Problems Identifying VCCS circuit

Discussion in 'General Electronics Chat' started by Cannonball2134, Jul 28, 2012.

  1. Cannonball2134

    Thread Starter New Member

    May 9, 2011
    18
    1
    Hello everyone,

    I am having problems identifying and creating the below circuit. I know it is a Voltage controlled current source(VCCS), but i don't know how you achieve a 350uA + and - current from it. I have created Howland based VCCS's before but i can't figure this one out.

    Any help would be great.

    [​IMG]
     
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,962
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    But we don't see any diagram. So how can we help?
     
  3. lokeycmos

    Active Member

    Apr 3, 2009
    432
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    please repost the attachment
     
  4. Cannonball2134

    Thread Starter New Member

    May 9, 2011
    18
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    Sorry, the image disappeared after i posted the thread.

    [​IMG]
     
  5. crutschow

    Expert

    Mar 14, 2008
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    3,244
    Edit: A Howland current source will generate a plus or minus current output. Here's a detailed discussion of their operation.
     
    Last edited: Jul 29, 2012
  6. crutschow

    Expert

    Mar 14, 2008
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    Just a bump so you will notice the edit to my previous post. ;)
     
  7. Cannonball2134

    Thread Starter New Member

    May 9, 2011
    18
    1
    Hi crutschow,

    Thanks for the link, i have had a look over that document before.
    I know that a Howland controlled current source can output a + - current, but it
    seems from the above circuit diagram that it is outputting the + current on one line, and the - current on another. How do you do that with a Howland current source?
     
  8. Cannonball2134

    Thread Starter New Member

    May 9, 2011
    18
    1
    I might be wrong, but the + output might actually be the + and - current output, and the - output might be the return path? I still don't understand where you would attach the return path to, maybe to ground? Overall the diagram is not very clear.

    Here is a little more information about the circuit:
    I found it while reading a paper on bioimpedance, which injects a small current via a VCCS into the user through two electrodes, and measures voltage potential from another two electrodes.
     
  9. crutschow

    Expert

    Mar 14, 2008
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    Now I understand your question. The symbol is showing a differential type output isolated from ground. You can connect either output to ground to generate a ground-based current, which is what a Howland source provides. For your application I would think it should work fine to connect the output of the Howland source to the patient with the return line going to circuit common.

    If you actually need a true differential output current source you could use two Howland sources. Connect the drive signal to the positive input on one and the negative input on the other.
     
  10. Cannonball2134

    Thread Starter New Member

    May 9, 2011
    18
    1
    Thanks for your help crutschow,

    Why does it seem that the return path is attached to the VCCS? From what you are saying, the circuit would have made more sense if they had set the return path to ground/common.
     
  11. crutschow

    Expert

    Mar 14, 2008
    13,052
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    That's because the output is isolated from ground, so you can connect the two output terminals anywhere you like in the circuit, which makes the device more universal. All sources need a return path and where the return is connected is up to you. Thus you have the option of connecting one output to ground or not as the circuit may require.
     
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