I'm studying how to properly design the ADC input of AD9258 and AD9268.
I downloaded S-parameters files available on the website and I would like to understand why those 2 ADCs show a different type of impedance: one rather capacitive, and the other one rather inductive:
AD9258 shows at 36MHz as impedance : 100 + j 1230 : a resistance and an inductance in series
AD9628 shows at 36MHz as impedance : 40 - 1270 : a resistance and a capacitor in series
On the other hand,
Page 3 of AN-935 (attached), they get as an equivalent model for the ADC input a resistance in parallel with a "negative capacitor" of - 4pF that turns to be consider as a "positive Capacitor" of +4pF which is compenasted by an inductance. That looks really weird to me.
Does anyone have an explanation about those 2 points?