AD7710 input impedance different to what is explained in AN-338?

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AD7710 input impedance different to what is explained in AN-338?

The datasheet for the AD7710 states that the input impedance is ver high.
However, in AN338 there is a different statement, the input impedance is gain
dependent and can be 288k for Gain =8 and higher, and is never higher than a
few MHz. The error we have is consistent with the 288kOhm.

Can you confirm that AN388 is correct and give any indication of how much that
288k might vary, for our application 10% variation would not matter (and maybe
even if it was more), we just need to know that it will not vary dramatically.

 

AN-388 is correct, assuming a sampling Capacitance of approximately 11.13pF.

However, the AD7710 datasheet is also correct and does state that the gain and
frequency influence the input impedance. The AD7710's d.c. input impedance is
>1Gohm.
The input impedance varies depending on the sampling frequency,fs, and this
frequency varies relative to the gain, which is:

fs = gain/(fclkin/256)

The input impedance, Zin = 1/(Cxfs) and if you refer to Table III, the input
impedance is as follows, where fclkin=10MHz and C=11.5pF (i.e. Cint in Figure
7) :

Gain=1, fs=39KHz, Zin = 1/(11.5pF*39Khz) = 2.229Mohms

Gain=8, fs=312KHz, Zin = 1/11.5pF*312KHz) = 278.7Kohms

As per the datasheet, the sampling capacitor's maximum value is 20pF, therefore
with Cs=20pF

Gain=1 => Zin = 1.6Mohms
Gain=8 => Zin = 1.28Mohms

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