Current Offset for Solar Cells

Goal: Cancel leakage current from the solar cells using a current offset source.

At IN2P3 the current offset is controlled with a current offset box that can be tuned manually. For the LSST CBP, we want to be able to control the current remotely. As such, we will use the voltage source on the Keithley 6517B. From the manual:

The V-source is isolated (>1 GOhm) from the measurement circuits of the Model 6517B when V-source LO can be (but does not have to be) internally connected to ammeter LO.

As sketched by Chris on the board on 9/23/2022 (note - there's still some debate about the proper resistors for the voltage dividers - could be e.g. 500k and 50):

In the above picture, L refers long cable. One key point is that the connection to ground should be at one point, and this point should be at the electrometer input. However, there is the option of connecting the Ammeter Lo to V-source LO. We probably don't want to do that (because we need a return cable from the solar cell anyway). Jim agrees that we should not use the internal connection and we should have the external ground connection in part because it will be easier to test things.

Generally, we want to be able to cancel about 2-5 nA of leakage current, which with a few kOhm solar cell resistance is ~ uV of applied voltage. If we use  300k/100 ohm resistors for the voltage divider, 1 uV corresponds to ~ 3 mV on the voltage source. That's lower than we'd like. If we do 500k/50, then we're at 10 mV at V-source, which still isn't really good enough resolution. But if we did 500k/10 for the divider, I think we'd be fine.