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newbie -- DAC 4-20ma output

Discussion in 'Electronic Design' started by Grenik, Jul 17, 2007.

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  1. Grenik

    Grenik Guest

    Hi all

    I am new to electronics and I am looking for a DAC that has 4-20ma

    I googled and one option I got was the AD421 but the problem is that
    it uses a mosfet that I cannot find anywhere the DN25D FET transistor
    from Supertex1.

    What solutions can you suggest to my problem? Are there any
    alternatives both to the AD421 or the fet?

    Thank you

  2. SP

    SP Guest

    Supertex depletion mode FETS are stocked by Mouser.
    Just add a voltage-to-current converter to any old DAC you like, or
    use PWM of a reference and a filter for your DAC.
  3. gravpoet

    gravpoet Guest

    What resolution do you need, what is the compliance voltage of your
    supply, what type of interface do you need for the D/A...?

    I don't recall there being any all in one solution when I researched
    this about a year ago. Most folks are building there own, myself
    included, with off the shelf D/A's, an op amp and MOSFET/transitor.

    If you are not comfortable building one, Google "voltage controlled
    current source" or you can try to Google "USB-3102" for a complete USB
  4. MooseFET

    MooseFET Guest

    You don't need a FET.
    Are you trying to make a "two wire" system where the power to run it
    comes from the 4mA?

    The basic current sink looks like:

    Vcontrol ---------!+\ !!------- Sink
    ! >----!!- N MOSFET
    --!-/ !!-
    ! !

    U1 is an op-amp that can swing to ground if needed and has an input
    range including ground. The MOSFET is just about any small power

    Parts are commonly added for scaling and stability.

    In self powered 4-20mA systems the resistor can't be as shown. It has
    to be in the power wiring of the whole system. The concept of the
    circuit is the same but there isn't the thing called "GND"

    Vcontrol ---------!+\ !!--------------- +Line
    ! >----!!- N MOSFET
    --!-/ !!-
    ! B !
    Local power -------------+
    Local GND --------------+-------------- -Line

    There is a problem with this in that VControl and U1 must all be above
    the local power. The second part is easy to solve by running U1 on
    +Line and the point labeled "B".

    Solving the first is easiest if U1 is a dual op-amp. The other
    section can be used as part of the circuit that translates the control
    signal up to the local levels. This can be just a current source
    feeding an inverting amplifier.
  5. Guest

    The FET sounds as if it is a Supertex deplection mode MOSFET

    Their current product range doesn't include any DN25D part, but it
    could have done in the past - e-mail them to ask for a data sheet.

    As other people have mentioned, any DAC can be used to set up a
    voltage which can be translated into a current by a voltage to current
    converter such as the Howland current source - to generate a unipolar
    4mA to 20mA current, the Howland current source is something of an
    expensive overkill and the sort of solutions shown in National
    Semiconductors application note AN-20 at figures 17 and 18 actually
    work better.

    AN-29 covers the Howland circuit at figure 27.

    hope this helps.
  6. Grenik

    Grenik Guest

    Hi again

    thank you for your answers

    I am new to hardware issues so I did not know all these details...I am
    working on a simple project with parallax basic stamp...I have to
    control a vfd that accepts 4-20ma...
    I searched on the net and I found that the ad421 was suitable but then
    I found out tha I cannot find the FET....

    Basically I was thinking to use a DAC chip controlled by the basic

    the resolution at the moment is not a problem....8 bit will suffice I
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