# Problems Identifying VCCS circuit

#### Cannonball2134

Joined May 9, 2011
18
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.

#### Jony130

Joined Feb 17, 2009
4,925
But we don't see any diagram. So how can we help?

#### lokeycmos

Joined Apr 3, 2009
431

#### Cannonball2134

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

#### crutschow

Joined Mar 14, 2008
23,140
Edit: A Howland current source will generate a plus or minus current output. Here's a detailed discussion of their operation.

Last edited:

#### crutschow

Joined Mar 14, 2008
23,140
Just a bump so you will notice the edit to my previous post.

#### Cannonball2134

Joined May 9, 2011
18
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?

#### Cannonball2134

Joined May 9, 2011
18
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.

#### crutschow

Joined Mar 14, 2008
23,140
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.
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.

#### Cannonball2134

Joined May 9, 2011
18