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Differential mode analog active filter?

Discussion in 'Electronic Design' started by Vladimir Vassilevsky, Jun 30, 2007.

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  1. Hello All,

    There is a high precision ADC with the differential input (SNR ~ CMRR ~
    100dB). The incoming analog signal is differential also. However there
    should be the antialiasing filter in front of the ADC. The filter is
    Bessel 4th order. This is the high performance design, and the cost of
    the components is not a big issue.

    What would be the best way to do this circuit without sacrificing the
    SNR and CMRR performance?

    If we make the two identical analog filters for /+/ and /-/ paths, then
    the CMRR will suffer because of the limited tolerance of the components.
    I would expect problems mainly due to the accuracy of the capacitors.

    Making a filter with fully differential opamps does not seem to be very
    different from the two separate filters. There is the same limitation
    because of the tolerances.

    It is possible to collapse the input differential signal into the single
    ended using an inamp, then filter it and convert it back to the
    differential in front of the ADC. The CMRR will be limited to that of
    the inamp, and the SNR is in danger because of the conversion from the
    differential to ground referenced and back.

    I tried to design a schematic similar to the conventional filter but
    with the floating ground. A simulation shows problems with the CMRR and
    the frequency response; the bootstrapping of the power rails of the
    opamps towards the floating ground helps somewhat, however it rather
    complex and doesn't look as a very reliable solution.

    What topology should be the good approach to the problem? Can you
    suggest a book, an appnote or any other information?

  2. Tim Wescott

    Tim Wescott Guest

    The only concrete suggestions I have are negative -- if it's a
    sampling converter, watch out that whatever solution you adopt is low
    impedance on the output, and damps transient currents well. The
    high-performance, differential input SARs that I have worked around (but
    not on) inject a bit of charge back into their input pins when they
    sample; this charge is more or less random, and if it's effects persist
    until the next sample then it's just another thing that degrades accuracy.

    I'd help you more, but on the projects that I've done using high
    performance ADCs I've had analog circuit designers who were eager to
    interpose themselves between me and the signal acquisition chain, so I
    haven't had the opportunity to design one yet.

    Tim Wescott
    Control systems and communications consulting

    Need to learn how to apply control theory in your embedded system?
    "Applied Control Theory for Embedded Systems" by Tim Wescott
  3. Have you looked at passive LC filters with symmetry and no
    ground connections? I think the matching problem with this
    sort of design involves the stray capacitances to the
    common, not the tolerance of the parts. Of course, the
    filter will not remove common mode energy, so you are
    relying completely on the differential A/D input to remove
    the common mode signals.
  4. Guest

    When I was at IDT, we had a flash converter with performance very
    dependent on the op amp driving it. My recollection is on the DUT
    board we used some damn expensive Comlinear part which cost multiples
    of the DUT itself.

    I'd design the filter single ended using leapfrog design. Such designs
    tend to have the lowest THD due to the op amp being used with one
    input at ground.

    I've done fully differential leapfrog designs in switchcap
    implementation, where I have full control over the op amp. Obvious to
    most but just to be sure everyone is on the same page, the fully
    differential designs need to set the output common mode of each op
  5. colin

    colin Guest

    If all you want is an anti aliasing filter then if you make a filter with no
    reference to ground then you should not introduce any CMRR problems. maybe
    just a differential mode torroid choke and a capacitor and some resistance
    for damping.

    However if you want to reject common mode noise to improve CMRR this might
    be more difficult,
    but ordinary LP filters on each input should help and not affect the CMRR
    below their cutoff frequency.

    Colin =^.^=
  6. Fred Bloggs

    Fred Bloggs Guest

    It takes more to make a high performance design than just throwing high
    performance and expensive parts at it. You seem to have no concept of
    SINAD degradation of SNR, nor is it important to you to mention
    bandwidth. You either decouple the 4th order Bessel requirement from the
    anti-alias function or drown under the weight of the competent invention
  7. John Popelish wrote:

    The required inductance is at the order of several mH. Besides the
    inevitable problems with the mismatch, the inductors of that kind will
    act as antennas picking up noise.

    That's is exactly what happens when the conventional single ended active
    filter is used for the differential operation with the floating ground.
    The bootstrapping of the power rails helps, however it is cumbersome and
    the performance is limited.

    Straight to the point!
    Actually, it is a bit more complex. In the short, the analog filter has
    to suppress only the critical bands near the multiples of the sample
    rate; the rest of the filtering is done by the DSP. In the "low" bands,
    I can rely on the common mode suppression of the ADC. The "high" bands
    are attenuated by the other filters in /+/ and /-/ paths; the cutoff is
    high enough not to worry about the CMRR degradation in the band of interest.

