Using op-amp to convert from current to voltage

Discussion in 'General Electronics Chat' started by Dritech, Oct 10, 2015.

  1. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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    Hi all,

    I need to convert 4 to 20mA to 0 to 10V using an op-amp. When web searching I found Current to Voltage op-amps like the one in the link below:

    https://upload.wikimedia.org/wikipedia/commons/e/ee/Current_Voltage_Converter.svg

    Why are all the current to voltage converter circuit inverting? Can it be used in a non-inverting setup?
    Also, if I use a 500ohms resistor I will get 10V for a 20mA input but 2V for 4mA input current. Is there a way to scale the output from 2V to 10V to 0V to 10V without using an MCU?

    Thanks in advance.
     
  2. ericgibbs

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  3. dannyf

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    Sep 13, 2015
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    Just put another inverter after the i-v converter and you are done.
     
  4. MikeML

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  5. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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  6. ericgibbs

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    Dritech likes this.
  7. ericgibbs

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    hi Mike,
    Its common practice to connect the bottom end of the Loop load resistor to 0V at the OPA end.
    Never had a problem doing it this way in a number of marine applications.

    Do you have an alternative method.?

    Eric
     
  8. MikeML

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    I always thought that the receiver for a 20mA loop should have a differential input with a relatively high common-mode capability?

    Can you have two or more receivers in a loop or is there always only one?
     
  9. Alec_t

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    Sep 17, 2013
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    Presumably if you had 2 receivers then one could ground its sense resistor but the other would consequently have to use a differential input?
     
  10. ericgibbs

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    hi,
    The marine Tidal Height products I designed and manufactured used a single immersed water pressure sensor, connected via a twin screened cable, with a internal atmospheric air vent pipe.
    The immersed sensor was powered via the same cable, with 12Vdc thru 24Vdc, the sensor modulated the loop current drawn by the sensor, from 4mA thru 20mA which was proportional to the Tidal height [immersed water depth].

    Eric

    added 'single'
     
  11. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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    Is it possible to post a screenshot of the circuit please?
    I downloaded the software but some of the components are not showing. Thanks.
     
  12. ericgibbs

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    This a screen clip of one of the circuits, let me know if its not the one you want.E
    upload_2015-10-10_18-50-55.png
     
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  13. MikeML

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  14. cadamhill

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    Oct 10, 2015
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    Can you explain why that is please?
     
  15. Roderick Young

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    Feb 22, 2015
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    Not to overly complicate things, but if you want it to be truly independent of voltage, you can use a high linearity optoisolator at the input. They're like 25 to 50 cents, and have a current output proportional to the current input, very good for the range of current you're considering (20 mA).
     
  16. ericgibbs

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    I would agree if I was using an Instrumentation Amp configuration for sampling across the 250R load resistor, but with the single OPA circuit which is commonly used, it is not essential.
     
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