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Max038 Help

Discussion in 'Electronic Design' started by Motty, Oct 14, 2003.

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  1. Motty

    Motty Guest

    I am using a Max038 function generator IC in the configuration
    that the data sheet specifies for normal operation. I am just
    prototyping right now so I am using a 100k pot tied to Vref and ground
    to generate the Iin currents. For some reason my Vref is not at 2.5
    volts. It is around 2.1 V. I took the cap out that is tied from Vref
    to ground and the voltage went up to about 2.25. I am under the
    assumption that Vref should ALWAYS be 2.5V. Is this correct? Again,
    am prototyping on a proto-board so I know there are probably
    noise issues, but I've tried to be as careful as possible making
    short and keeping rogue capacitance to a minimum.

    I also had this problem some time ago when I first started this
    I was wire wrapping on a regular breadboard at that time. The same
    problem was occuring. The circuit seems to be working corectly
    I've tested it through an amplifier and it is defintely producing a
    sounding sine wave and the frequency is variable, but I can't
    design the Iin current, and thus the frequencies I want, if I don't
    what the Vref is going to be.

    I have three IC's and have tried them all. They all produce the same
    results. I have also looked at the evaluation board and how it is
    routed. I can't think of why the Vref is not 2.5V. It's an internal
    voltage reference so external parts shouldn't have an effect on it
    right (unless you exceed the 4mA Vref current rating)? Power is being
    supplied by lots of batteries and 7x05 voltage
    regulators that can supply the needed current to the chip. I might
    not have been as careful bypassing the power supplies of the Max038,
    but that shouldn't have a detrimental effect on Vref I wouldn't think.
    Please help. This is
    going to be a cool guitar effect pedal one day!
  2. There is supposed to be a cap from the reference_out to GND.
    You have that ?
    And it doesn't supply more than 4mA.

  3. Tim Shoppa

    Tim Shoppa Guest

    It's a buffered bandgap reference, so it really should be much closer
    to the 2.5V.

    Are you grounding all the ground pins? 6,2,9,11,15, and 18 all have to be
    at ground, and preferably at a low-impedance ground (something not
    necessarily easy on a breadboard). Check pin 17 to be
    sure you're really giving it +5V too.

  4. Motty

    Motty Guest

    Well, it basically boils down to me being an idiot. Most things do.
    I ran out of space on my protoboard and used a wierd place for another
    ground connection and forgot to tie a couple things to it. Great.
    Now I am getting around 2.475 - 2.499 volts. The low side is still
    lower than the data sheet says the minimum should be. I hope I
    haven't messed up the internals by leaving something floating...still
    getting a good sine wave out though. Thanks for the reply.
  5. SioL

    SioL Guest

    Good practice in troubleshooting your new design is to check the power
    and ground pins with an oscilloscope or a voltmeter. Oscilloscope may
    show you that you've forgotten to decouple a power line.

    If this is a very expensive chip or you only have one, install a socket and
    check the pins before you insert it. 5V parts usually don't like 12V.

    Helps eliminate the most obvious errors, only than the real troubleshooting begins.

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