    Vladimir Vassilevsky

    DSP and Mixed Signal Design Consultant
  8. This is little problem.
    Toroidal designs are pretty good in that respect.
    That you need a Bessel design and not an elliptical or
    Chebychev (lower Q resonators) helps a lot.
    With a floating differential filter, I would enclose it in a
    shield driven by the common mode voltage.
  9. colin

    colin Guest

    You can have a torroid with two equal windings wich should not present any
    1mh isnt hard to acheive. (bit like a common mode choke but with one winding
    the other way round).
    but above the cuttoff frequency of the differential filter the CMRR will not
    be so badly affected by the mismatch of two low pass filters before it.

    Colin =^.^=
  10. Hello Vladmir,

    This is an interesting problem! I recommend you read James Karki's
    app note SLOA54D especially the section about Multi Feed Back topology
    and enhanced CMRR. (Maybe you have read this already?) By the way,
    the THS4509 kicks ass!!!* It is an amazing fully differential
    amplifier. Look at the GBW product on that baby. I used it for an
    application and was extremely impressed with its performance. Oh
    yeah, use the most accurate caps and resistors you can find! but you
    know that. Good luck.

    Kadir "Solid Gold" Suleyman
    *No I don't work for TI.
  11. Fred Bloggs

    Fred Bloggs Guest

    Lessee 0.1% match preserves 60dB so ummm...0.001% match for 100dB CMRR?
  12. Yeah good luck measuring the capacitors that accurately, right? I
    didn't read that he needed 100dB CMRR just that his ADC was capable of
    this and he didn't want to 'sacrifice' the performance of his ADC.
    The graph of CMRR performance using the test circuit of the 4509 data
    sheet shows 90dB @ 100kHz using 5V supplies. At higher frequencies
    it is worse but it sounds like for his application the higher
    frequencies are of less concern. He never specified bandwidth. For a
    part that costs less than $4 in 1k quantity, I think the 4509 has
    pretty darn good performance but he's not going to get 100dB CMRR
    performance out of it without a having board manufacturable only by
    thaumaturgic obeissance.
  13. Guest


    Here is an example of the kind of inductors I am talking about:
    20 H max inductance.
  14. Jim Thompson

    Jim Thompson Guest

    Take a look at the gyrator-based filter configurations on the SED page
    of my website. You should be able to easily extend these to

    ...Jim Thompson
  15. Jim Thompson wrote:

    Thank you very much for the good advice. Yes, the gyrator topology works
    well for the differential filter.

  16. Jim Thompson

    Jim Thompson Guest

    And you can synthesize BIG inductors ;-)

    ...Jim Thompson
  17. I've been thinking about your differential input A/D
    converter and wondered if you might be able to make use of a
    differential amplifier with a differential output as a front
    end, something like this:

    ___ ___
    in+ -|___|-+----|___|-+
    | |
    | |\| |
    +----|-\ |
    | | >--+ out-
    | +-|+/
    | | |/|
    | |
    | |
    ___ | | ___
    in- -|___|----+-|___|-+
    | | |
    | | |\| |
    | +-|-\ |
    | | >--+ out+

    This circuit removes the common mode part of the signal to
    the extent that the two resistor pairs match in ratio. You
    can turn this into a one pole low pass differential filter
    by paralleling each of the feedback resistors with
    capacitors, or extend the concept into a pair of MFB 2 pole
    Bessel filters.

    I came up with this variation on the single opamp
    subtractor, today, but I don't remember ever seeing it
    before. Surely, something so simple and useful has a name.
    Does anyone recognize it?
  18. Rich Grise

    Rich Grise Guest

    Well, if it is new, you only have 364 days left to file for a patent. ;-)

  19. john jardine

    john jardine Guest

    AD have started pushing diff in, diff out, amps (eg ADA4938) for their fast
    ADCs but it's anyones guess what's inside.
    In LTspice I'm seeing curious results both in .AC and .TRAN, if the Rs are
    in the 10 to 1000 ohm area.
    The circuit is perfect using a couple of VCVS' (1Vpk inputs) but LTspice
    "ideal" single pole opamps, 25ma limit, Vos=0, seem to develop an offset
    output signal and 10% second harmonic. I've set something wrong but don't
    know what.
  20. Jim Thompson

    Jim Thompson Guest

    I see a positive feed back loop.

    ...Jim Thompson
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