Hello Don,

    thank you very much for your reply.

    My circuit have to measure voltage at the output of a transformer. The
input of the transformer is wired to the output of a (mains) power
generator. The power generator can be very bed, with lot of noises,
frequency flutuations and so on. In this situation, a perfect power
generator would give a perfect sine at 60Hz but this simply isn't true in
this case.

    The output of the transformer goes to a voltage divider (two resistors)
and to a full-wave precision rectifier with an op-amp. So, I don't need to
calculate the DC offset and my reading will always be positive.

    The PIC ADC will "see" something like this (sorry for the ugly
graphics):

        |
        |    **         **         **
        |   *  *       *  *       *  *
        |  *    *     *    *     *    *     *
        | *      *   *      *   *      *   *
        |*        * *        * *        * *
        +----------*----------*----------*---

    If I sample between zero crossings like:

        |
        |    **         **         **
        |   *  *       *  *       *  *
        |  *    *     *    *     *    *     *
        | *      *   *      *   *      *   *
        |*        * *        * *        * *
        +----------*----------*----------*---
                   ^---------------------^

    I will have a very precise reading. Now, what about if I sample during
this time:

        |
        |    **         **         **
        |   *  *       *  *       *  *
        |  *    *     *    *     *    *     *
        | *      *   *      *   *      *   *
        |*        * *        * *        * *
        +----------*----------*----------*---
                ^-----------------^

    The result wouldn't be meaningless?

    Best regards,

    Brusque

-----------------------------------
Edson Brusque
Research and Development
C.I.Tronics Lighting Designers Ltda
(47) 323-2138  /  (47) 9993-6453
Blumenau  -  SC  -  Brazil
www.citronics.com.br
-----------------------------------

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