filter, and probably not too
    many fewer for a Chebychev.

    Is there any way that you can push up the sampling rate? You should be
    able to get 12-bit ADCs that will do in excess of 1MS/sec; if you
    sampled faster then you could have a filter/decimate step in your
    digital hardware. The overall cost with the faster sampling rate may be
    lower than fiddling with such a sharp filter.

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Posting from Google? See http://cfaj.freeshell.org/google/
     
  3. Tim Wescott

    Tim Wescott Guest

    Forgot to mention -- there is a 5-pole continuous (_not_ switched
    capacitor) active filter out there. I just can't remember who make it.

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Posting from Google? See http://cfaj.freeshell.org/google/
     
  4. Elektro

    Elektro Guest

    Sorry, I meant a 16-bit SAR, not a 12-bit, sorry.

    And you are right about 390 kHz, I didn't think of that. So I need
    -98 dB at 390 kHz ideally.

    I must check if it's possible to select a different ADC with higher
    sampling frequency.
     
  5. Tim Wescott

    Tim Wescott Guest

    The 16-bit part makes it harder to find a higher sampling rate; there
    are parts out there but they're expensive.

    While you're pondering attenuation vs. frequency you may also want to
    ask if you need 98dB of alias rejection at the high end of your
    frequency range -- you may find that you need high accuracy at DC but
    that various other forces conspire to screw up your accuracy at high
    frequencies anyway, allowing you to relax the spec.

    And check the LTC1563 -- it's a monolithic 4th-order filter who's
    frequency you can set with one resistor; with another resistor you can
    turn it from a Butterworth into a Chebychev. With two or three of those
    you should be able to build your filter pretty quickly.

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Posting from Google? See http://cfaj.freeshell.org/google/
     
  6. Tam/WB2TT

    Tam/WB2TT Guest

    You might want to look at a combination of active and passive filters.

    Tam
     
  7. Sounds like an active Chebyshev to me ... given the limited information.
     
  8. Tom Bruhns

    Tom Bruhns Guest

    A sixth-order elliptical will do it, with 1dB or less passband ripple.
    We used to do things like that with op amps, but beware that you need
    to use a pretty fast op amp to get good performance. Presumably, you
    want distortion products down 100dB from full scale, too, or you're
    wasting effort on the AAF part. There are some freeware and trialware
    filter design programs out there which could help you explore both
    active and passive (LC) filters. The free filter program from aade.com
    is very decent for LC filter designs.

    But for a 110kHz bandwidth and 16 bits, could you just use a
    delta-sigma converter and make your antialias life much easier? I
    certainly understand that there are times when a delta-sigma isn't
    appropriate, but maybe it's at least worth asking.

    Cheers,
    Tom
     
  9. Ken Smith

    Ken Smith Guest

    I think you are thinking of Linear. The make a filter that can be shifted
    between 500KHz and 1MHz cut off.
     
  10. Iwo Mergler

    Iwo Mergler Guest

    That's quite steep. You may want to consider sampling faster,
    with less bits, and do some of the filtering and downsampling
    digitally.

    Kind regards,

    Iwo
     
  11. Elektro

    Elektro Guest

    I found a free filter design program at Linear called FilterCAD. It
    suggested a 1596-7 filter IC. It is a "Linear Phase, DC
    Accurate,Tunable, 10th Order Lowpass Filter" :) You set the cut of
    frequency with a resistor.
     
  12. Joseph2k

    Joseph2k Guest

    That is in the class called "brick wall filters" almost always implemented
    digitally.
    The high attenuation is in and of itself is difficult to realize
    practically. In the real world you would want to do about 24 dB of this
    filter in the analog world and do the rest digitally.
     
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