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4-20 ma ADC IC needed

Discussion in 'Electronic Design' started by RG, Nov 15, 2007.

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  1. RG

    RG Guest

    I need to read the pressure from a transducer that puts out 4-20 mA. I
    was planning on putting an ADC across a precision 250 Ohm resistor to
    read the voltages from 1v to 5v. I need atleast 16 bits of
    resolution. I have been searching data pages for a good 16bit ADC but
    most ratiometric ones are limited to 2.5v. I use an AD7730 24 bit for
    some projects but it is limited to 80 mv I believe. Do I need to use
    a Voltage divider to get the voltages low enough or are there any 16
    bit ADC that can handle the 1-5V? I dont need very fast output, say
    less than 20 Hz.
  2. John Larkin

    John Larkin Guest

    Why not use a 100 ohm resistor and a 2.5 volt ADC? AD7793 would work,
    and has an internal reference.

  3. Fred Bloggs

    Fred Bloggs Guest

    No, don't do that. It is much simpler to stick with that 250 ohm resistor...
    There should be, but, if not, you can always go custom...The important
    thing here is to avoid off-value resistors or voltage dividers.
  4. LOL. What a card you are. ;-)

    Best regards,
    Spehro Pefhany
  5. john jardine

    john jardine Guest

    The serial AD7680 does 16 bits at upto 5V.
  6. whit3rd

    whit3rd Guest

    Precision resistors don't benefit from heating; your peak dissipation
    in that resistor would be a tenth watt, which isn't (IMHO) low
    Remember, a 0.1% resistor has temperature dependence up to
    six bits of your 16-bit converter!

    I'd also be uncomfortable at changing the voltage bias on the
    transducer by four volts; an op amp, couple of resistors, and a
    pass transistor makes a better receiver, and holds the sense
    terminal at pseudoground. Such a receiver is inverting, and
    a second (also inverting) gain/offset stage can condition the
    signal for your A/D converter.
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