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Analog signal through a digital CMOS switch

Discussion in 'Electronic Design' started by MM, Apr 3, 2008.

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  1. MM

    MM Guest

    Hi all,

    I tried asking a more specific question about using a LAN mux/demux but
    nobody replied. So, here is a very basic question. Is it OK to have biasing
    on one side of the switch only while having AC coupling capacitor on the
    other side? I don't care about transition time, switching happens only once
    before any operation starts, so "clicks" or "pops" are not an issue here.

  2. Guest

    This isn't the clearest post, but the cap coupling should not be an
    issue IF you someone establish a path to DC (generally ground).
    Floating nodes are not a good idea.

    LAN? Er, as in local area network?
  3. MM

    MM Guest

    Please read my other post titled "Applying Pericom PI3L301D-A (10/100/1000
    BaseT LAN Mux/DeMux)" from a day before.

    The purpose of the circuit is to mux/demux PHY between two LANs. Usually a
    transformer is used between a PHY and a cable. Some older PHY's required the
    center tap of the transformer to be connected to a power source, however the
    PHY I am using (VSC8201 by Vitesse) has fully differential active drivers
    that can both source and sink current, so when a transformer is used with
    this PHY, the center tap is AC coupled to the ground. The PHY outputs are
    biased to about 1.5V. Now, imagine that we insert a switch between the PHY's
    pins and the transformer (or AC coupling capacitors if they are used instead
    of the transformer). The side of the switch connected to the PHY will have
    biasing from the PHY, while the other side will not. The question is whether
    it is a problem from the switch's point of view (The switch can only pass
    positive voltages, hence the biasing is required)? As far as I understand
    when the switch closes I should see the same DC offset at both sides, so I
    am safe, right? Or not?

  4. MM

    MM Guest

    Well, whether there is need or not is another question isn't it? If there
    was no need TI and Pericom wouldn't have parts designed specifically for
    this purpose.

  5. MM

    MM Guest

    Nope. Check the datasheet and the Pericom appnotes (TI doesn't seem to have
    any)... It is intended for the PHY lines themselves. The part numbers I am
    particularly interested in are TI TS3L301 and Pericom PI3L301D-A, but there
    are more...

  6. MM

    MM Guest

  7. MM

    MM Guest

    So, in your opinion, is it a concern for the mux or for the PHY? The
    particular PHY I am using is a voltage mode and it doesn't require DC path
    through the transformer. It can happily work through capacitors. I just have
    to make sure the mux is happy too...

  8. MM

    MM Guest

    If you send me your email address I will forward to you some appnotes
    showing capacitor coupling. One obvious application is LAN over a backplane
    as in PICMG 2.16.

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