4-20mA to 0-3.3V conversion

Discussion in 'Embedded Systems and Microcontrollers' started by schnnegi, Sep 24, 2008.

  1. schnnegi

    Thread Starter New Member

    Sep 24, 2008
    9
    0
    Hi I am sachin.
    I am doing my project work using Zigbee protocol for wireless data transfer.
    Here I am using 3.3V power supply as it is low battery power application.
    I have to connect different sensora at ADC channels of PIC18LF4620 micro controller.Sensors output is in 4-20mA standard.
    How can I got 0-3.3V signal from 4-20mA signal.
    I made a circuit whose output was in this range but as I applied this to my ADC the voltage is dropped from 3.3V to 1.28V.
    Plz provide me circuit where there is no drop when applying this to ADC channel.
    Thanx
     
  2. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
    1
    Hi,

    I asked this question a little while ago and we never really got to the bottom of it. Basically, the easiest way is to stick with the topology shown in the following link.

    http://www.maxim-ic.com/appnotes.cfm/appnote_number/823

    You may use pretty much any high-side current measurement opamp combined with a subtractor that takes out the 4mA offset. You can use the circuit in the appnote, except you need to change the 472 to something available and reduce the gain level to give the 3.3V, instead of 5V.

    Steve
     
  3. schnnegi

    Thread Starter New Member

    Sep 24, 2008
    9
    0
    Sir Can we use LM324 for I-V.I used this but the output is not varying with varying the resistance on feedback.
     
  4. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
    1
    Can you post the circuit you used?

    Steve
     
  5. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
    1
    Oh, do you mean replace the MAX472 with the LM324? The MAX472 is a special opamp that can handle high common-mode voltages, I don't think most opamps can do that.

    Steve
     
  6. schnnegi

    Thread Starter New Member

    Sep 24, 2008
    9
    0
    Sir first i used a crud method directly connecting calibrator across 165Ω resistor,then see output voltage.For 20mA output is 3.3V.thats ok
    When i connect this to ADC channel(ADC & μC are operating at 3.3V and 8 bit ADC is used) i should get 255 for max voltage 3.3V.But here is drop and i am getting 99 (1.28V).
    when i am applying 3.3V from regulated power supply output is 255.

    Second i used LM324,where i gave 3.3V supply and simply operate this as inverting mode.Applied current source a/c inverting i/p and GND.And non inverting i/p to GND. And used a 1K preset pot on feedback.
    but I saw no change when i am varying pot setting.Output for 4mA was 1.2V and for 20mA it was 5.54V.I also applied different resitor instead of pot but reading is same.

    i am also trying with LM358,OP07 but no satisfactory output.
    Thanx to support me
     
  7. mindmapper

    Active Member

    Aug 17, 2008
    34
    0
    You need an instrumentation amplifier to be able to measure signals that is floating in respect to ground. I've been using Burr-Brown INA 122 to solve similar measurments.
     
  8. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
    1
    Are you sure your still within your common mode range? A lot of opamps cannot tolerate such high common mode voltages.

    Steve
     
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