The EXC input for the digital injection is indeed an equal-ratio summing amplifier; it adds the excitation in along with the loop signal (admittedly, I'm not quite sure how it works when the invert is flipped, but this is how it is set up with Rana's modification to the current box, also). The input resistance for each of the inputs is 10K.

(By the way, the connections to the inversion relay on the first schematic I posted were wrong. They are right now.)

What I meant was that in addition to having this summing injection it would be nice to have a local sweep for driving the PZT while we are doing things like optimizing the error signal (with the loop open). This is actually the only "sweep" functionality that the universal box has by design, and Rana modified the box to enable the sweep input to go through even when the switch was off. I am just proposing that we implement these two things separately so that we don't see a weird effect when we switch the invert on and off (like we do now). Of course, we can always just drive the PZT or VCO directly with a FG, so I can understand your point that we don't really need to have the local sweep option. I guess we should talk about this some more and then decide if it's worth it or not.

About the AD829, I will read the data sheet in detail and figure out where we need room for more components.