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Single Supply Salen Key

Discussion in 'Electronic Design' started by [email protected], Aug 11, 2013.

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  1. Guest

    This is for an anti-aliasing filter. I found a good ref here

    http://www.eng.yale.edu/ee-labs/morse/compo/sloa058.pdf

    page 16 which uses split supply with two resistors as per usual.

    How does that compare with say using one of those ICs that generates a negative supply and using that instead? (so that the op-amp has +=0=- supply lines from a single positive supply)
     
  2. Phil Hobbs

    Phil Hobbs Guest

    That circuit on P. 16 of the app note isn't just a lowpass filter--it's
    AC-coupled at the input. That's why it needs its own bias resistors.

    A plain-vanilla Sallen-Key lowpass is a DC-coupled noninverting
    amplifier with unity DC gain, making it a reasonable choice for
    antialiasing. Its output basically just follows its input, so you
    shouldn't need a separate source of bias.

    You do need to pay attention to what happens to the op amp's output
    impedance at high frequency, though. "Rail-to-rail" outputs are
    intrinsically high impedance, and rely on feedback to get low
    closed-loop output impedance. (LDO regulators have the same issue, for
    the same reason.) That means that the op amp's output impedance rises
    more or less linearly with frequency, i.e. it looks like a lossy
    inductor. At some frequency that will mess up your filter
    characteristic, so you need to calculate it fairly carefully.

    Some RRIO amps (e.g. the AD8605) use fast internal feedback around the
    output stage, which makes them quite a bit stiffer. Those ones make
    much better ADC drivers as well as better filters, compared with
    normal RRIO amps.

    If you use an amp with totem-pole outputs, you'll need that charge pump
    to get the output to swing to ground, but with an RRIO with stiff
    outputs, you should be able to get away without it.

    One very useful tip is to make sure that you have a small positive
    offset, i.e. that zero signal gives you an on-scale reading from the ADC.

    Cheers

    Phil Hobbs

    --
    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC
    Optics, Electro-optics, Photonics, Analog Electronics

    160 North State Road #203
    Briarcliff Manor NY 10510 USA
    +1 845 480 2058

    hobbs at electrooptical dot net
    http://electrooptical.net
     
  3. So, fig. 3 was ok?
    Hey Jim, don't take any wooden nickels.

    Harry
     
  4. Guest

    Not sure what you are saying here. I take a plain Salen Key low pass - but that needs +0- dual supplies. How do I run it from one supply? Just split the 0v with two resistors? Then I get a bias of V/2 at the output hence the ac coupling at the output to get rid of the dc bias - and at the input as you pointed out.
     
  5. Guest

    Ok what about this one in Ficure 12

    http://ww1.microchip.com/downloads/en/AppNotes/00682c.pdf
     
  6. Guest

    How about you just quietly returning to your coffin Vlad, I rarely if ever use MatLab.
     
  7. Phil Hobbs

    Phil Hobbs Guest

    From the op amp's POV, there's nothing special about single-supply vs
    bipolar supplies--all it cares about is that the inputs and outputs stay
    in range. (Op amps don't have a pin called 'ground'.) A "single
    supply" op amp is one whose input common mode range includes the
    negative supply rail, and whose output can swing nearly to the negative
    rail as well. That makes it convenient to run from a single positive
    supply, but no magic is involved.

    Since you're building a filter, it has to get its input from some
    earlier stage. I assume this input signal is also unipolar because you
    didn't ask about level shifting (e.g. going from a +-5V signal range to
    a 0-2.5V digitizer).

    The basic Sallen-Key lowpass is a voltage follower at low frequency, so
    if its input signal range is centered around 2.5V, say, so will its
    output be.

    Cheers

    Phil Hobbs

    --
    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC
    Optics, Electro-optics, Photonics, Analog Electronics

    160 North State Road #203
    Briarcliff Manor NY 10510 USA
    +1 845 480 2058

    hobbs at electrooptical dot net
    http://electrooptical.net
     
  8. miso

    miso Guest

    You are over thinking this. For the Salen and Key g=1, the op amp is
    just a follower (buffer). In fact, the wiki does a better job than that
    app note.
    One cap goes to ground, but any AC ground will work since it is just a
    capacitor. Obviously you would use the quietest ground you have.

    For any other Salen and Key, you would need the psuedo ground, but not
    for g=1.

    While Jim is right that the Salen and Key is a shitty topology due to
    component sensitivity, it may be suitable for anti-aliasing oversampled
    ADCs. That is, the ADC has its own anti-aliasing up to a point, thanks
    to the digital filter, but it is still subject to aliasing artifacts at
    the Nyquiest frequency of the sigma-delta modulator in the ADC. But
    thanks to oversampling, you don't need much of a filter (2 poles is
    fine) AND the corner frequency will be in the stop band of the digital
    filter. Basically the anti-aliasing filter corner will be far enough
    away from the in-band signal that you don't care too much about the
    crappy accuracy.

    Here is the procedure:
    1) Determine the sigma-delta modulator sampling frequency.
    2) Divide that frequency by two to get the Nyquist frequency.
    3) Pick an anti-aliasing type: Butterworth, Gausian, Bessel, etc. [Note
    the 2nd order Bessel doesn't ring if that is your criteria.]
    4) Using a table or hacking around with spice, insure that at the
    Nyquist frequency, your filter is down (attenuating) sufficient for the
    noise floor of the ADC. If you use some sexy 24 bit ADC, that would be
    around 110db.
    5) Since the Salen Key component sensitivity is horrible, see if you can
    live with the error. Alter the components for a worst case. If you are
    doing multiple channels, look at phase difference.

    If this Salen Key filter is for anything other than an oversampled ADC,
    you are just kidding yourself.
     
  9. Guest

    My ADC runs + o - 10v ie bipolar input and runs from a 5v supply. The source
    is from a stereo DAT player which I assume has been ac coupled at the output to remove dc.
     
  10. Guest

    Don't have sigma-delta adc on this board. I did on another one and it was easy to deal with. Won't be a single op-amp, I will need at least a 4th order or higher and probably to sacrifice a lot of bandwidth - I know its not what I would have hope for. Before sigma-delta we managed ok for a good 30 odd years. It is annoying that designers aren't putting in sigma-deltas as standard.
     
  11. miso

    miso Guest

    Don't have sigma-delta adc on this board. I did on another one and it was easy to deal with. Won't be a single op-amp,
    Those sigma-delta bastards made me find a new specialty. It killed the
    SCF business. The technology is good, especially for control systems
    since there are no missing codes.

    It is annoying that designers aren't putting in sigma-deltas as standard.
    The only place a shitty filter like a Salen Key is useful is before an
    oversampled data converter. In fact, if gain error is a big deal, I can
    make an argument that a g=1 SK is the right filter.

    But it appears now you are not using an oversampled converter. The
    filter error is now part of the error budget. Expect lots of variance
    due to using the wrong filter topology.

    Have you priced 1% caps?

    I'm outta here.
     
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