Switched Capacitor ADC Input Impedance


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?


  • 0
    •  Analog Employees 
    on Feb 8, 2013 7:40 PM over 7 years ago

    Hi YYHH,

    The input impedance of these high speed ADCs can be modeled as a parallel RC combination of typically several kohms in parallel with a few pF of capacitance.  There seems to just be some slight inconsistencies across different products over time in how the impedance is presented, but in all cases it can be modeled as a parallel RC equivalent.Both the AD9258 and AD9628 should be treated in this manner.  I hope this helps.