+ Post New Thread
Results 1 to 9 of 9
  1. #1
    Member level 1
    Points: 808, Level: 6

    Join Date
    Feb 2011
    Posts
    41
    Helped
    1 / 1
    Points
    808
    Level
    6

    Current to voltage converter using single supply OP-AMP

    Hi,

    I am trying to interface a pressure sensor with 4-20 mA output with an ADC. So I have to convert this current to voltage.

    But the problem is that I have a single supply of 5V and I want to use LM358 single supply OP-AMP for this conversion. I successfully implemented the circuit using dual supply(+5V and -5V) in inverting amplifier mode, but I was unable to implement using single supply(+5V).

    Please suggest how to implement the same circuit in non-inverting mode, so that I can convert this 4-20 mA to +ve voltage.

    Thank you.

  2. #2
    Administrator
    Points: 38,285, Level: 47
    Achievements:
    Created Album pictures Created Blog entry 7 years registered
    alexan_e's Avatar
    Join Date
    Mar 2008
    Location
    Greece
    Posts
    11,895
    Helped
    2093 / 2093
    Points
    38,285
    Level
    47
    Blog Entries
    6

    Re: Current to voltage converter using single supply OP-AMP

    Can you explain what was the problem with the configuration you tried?

    You have a 4-20mA source, you can use a resistor to convert it to a voltage and a differential amplifier to apply an offset and amplify the voltage or maybe you don't need the offset if the output is going to a microcontroler ADC and can be compensated in the code.

    Here is a differential amplifier
    Click image for larger version. 

Name:	DiffAmp.jpg 
Views:	43 
Size:	13.9 KB 
ID:	75004]

    Alex
    Please don't make requests for help in private using PM. Create a thread in the forum so that other members can benefit from the posted answers.

    Consider reading this before posting : How To Ask Questions The Smart Way


    1 members found this post helpful.

    •   AltAdvertisement

        
       

  3. #3
    Advanced Member level 5
    Points: 11,015, Level: 25

    Join Date
    Apr 2012
    Posts
    1,981
    Helped
    637 / 637
    Points
    11,015
    Level
    25

    Re: Current to voltage converter using single supply OP-AMP

    The opamp's input common-mode range includes ground, so you could use a circuit like the one below. The sensor's connections may need to to reversed, depending on whether it sources or sinks current.




  4. #4
    Member level 1
    Points: 808, Level: 6

    Join Date
    Feb 2011
    Posts
    41
    Helped
    1 / 1
    Points
    808
    Level
    6

    Re: Current to voltage converter using single supply OP-AMP

    Hi,

    I am thinking of two ways to implement the above circuit.

    First is in the first image, where I use a resistor to convert current to voltage and use Op-Amp buffer.

    Second is in the second and third image, where I use a differential amplifier to shift my output voltage to get better range for my adc, and use the formula mentioned in the image to calculate the output voltage. I think the circuit in Ck2 is much better than the circuit in Ck1.

    Am I correct? Please suggest.



    Click image for larger version. 

Name:	Ckt3.JPG 
Views:	63 
Size:	21.7 KB 
ID:	75024Click image for larger version. 

Name:	Ckt1.JPG 
Views:	60 
Size:	12.3 KB 
ID:	75025Click image for larger version. 

Name:	Ckt2.JPG 
Views:	57 
Size:	19.9 KB 
ID:	75026


    1 members found this post helpful.

    •   AltAdvertisement

        
       

  5. #5
    Super Moderator
    Points: 260,190, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    45,437
    Helped
    13823 / 13823
    Points
    260,190
    Level
    100

    Re: Current to voltage converter using single supply OP-AMP

    A typical 4-20 mA current loop configuration uses a positive supply of 15 - 30 V and a ground referenced shunt. Self powered 4-20 mA outputs are often ground referenced.

    But its's not the preferred way to pass the full loop current through the input amplifier. Many PLCs are simply using a 250 or 500 ohm shunt as passive I-V converter and no amplifier at all. Alternatively you can use a lower resistance value (e.g. 50 ohm) to reduce shunt power dissipation and achieve better loop supply voltage margin. Then a single supply non-inverting amplifier can scale the shunt voltage to the intended range and optionally apply an offset to get zero output at 4 mA.

    P.S.: I see, that your latest post is going the same way. All circuit variants are basically correct.
    Last edited by FvM; 29th May 2012 at 09:11.


    1 members found this post helpful.

  6. #6
    Advanced Member level 5
    Points: 11,015, Level: 25

    Join Date
    Apr 2012
    Posts
    1,981
    Helped
    637 / 637
    Points
    11,015
    Level
    25

    Re: Current to voltage converter using single supply OP-AMP

    I think the type of circuit to use depends on the kind of sensor you have. Is it OK to just pass the current through a resistor to create a voltage drop, or should the sensor be connected to a virtual ground?



  7. #7
    Member level 1
    Points: 808, Level: 6

    Join Date
    Feb 2011
    Posts
    41
    Helped
    1 / 1
    Points
    808
    Level
    6

    Re: Current to voltage converter using single supply OP-AMP

    The sensor has to be provided with a supply and ground.

    I want to use the op-amp to reduce any non-linearity that might occur. The op-amp buffer or differential amplifier will reduce such possibilities and provides good output impedance.



    •   AltAdvertisement

        
       

  8. #8
    Super Moderator
    Points: 260,190, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    45,437
    Helped
    13823 / 13823
    Points
    260,190
    Level
    100

    Re: Current to voltage converter using single supply OP-AMP

    The sensor has to be provided with a supply and ground.
    You mean, it's a three-wire sensor: V+, Gnd and current out?



  9. #9
    Advanced Member level 5
    Points: 11,015, Level: 25

    Join Date
    Apr 2012
    Posts
    1,981
    Helped
    637 / 637
    Points
    11,015
    Level
    25

    Re: Current to voltage converter using single supply OP-AMP

    Yes, as shown in the pictures in post 4.



--[[ ]]--