PCI6115 input impedance

[Dekay]

I'd like to model the input of the PCI6115 with LTspice. However, I'm having some difficulty decyphering the specs, in comparison with the manual's description of the AI connection. 

The specs (<10V) are:

AI+ to AI-:  1Mohm in parallel with 100pF

AI- to AIGND: 100Gohm in parallel with 10nF

AI+ to AIGND: 100Gohm in parallel with 100pF

 Fig 4-2 of the manual doesn't appear to reflect the specifications. For example, the figure doesn't show the 1Mohm resistor in parallel with 100pF, but has 1Mohm in series with 10nF from AI+ to ground.

A circuit diagram showing the relevant impedances, or an explanation of Fig 4-2 (p. 4-9) would be very helpful. Thanks.

[thuyanhl]

Hi Dekay,

Which manual are you talking about? I don't seem to see which figure you are refering to. Could you post the link to the document you are looking at? 

Thanks, 

[Dekay]

I was referring to the "NI 6115/6120 User Manual", Nov 2002 edition, part number 322812C-01. It's also in the S Series User Manual, Fig. 4-4 on p. 4-11: 

http://www.ni.com/pdf/manuals/370781h.pdf

[thuyanhl]

Hi Dekay,

Looking at figure 4.4 on page 4-11 of the link you posted, it seems that it corresponds to the specifications of the 6115. The input impedance between AI+ and AI- is 100pF in parallel with 1M. The input impedance for AI+ to ground is not shown in the diagram. Please find attached the diagram I am talking about.

Best Regards,

[Dekay]

It appears that the1Mohm resistor is in series with 10nF, where the 1Mohm resistor is connected to the + input but not the - input. From the specs I would expect it to be between the + and - inputs directly. Maybe the 'solder blob'  between the 1Mohm resistor and the 10pF cap is not shown in the right place.

[thuyanhl]

Hi Dekay,

You are correct. It seems like the 'solder blob' is not shown in the right place. 

Thanks for the feedback.

Best Regards,