Increasing input impedance on a PXIe-6535?

[CurtisHx]

I need to significantly increase the input impedance on the IO lines while maintaining rise and fall times.  I was thinking of adding 1-2 meg ohm resistors in series with the IO lines on the breakout boad, but if the parasitic capacitance on the PXIe-6535 is too high, I would be forced to a lower sampling rate, due to the charge/discharge time. 

Or is there a way to make the 6535 completely "let go" of an IO line?  Tri-stating forms a resistor divider when an IO line has an external 20k pull up.  I need the PXIe-6535 to drive some IO lines that have very high-value pull ups with open-drain.

[menay]

Hey CurtisHx!

It looks like you may have posted this question on the wrong board for the product type. If you repost this question to the Digital I/O board, you are more likely to receive help from the community members that are familiar with the high speed DIO cards!

[CurtisHx]

I was under the impression that the PXIe-6535 was classified as a DAQ card since it uses the .NET DAQmx library.  Let that be a lesson in making assumptions......

[Henrik_Volkers]

The most important information(s) are missing!

Needed timing resolution ?

If you need ns high speed you need matched impedances (and these are usually low impedances 50 Ohm to 100 Ohm ... and a meter cable is about 5ns 😉 )

So: What do you want to measure?  Source ? Impedances?

(one solution migth be a 'simple' DIY FET probe adapted to DC ...  atually a FET voltage follower ) 

here is an AC version .... but you would need to adapt it to DC and match the impedances to a DI ...  (wasn't that 100 Ohm?  Than a twisted cable pair (Cat-cable) should fit)

http://www.seekic.com/blog/project_solutions/2011/08/03/Poor_Man%E2%80%99s_1_GHz_Active_Probe__DIY_S...

http://teledynelecroy.com/doc/probes-probing