# 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.

2. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,962
1,097
But we don't see any diagram. So how can we help?

Apr 3, 2009
432
7

4. ### Cannonball2134 Thread Starter New Member

May 9, 2011
18
1
Sorry, the image disappeared after i posted the thread.

5. ### crutschow Expert

Mar 14, 2008
13,052
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
13,052
3,244
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.

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
13,052
3,244
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.

May 9, 2011
18
1