I use two ADL5201 in cascade as presented in the figure.
The question is that the second stage performs well whose gain margin is -11.5dB~+21dB, while the gain margin of the first stage is only -17dB~+14dB, which is 5dB smaller than the standard. When testing, I compare the differential output of ADL5201 with the differential input of it.
The layout of the two stages are almost the same, and the first stage has the same result when it is tested alone. So I think there is something wrong with the design of the first stage, but I can't figure it out. Could you give me some suggestions?