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Oscillator using AVR on chip comparator ?

Discussion in 'Electronic Design' started by richard mullens, Jan 30, 2005.

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  1. The AT90S2313 has two pins which can be used as the inputs of an analogue comparator.

    I propose to create an LC oscillator by placing a parallel resonant tuned circuit across these pins and by supplying energy via
    a resistor from an output pin (which would normally be tristated) when the comparator switches state.

    I am thinking of a 20kHz frequency with a capacitor between 10nF and 1 micro Farad with a 3V supply.

    Do I need to protect the inputs from the stored energy in the tuned circuit when power is removed ?
    Would it be good (or bad) practice to include a resistor between the tuned circuit and a comparator input.
    The configuration I suggest leaves the comparator inputs floating - is this a bad idea and if so what should I do about it ?

    I would like to achieve a stable frequency - as I would like to detect changes in the frequency when metal comes into the
    vicinity of the coil. Should I be employing a different oscillator configuration - for example a "centre tapped" capacitor like
    in a colpitts oscillator.

    On a more theoretical note, presumably such an oscillator will generate electromagnetic radiation - which I suppose one could
    model by a resistor in series or across the tuned circuit. Is this a misunderstanding on my part - or is it so negligable that
    that is why one doesn't see it discussed in models of a tuned circuit.

    Many thanks for your comments.
     
  2. Or at the expense of software complexity driving L & C from the same pins configured temporarily as outputs.
     
  3. Gert Baars

    Gert Baars Guest

    A good quality L and C might take the AC swing in excess.
    Putting 2 zenerdiodes in antiseries over the LC combination
    will limit the AC voltage. Putting a resistor across will
    also lower the effect of metal.

    Regards,

    Gert Baars
     
  4. Gert,
    Many thanks for your reply, it hadn't occurred to me that I might drive the resonance above the power supply rails.
    Richard
     
  5. Gert Baars

    Gert Baars Guest

    No big deal. If the opamp has J-FET inputs there is no
    significant load that's why. You might check for absolute
    maximum ratings.

    Regards,

    Gert Baars
     
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