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+5v supply ... but need -5v to drive a DAC

Discussion in 'Electronic Design' started by pronforall, Apr 10, 2006.

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  1. pronforall

    pronforall Guest

    I have a circuit thats driven by a +5v supply ... but I need -5v
    (I could also use values between +-18v but since I have 5v ... I want to
    see what I can do with that)

    Does anyone have an idea how I might rig this up without having to buy
    a dc converter or changing my power supply?

    Thanks
     
  2. John_H

    John_H Guest

    Without a DC-DC converter (such as a voltage inverter, either inductor or
    charge-pump based) or changing your power supply, there is no way to
    accomplish your goal. You ask for a method to produce a voltage when all
    you have is another voltage; that's DC-DC conversion.

    If you mention your current requirements at -5V, someone might be able to
    recommend a voltage inverter or simple genereic DC-DC that will fill your
    needs.
     
  3. Tim Wescott

    Tim Wescott Guest

    With a tight interpretation of those constraints -- no.

    You need to allow for a DC-DC converter of some sort. These can be done
    quite economically and quietly, however, so they shouldn't be ruled out.
    It's been a while since I've had to care, but IIRC both National and
    Linear Technology have easy to use switchers that can be made quite
    quiet just by following their app notes. In the project that I saw them
    used we had them co-resident on a board with a bunch of 16-bit ADCs,
    with no apparent problem.

    You could also look at charge pumps. I couldn't guarantee that by the
    time you got one to be quiet in operation that it wouldn't be as big and
    complex as a switcher, but the actual converter part is simple and
    doesn't need inductors.

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Posting from Google? See http://cfaj.freeshell.org/google/
     
  4. Depends on the current you need. If you only need a few mA then you could
    use a charge pump. ICL 7660 or similar devices from Maxim. That's in deed
    same sort of dc/dc converter but not that kind you probably think of.
     
  5. no_one

    no_one Guest

    are the inputs to the ADC floating? (is the negative input lead isolated
    from "ground"?) if so just reverse the leads on the +5v supplied to the
    input to get - 5v.
     
  6. John Fields

    John Fields Guest

     
  7. pronforall

    pronforall Guest

    How much current do you need from the -5V supply?

    2ma should suffice
     
  8. pronforall

    pronforall Guest

    are the inputs to the ADC floating? (is the negative input lead isolated
    I tried this and it didnt work for me. Only +5 -5 (from another supply)
    worked
     
  9. pronforall

    pronforall Guest

    If you mention your current requirements at -5V, someone might be able to
    Point taken!
    2ma should suffice.
    Believe it or not, I was thinking of tapping the negative voltage from a
    PC' serial port ... since I only need it for the DAC and opamp, and I
    happen to have a lot of the PC mountable db9's lying around.

    Of course Ill invest in a dc converter ... perhaps for the next project.
    Looking forward to any and all advice.

    Thanks
     
  10. John_H

    John_H Guest

  11. Kryten

    Kryten Guest

    I'm reminded of the RS application note for the ancient ZN427.

    That chip also needed a negative voltage, nominally -5V, but this did not
    need to be very accurate as it was just providing bias for the comparator
    tail.
    See page 16 of this:
    http://www.web-ee.com/Electronic-Projects/data/zn427.pdf
    for a circuit that generates a continuous -5V rail.

    It was actually only needed during the A to D conversion, so a simplified
    circuit can just use the !CS signal to generate a single pulse of -5V.

    I can't find the circuit diagram right now but I do have a board that used
    that circuit. I can buzz the connections for you if you wish. It's just a
    transistor, a diode and a capacitor IIRC.
     
  12. Kryten

    Kryten Guest

  13. Ken Smith

    Ken Smith Guest

    He may be able to do it with a marking pen.

    Relabel the +5V as ground and the ground as -5V. Now he doesn't have a
    +5V. He may not need one if the part he is using is assuming ECL supplies
    and signalling.
     
  14. Ken Smith

    Ken Smith Guest


    Do you also want an RS232 I/O? If so add a LT1081 and a 3 pin regulator.
     
  15. jack

    jack Guest

    Depending on the amount of current you need you can derive + and - voltages
    from as low as 4 VDC supply using an LT1026 -- I have a couple tubes of
    these if you need http://www.tech-diy.com/smpschips.htm go 1/2 way down the
    page. It's a good idea to postfilter the output, and they like to work with
    a load of several ma.

    You can also derive an artificial ground with an opamp -- if you need more
    current you can use one of the huskier varieties from Burr-Brown.
     
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