Current to voltage conversion with level shifting for the current

Discussion in 'General Electronics Chat' started by Ajay1989, Feb 1, 2016.

  1. Ajay1989

    Thread Starter New Member

    Feb 1, 2016
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    0
    Hi Guys and Gals,

    I am working with a sensor which outputs current in the range of +/- 72 mA.
    I want to amplify (1.1976mA to 1.2024 mA) of the sensor output to (0 to 5V).

    I am comfortable with current to voltage amplification but I am not sure how to do level shifting for the current i.e. 1.1976 mA corresponds to 0V and 1.2024 mA corresponds to 5V.

    What I was thinking was, to connect the output from the sensor to a 1.1976 mA current sink (so that my 1.1976 mA to 1.2024 mA becomes 0 to 4.8 uA), and then apply 10^6 amplification using op-amp transimpedence amplifier. But I can not find any more information if this can be done.
    Any seniors could shed some light if this can work? Or is there any other simpler way to achieve this?

    Thank you in advance and please let me know if you need any more clarification from my side

    Regards
    Ajay
     
  2. PeterCoxSmith

    Member

    Feb 23, 2015
    148
    38
    Take a look at the instrumentation amplifier AD623. Apply a reference of 2.5V and the output will swing from 0 to 5v, centred on 2.5V, with an input scaled to +/-0.5V and a gain of 5.
     
  3. Ajay1989

    Thread Starter New Member

    Feb 1, 2016
    6
    0
    Thank you Peter for your reply.
    AD623 will very well work if the output from the sensor is voltage (please correct me if I am wrong)

    But my sensor generates +/- 72 mA and I have to extract 1.976 to 1.204 mA and amplify to 0 to 5V.

    Thanks
    Ajay
     
  4. PeterCoxSmith

    Member

    Feb 23, 2015
    148
    38
    use a resistor to convert current to voltage, calculate the resistance using ohm's law to scale +/-72mA to +/-0.5V...
    R=0.5/0.072...6.94R...nearest value is 6.98R 0.1% tolerance.
     
  5. Bordodynov

    Active Member

    May 20, 2015
    637
    188
    See CurrLim.png
     
  6. Ajay1989

    Thread Starter New Member

    Feb 1, 2016
    6
    0
    Dear Bordodynov,

    Thank you very much for your diagram. I assume I1 you drew is current sink of 1.2 mA. And I understood the 1 stage amplification circuit.
    But I still have couple of doubts.

    1. What is the use of R2, R3, U4 and BAT54S in this circuit
    2. How do we realize 1.2mA current sink in actual circuit i.e. how to realize I1. I saw one sample design and is attached here. Will this work?
    upload_2016-2-2_9-27-9.png

    Thank you

    Ajay
     
  7. dannyf

    Well-Known Member

    Sep 13, 2015
    1,778
    360
    I would add it. Analog solutions are dicy, as the resolution is high.

    An act with 20x gain plus Pam can do this easily, in a 8dip package.
     
  8. Ajay1989

    Thread Starter New Member

    Feb 1, 2016
    6
    0
    Thank you Danny for the reply. But I didn't understand your explanation here. Would you be able to elaborate a bit more?

    Thank you
    Ajay
     
  9. Bordodynov

    Active Member

    May 20, 2015
    637
    188
    On the twin Schottky diode limiter made. This protects the input of the amplifier. When operating in the linear range of the amplifier at the inverting input voltage is substantially zero. Therefore there is no need for precision current generator! Current generator can be replaced by a resistor. Used reference diode napyazheniem ~ 1.22V. For example LM4041-1.2, ZXRE125. With stable voltage across the resistor goes stable current. For the flowing currents of the current scheme is necessary to turn the resistor. It is also necessary to add an inverting amplifier. What is the current direction you?
    Current resistor I have divided into two parts. This is the usual resistor and multi-turn potentiometers.
    If you wish to limit the output voltage range (0V-5V), you'll need to add a precision limiter with operational amplifiers.
     
    Last edited: Feb 2, 2016
  10. Ajay1989

    Thread Starter New Member

    Feb 1, 2016
    6
    0
    Thank you so mucj Bordodynov for your explanation.
    Could you please share the .asc file for Ltspice IV ?

    Thank you once again

    Ajay
     
  11. Bordodynov

    Active Member

    May 20, 2015
    637
    188
    Get scheme and another. And many models of AD.
     
  12. Ajay1989

    Thread Starter New Member

    Feb 1, 2016
    6
    0
    Thank you for the files.
    Could you also share the models for BAT54S, ShuntRef and Pot. When I opened the file it was showing error as shown in the pic below.

    Thank you
    Ajay

    upload_2016-2-4_17-4-52.png
     
  13. Bordodynov

    Active Member

    May 20, 2015
    637
    188
    I am sorry. Replace standard.dio my file.
     
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