AD7634 Pin capacitance

Question

Hi everyone, 
 On page 20 from AD7634 datasheet it's stated that "during the acquisition phase for ac signals, the impedance of the analog inputs IN+ and IN-, can be modeled as a parallel combination of Capacitor Cpin and the network formed by the series connection of Rin and Ci n. Cpin is primarily the pin capacitance ". This is showed in Figure 28, but there is no information about the value of Cpin through the entire datasheet 
 Is this value just missing, or it should be measured by me? In any case, could someone give me some kind of expected value for it? I'm currently designing the front-end for this ADC, and I need this information to simulate some circuits. 
 Thanks in advance, 
 Gabriel

KarenNE · Accepted Answer

Hi, Gabriel. 
 The analog input s-parameters data which tabulates the analog impedance values over frequency is available for specific products only. We do not have one for AD7634. 
 What you can do is try the mathematical approach. The first attachment in the thread I mentioned talks about this and has supporting graphs showing that measured and mathematical approach have similar results. Although not exact, it gives a fairly good approximation. 
 The AD7634 datasheet states that because of its high input impedance, it can be directly driven by a low impedance source without gain error. An RC filter is also recommended to improve noise filtering. Anyway, for your simulations, CPIN is typically a few pF, ~5pF should be ok. 
 Regards, 
 Karen

KarenNE · Answer

Hi, Gabriel. 
 CPIN, which is primarily the pin capacitance should be measured. It is typically a few pF. 
 There is a thread here discussing the importance of the input impedance with attached technical notes which might be of help: http://ez.analog.com/message/25508#25508 . 
 Regards, 
 Karen

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AD7634 Pin capacitance