68 pin connector capacitance for PXI-6551

[ajog]

Hi All,

I currently use the PXI-6551 with the flying lead cable connected to the 68 pin connector.  The coaxial cable is specified at 30pF per foot and we note a 150pF total capacitance during measurements.  

How much of this 150pF comes from the cable alone and how much is contributed by the 68-pin connector and housing?

We are looking at making a cable-free connection (a 68-pin connection that mates directly with the PXI-6551 with the aim of reducing the capacitance at each digital IO pin to a minimum.

Please advise.

Thanks.

Anand

[Brandon_B]

Hello Anand,

Thank you for using NI forums.  What kind of connector block are you using and how long is your cable?

Regards,

[ajog]

Hi Brandon,

We are using the NI SHC68-H1X38 cable (flying lead cable) to connect to the HSDIO card.

Thanks.

Anand

[Brandon_B]

Hello Anand,

Are you using an attenuator at all?  The reason I ask is because with an attenuator you can generally achieve 1-2pF of input capacitance.  Based off of the specifications you gave in your first post it sounds like almost all of your capacitance seen is coming from the cable.  You mentioned that there is 30pF per foot and a total capacitance of 150pF.  The cable is 1.5m long which would give a total capacitance of 147.6pF.  Is this what you were looking for or is there something I may have misunderstood?

Regards,

[ajog]

Hi Brandon,

We currently don't plan on using attenuators.  So from your calculations, I should subtract 147.8pF from the capacitance I measure to obtain that introduced by the connector.  Thanks.

Anand