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Multiplying DAC: Current Out

Discussion in 'Electronic Design' started by Uday Godbole, Apr 4, 2006.

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  1. Uday Godbole

    Uday Godbole Guest

    Hi,

    I have a board that needs eighty 8-bit MDACs with parallel load and current
    out, 4-quadrant multiply with individual Vref and feedback resistor. Analog
    Devices' DAC8408 (quad) fits the bill, but I am not happy with the
    footprint. (28-pin SOIC / PLCC). Twenty chips take up way too much space.
    Looking for something much smaller. Google throws up thousands of pages,
    but most of them point to the 8408. (Interestingly, DAC8408 doesn't even
    appear on Analog Devices parametric search results, even with "List all
    DACs" option!)

    I'd appreciate any pointers.
     
  2. Unless extremely high speed is an issue, the normal
    DACs are serial load. A octal DAC is available in a
    TSSOP16 case. Eg the LTC1665 for 3.5$ or so.

    Rene
     
  3. Uday Godbole wrote...
    The dac8408 (or dac-8408) is an ancient PMI part, introduced in
    1990 or before, portions of which Analog Devices discontinued in
    2003. It might be unwise to use it in a new production design.

    You mention size, but do you also have a price point you have
    to meet? I notice the ad7568 gives you eight dacs in an SMD
    package, but 12-bit parts do cost more. It uses serial load.
    Requirements with lots of qualifiers can be hard to meet. :)

    I suppose you considered and rejected octal 8-bit voltage-output
    4-quadrant multiplying dacs?
     
  4. Uday Godbole

    Uday Godbole Guest


    Thank you, Win,

    The dacs are needed to control gain of low frequency amplifiers and corner
    frequency of analog filters. So I do need the current output. Price is not
    a big deal: This is a low volume industrial product. Serial load is not an
    absolute no-no either, but I do need to load all dacs every 250 or 500
    microseconds (It is a multiplexed application), and the overhead of doing
    this would severely burden the onboard (8-bit) microcontroller. Of course I
    could use an FPGA to handle the loading, but that would negate the purpose
    of space-saving due to the large footprint.

    AD maintain the 8408 is still an active part, although the fact that it
    doesnt appear upfront indicates it is on the way out. As and when they kill
    it (hope its not anytime soon..) perhaps I'll have to make do with the 7568
    or 7564..

    If Maxim / Linear etc dont have an equivalent, I wonder what niche was the
    8408 designed to address..

    Uday
     
  5. Alternatively, there are R-2R network ladders, which
    together with a latch such as the 74HC573 form a
    multiplying DAC. Multiplying in the sense that the
    supply for the latch is positive within the family
    limits, which can be 1.8 to 5.5V for a more moderm
    one.

    Rene
     
  6. Uday Godbole wrote...
    If you use a filter configuration that employs summing junctions,
    you can use voltage-output dacs with output resistors into the
    summing junctions to get the dac-variable current gains you want.
     
  7. John  Larkin

    John Larkin Guest

    We did one product that used four 16-bit LTC mdacs. It turned out that
    they had way too much personality, and alternate mdacs are rare, so we
    just programmed the multiplies in an fpga and used simple bipolar
    dacs. At the 16-bit level, nobody can tell the difference. So, can you
    use some 12 or so bit dacs, and multiply numerically?

    John
     
  8. John Larkin

    John Larkin Guest


    Oops, I see now that you're controlling analog signal paths. Never
    mind.


    John
     
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