Question regarding 4-20mA current loop input

Discussion in 'Analog & Mixed-Signal Design' started by RamanathanR, Sep 3, 2016.

  1. RamanathanR

    Thread Starter New Member

    Sep 3, 2016
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    0
    Hi,

    I am programming a c8051F850 to take in both a potentiometer and a 4-20mA current loop input through a ADC (0-3.3v mapped to 0-1023 10-bit range). However not being a electronics/board design engineer i would appreciate some help on the following circuit (see attached schematic). I read P1.3 with potentiometer input (from J12 with pin 2 not being used) and everything works ok. However i am not quite clear on how the current loop input (from J11) gets mapped to my expected 0-1023 range within the program. As i understand from the schematic, the 4-20mA input is sent over a 10ohm resistor thus converting it to a 40-200mV range (subset of the 0-3300mV range the ADC is configured for) and thence to a subset of the 0-1023 range to pin P1.0. Is my understanding correct? What then is the circuit on P1.1? How is it related to the circuit on P1.0?

    --Ram
     
  2. AlbertHall

    Distinguished Member

    Jun 4, 2014
    2,603
    526
    The 4-20mA passes through R15, 120Ω, which gives 0.48V - 2.4V. R14/C15 is a low pass filter but doesn't affect the DC voltage.
     
  3. RamanathanR

    Thread Starter New Member

    Sep 3, 2016
    4
    0
    Thank you. Is this (mapped to subset within 0-1023 ADC range) what i get from P1.0? What is P1.1 then? J12 has note saying "+ Switch" so i am wondering as to how that interacts with J11 (if any).
    Appreciate any and all insights/suggestions. Our hardware guys are all gone and hence the software guy (i.e. me) is now learning electronics to get this to work. I have got everything working other the current loop input :)
     
  4. AlbertHall

    Distinguished Member

    Jun 4, 2014
    2,603
    526
    P1.1 and P1.3 are signals from that box labelled 'variable resistor + switch'. As I have no information on the connections inside that box what follows is my best guess.
    P1.1 is 0 - 3V from the potentiometer
    P1.3 is 0V or 3V depending on the position of the switch.
     
  5. RamanathanR

    Thread Starter New Member

    Sep 3, 2016
    4
    0
    J12 (pin 2 is unused) is just a potentiometer connector. I read my MCU ADC via pin P1.3 in code for potentiometer state and process accordingly. This works. The confusion is w.r.t. J11. I think i need to read P1.0 for the current loop ADC. If so, what is the voltage range it is mapped to?
     
  6. AlbertHall

    Distinguished Member

    Jun 4, 2014
    2,603
    526
    P1.0 will give 0.48V - 2.4V for input currents of 4mA - 20mA. If your ADC is using a 3,3V reference that will give you readings of 149 - 744 from the ADC
     
  7. RamanathanR

    Thread Starter New Member

    Sep 3, 2016
    4
    0
    Thanks you.


    I got It to work by reading P1.0. 4-20mA over 120ohms gives me 480-2400mv. I configured ADC to range between 0-2.4v and adjusted the lower bound of 10-bit ADC from 0-1023 to 205-1023 in code.


    Still don't understand the schematic but that can only be learned in time :)
     
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