Problems Identifying VCCS circuit

Thread Starter

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.

 

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:

Thread Starter

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?
 

Thread Starter

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.

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.
 

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.
 

Thread Starter

Cannonball2134

Joined May 9, 2011
18
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.
 

crutschow

Joined Mar 14, 2008
23,140
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.
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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