The input bias current effectively adds to the measured current, a little like additional offset. One would see the sum of the voltage offset and the bias current. 23 µV (2.3 nA) looks like quite high. The typical current should be lower.
The bias current can vary between units and can also vary with the input impedance (in the higher frequency range).
In most cases on can tolerate a small offset and just subtract it. Some extra 2 nA should not be unnoticed in the mA range and hardly visible in the µAs.
One does not have to change both of the AZ OPs. Only the OP at the input is critical for the noise.
Ya, it seems that would make sence about the voltage offset, but why didn't dave see that voltage offest in his testing?
The decrease in noise is great improvement for it, but I think I want to change both ops because of their frequency range and the higher frequency of the chopper.
I figured it all out. I did away with the short functionality which allowed me to switch the +3V and the -3V lines at the same time. Here's what I did:
1. Used a 4 x AAA battery holder and added an extra wire (green) that's the center tap of the 4 batteries.
2. Cut away the ridges inside the plastic case using a chisel modelling knife and Dremel - the battery box just fits inside.
3. Removed U2, R6,R7,R10,C2
4. Replaced U1 and U4 with OP189IDBVRCT
5. Replaced U3 with TPS3809I50DBVR (4.55) volt version and changed R4 to 909 Ohms
6. Removed battery holder and cut tracks on PCB to remove shorting connection (see picture)
7. Soldered wires onto PCB (see picture)
On testing, I was able to set up a 0.5 uA current through the uCurrent and see 500 mV on the output (1mV/nA range) with my calibrated 34461A. It looks like it was all successful.
I've also got a question about this design change.
First, I think I'm going to use dual PS's at +/-10V instead of batteries because I'm using the PS's in my FG isolator anyway, so that would eliminate U2,R6,R7,R10,C2,U3,R4, the battery holder, and the LED. I'm thinking because the PS's would show the input voltages and therefore I don't really need a low voltage indicator.
What I don't understand is why you attached the negative to the switch and then cut the shorting traces to the switch. Doesn't this take the function of the switch away from Off/On/On(Shorted), to Off/On/On(Not Shorted)?
Can't the negative voltage wire simply go the negative battery terminal pad and leave the switch and traces alone?
Just in case, I included the schmatic of the uCurrent
Edited:
I figured out why you did that, because you are using batteries and not PS's that can be turned off.
I'm guessing that the ops will still drain the negative supply side of the batteries because their is no virtual ground.
I wasn't thinking about that because I had PS's in mind with their own on/off switch.