Okay than just a standard op amp may be able to do what you want.

Can the electrodes be isolated from ground or do they need a common ground connection?

 

Okay than just a standard op amp may be able to do what you want.

Can the electrodes be isolated from ground or do they need a common ground connection?

We usually use one of the electrodes as a reference electrode to be connected to gnd.
Note: all the electrodes are in a salty solution.

 

Note: all the electrodes are in a salty solution.

Are they all in the same solution which would mean they need a common ground?

 

Are they all in the same solution which would mean they need a common ground?

Yes all the electrodes are in the same solution, and one of them is used as a reference.

 

Okay, here's the circuit modified to deliver up to 25µA into a 200kΩ load.
It's the simplest circuit I could think of, using a quad op amp SMD package, and a quad transistor SMD package, so 4 packages for 8 channels.

I used a BJT instead of a MOSFET since they are cheaper and more readily available, but they will cause about a 1% reduction in the output current versus the input current due to their required base bias-current.
If that's a problem I can go back to the MOSFET.

The one requirement is that all the electrode commons (the solution?) must be connected to the -5Vdc supply instead of ground, but hopefully that will not be a problem.

The power supply voltages can be different, but the total magnitude of the plus and minus supplies summed must not exceed the 15V max rating of the op amp, with the minus supply voltage determining the maximum electrode current.

 

Okay, here's the circuit modified to deliver up to 25µA into a 200kΩ load.
It's the simplest circuit I could think of, using a quad op amp SMD package, and a quad transistor SMD package, so 4 packages for 8 channels.

I used a BJT instead of a MOSFET since they are cheaper and more readily available, but they will cause about a 1% reduction in the output current versus the input current due to their required base bias-current.
If that's a problem I can go back to the MOSFET.

The one requirement is that all the electrode commons (the solution?) must be connected to the -5Vdc supply instead of ground, but hopefully that will not be a problem.

The power supply voltages can be different, but the total magnitude of the plus and minus supplies summed must not exceed the 15V max rating of the op amp, with the minus supply voltage determining the maximum electrode current.

View attachment 281724

Dear crutschow,
I am very grateful for your help, I was looking for these parts for a while.
Thanks a lot for your great help