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Tue Oct 21 16:17:45 2008
AD797 Oscillation in the FSS board
I checked each op-amp's output in the FSS board to see if any indication of slew-rate saturation can be found.
PA85, which was the most suspicious one, actually has a very large slew rate limit (1000V/usec).
Its output swing was about 5V/usec. So PA85 was ok in terms of slew rate.
However, I found that an AD797 used at the first stage of the PC path was oscillating by itself, i.e. even without the loop closed.
The frequency was about 50MHz and the amplitude was large enough to reach the slew rate limit of this chip (the steepest slope was 30V/usec whereas the slew
rate limit of AD797 is 20V/usec).
I replaced it and another AD797 right after the oscillating one with AD829s. Just replacing the chips caused oscillation of AD829.
It was because there were no phase compensation capacitors connected to the pin 5 of AD829s.
Since the PCB was designed for AD797, there is no pattern for compensation caps. So I ended up putting Mica capacitors (47pF) across the pin 5 and the nearest ground point.
It worked and the oscillation stopped.
As I reported in an earlier elog, stopping the oscillation did not solve the problem of low FSS bandwidth.