[Larisa and Jenne]
We wanted to get rid of the awkward cart that was sitting behind the 1Y1 rack. This cart was supplying a +5V offset to the PZT driver, so that we could use the MC length signal to feedback to lock the laser to the MC cavity. Instead, we put the offset on the last op amp before the servo out on the Mc Servo Board. Because we wanted +5V, but the board only had +5, +15, -15V as options, and we needed -5 to add just before the op amp (U40 in the schematic), because the op amp is using regular negative feedback, we made a little voltage divider between -15V and GND, to give ourselves -5V. We used the back side of the voltage test points (where you can check to make sure that you're actually getting DC voltage on the board), and used a 511Ohm and 1.02kOhm resistor as a voltage divider.
Then we put a 3.32kOhm resistor in ~"parallel" to R124, which is the usual resistor just before the negative input of the op amp. Our -5V goes to our new resistor, and should, at the output, give us a +5V offset.
Sadly, when we measure the actual output we get, it's only +2.3V. Sadface.
We went ahead and plugged the servo out into the PZT driver anyway, since we had previously seen that the fluctuation when the mode cleaner is locked was much less than a volt, so we won't run into any problems with the PZT driver running into the lower limit (it only goes 0-10V).
Suresh has discovered that the op amp that we're looking at, U40 on the schematic, is an AD829, which has an input impedance of a measely 13kOhm. So maybe the 3.32kOhm resistors that we are using (because that's what had already been there) are too large. Perhaps tomorrow I'll switch all 3 resistors (R119, R124, and our new one) to something more like 1kOhm. But right now, the MC is locked, and I'm super hungry, and it's time for some arm locking action.
I've attached the schematic. The stuff that we fitzed with was all on page 8.