Help setting up USB 6009 for experiment

[Bob_Schor]

Ah, I see the venerable 741 is still in use!  It looks to me like you have three gain-of-10 amplifiers, with two in series for a gain of 100.  You also amplify the + and - inputs by 10 before subtracting them, meaning if the gains aren't precisely equal (for example, if + gain is 10.1 and - gain is 10.0), when you subtract you won't get the exact difference.  

I'll let someone with more EE knowledge than I comment on the choice of resistors and the overall circuit design.  Get out your breadboard ...

Bob Schor

[ahsan2]

Thanks for the feedback Bob. I've redone the circuit and attached it below.

[johnsold · Accepted solution]

That is a better configuration than your first one.

The major issue is how much current can flow through the electrodes. The typical input bias current for the 741 op amp is 80 nA at room temperature but the worst case is close to 1 uA.  If that is too much current for the electrodes/electrochemistry of the cells, then you will need to switch to an op amp with much lower input bias current.

Lynn

[ahsan2]

I will be testing the electrodes with varying salt solution concentrations. The smallest current that will be flowing through the electrodes is 10 uA. For my purposes, I think the 741 is sufficient. However, do you know of any other op amps that have a smaller input bias current and yet aren't vastly more expensive than the 741?

[johnsold]

I have used the TL07x family from Texas Instruments for many circuits. It has typical 65 pA input bias current and 7 nA max over temperature.

Lynn