Measure 8 channel 4-20mA; isolated

Discussion in 'Embedded Systems and Microcontrollers' started by Vindhyachal Takniki, Jul 5, 2015.

  1. Vindhyachal Takniki

    Thread Starter Member

    Nov 3, 2014
    1. Need to measure 8 channel 4-20mA. Ckt is 3.3V powered.
    2. One option is to measure by adding a resistor of 165ohms (or 150ohm for error margin) & get 3.3V.
    But problem is I have to ground all the resistor connection to that of ADC.I want to keep all of them isolated.
    3. I have read abt receiver RCV420, but its minimum voltage requirement is higher.
    4. Is there any way to do it isolated.
  2. ScottWang


    Aug 23, 2012
    If you think the Rcv420 is match what you want, it just only the voltage problem, maybe you still can using RCV420 to get +5V and adding the resistors voltage divider as :

    1) +5V → 330Ω → (pin 1)VR500Ω(pin 3) → 180Ω → GND (The standard resistors are 340Ω + 660Ω)
    2) +5V → 330Ω → (pin 1)VR200Ω(pin 3) → 500Ω → GND (The standard resistors are 340Ω + 660Ω)
    Vout = 5V*(660Ω/1K) = 5V*0.66 = 3.3V

    So you can get 3.3V from VR200Ω(pin 2) or VR500Ω(pin 2) and you can also adding an op amp to be the voltage follower after 3.3V, the op amp was used for isolation and increasing the current, VR200Ω and VR500Ω are trimpot.