Multifunction DAQ

cancel
Showing results for 
Search instead for 
Did you mean: 

PCI6115 input impedance

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.

 

0 Kudos
Message 1 of 6
(3,957 Views)

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, 

T. Le
Vision Product Support Engineer
National Instruments
0 Kudos
Message 2 of 6
(3,937 Views)

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

0 Kudos
Message 3 of 6
(3,935 Views)

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,

T. Le
Vision Product Support Engineer
National Instruments
0 Kudos
Message 4 of 6
(3,930 Views)

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.

0 Kudos
Message 5 of 6
(3,926 Views)

Hi Dekay,

 

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

Thanks for the feedback.

 

Best Regards,

T. Le
Vision Product Support Engineer
National Instruments
0 Kudos
Message 6 of 6
(3,896 Views)