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I don't have a circuit per se. Your quantity of interest is
the charge (and probably at multiple common-mode points
unless you have a very constrained application, such as an
always-ground-referred (-) input).
You would need to comprehend the application source
impedance and stray capacitance, if you want realism and
I would set up a very low input capacitance op amp in a
noninverting configuration, "sniffing" the input of interest
in a moderately high gain (A=100?) against a low-passed
image of that same input so you can measure at the op amp
output for step voltage. Your kickback charge is then
Ctotal*Vstep after the amp has settled (but some of the
input states might bleed the kick-charge quickly and you
might need to forego DC accuracy, for transient-following -
in this case, perhaps a low-C FET follower and calibrate it
yourself, using a charge injection network such as a 1pF
cap from a pulse gen).