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Switched Capacitor ADC Input Impedance

Question asked by YYHH on Feb 8, 2013
Latest reply on Feb 8, 2013 by J.Harris



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?