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NAND output saturation and malformed signal from LP filter

Discussion in 'Electronic Basics' started by Amanda Robin, Oct 8, 2006.

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  1. Amanda Robin

    Amanda Robin Guest

    I am breadboarding an amplifier/LP filter circuit using an INA125
    instrumentation amp, ECS 1 MHz clock to generate signal for the filter
    cutoff, a NAND gate to boost the clock output (thanks to Don Bruder for
    explaining this in an earlier post), and a Maxim 281 Bessel LP filter.
    The amp is powered at +- 15 V (using the breadboard's power supply), and
    I am using LM7805 and 7905 voltage regulators to provide +- 5 V for the
    rest of the chips.

    I breadboarded the amp, clock, and NAND gate successfully, with all
    components working properly. The trouble started after I added the Maxim

    At this point I have checked and rechecked my connections, but the
    following two problems remain: 1) the output of the NAND is saturated at
    +5 V rather than generating a square wave, and 2) the output of the
    Maxim filter is malformed. It is sinusoidal (like what I'm putting in)
    but it flatlines every time it crosses ground. The LP filter is working,
    however, with the right cutoff set.

    Like this, but with curvature to the sinusoidal part. (12 point Courier)

    * *
    * *
    **** **** ****
    * *
    * *

    It does this with other waveforms, too.

    I can input the clock signal directly into the LP filter, skipping the
    NAND, and the right cutoff is still set, but the output is still
    malformed. (Maybe I don't need the NAND, but since I'm eventually going
    to serve several circuits with the same clock, I'll keep it for now.)

    I have separated my analog and digital grounds (connected in one place
    only). Another thought I had was that since I am using a quad NAND gate,
    do I need to do anything to the other three gates? There are only 4 pin
    connections currently: 1A (input), 1B (grounded), 1Y (output), and

    I feel like it's an analog/digital problem, but I don't know how to
    isolate it since I have to have the clock chip running to use the Maxim.
    The thing I can think of to do next is to take the Maxim and its
    connections off the board to see if the NAND output corrects, and make
    sure it didn't get damaged by something I may have done. I don't want to
    just switch in another one in case my board is currently a Toxic NAND
    Gate Killer.

    Also I suspect the voltage regulators, but they do seem to be
    functioning correctly.

    I would be very grateful for any help.

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