Designing a 4-20mA AO transmitter

Discussion in 'General Electronics Chat' started by gdallas, Sep 25, 2013.

  1. gdallas

    Thread Starter Member

    Apr 25, 2012
    Hi, wondering if someone can help clear up a few issues for me. I have a 3.6VDC battery powered device which at its heart is a PIC 18F. it uses one of its AI to detect linear position movement of a valve and displays a local indication of this. At times, the client may wish to remotely see the position through his supervisory system therefore one of the requirements are to build in 4-20mA output from the transmitter. I’ve opted to use an AD420 which simplifies my circuit somewhat. The PIC just speaks serially to the AD420 current transmitter which in turns outputs my 4-20. Now the onboard battery power is obviously insufficient to drive this, therefore my intention was to makes use of client loop power. I understand this is typically only 2wire, however, to make it work in my design, I think I need a 3 wire system as per what I’ve sketched up (attached)
    I’m galvanically isolating the current transmitter electronics (AD420) from the main sensor electronics; PIC etc due to having to use two power sources at different potentials. This in turn makes leads me to the conclusion that I will indeed require a 3 wire system to include the clients gnd power my isolator and also my AD420 itself.
    Would you agree? Would appreciate feedback and guidance.

    Many thanks
  2. gdallas

    Thread Starter Member

    Apr 25, 2012
    Its ok, the model up from that, AD421 is more suited to my needs i think, and is a true loop powered device requiring only 2 wire. I'll use this instead.
  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
    Last edited: Sep 25, 2013
  4. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    The transducer is typically the loop source, and as such requires a DC supply and common, converting to signal current. Devices may then operate on the minimum current, each generating a voltage drop.