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Input impedance of NI 6602 Card

Hi All,

    Why is the Input impedance of the PFI's are Finite? In the data sheet it is mentioned that input impedance is 25k-500k ohms. I have measured the impedance of the PFI channel using a Multimeter and it shows that as 9k ohms. In any case (say even if 25k ohms), why is the input impedance finite?

When i connect my PWM signal to a PFI channel it is pulled down and so pulses ar not detected. I have had to use a pull up to count pulses. So, can any one explain why this input impedance is finite.

 

Thanks in advance

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