For the PXI-6551, what is the input capacitance of the CLK IN pin on the front panel of the 6551? If I were to supply an external clock using an off the shelf oscillator IC this information is important. I would like to pick a 8.192MHz oscillator to drive CLK IN.
Please advise as to what capacitive load it would normally present to an oscillator.
This is not something we specify for the NI 655x devices. The CLKIN input to the 655x devices is AC coupled using a 1uF capacitor. Your signal will hit this DC block before it reaches the configured 50ohm/1kohm input impedance. If you measure the capacitance at the SMB connector with an LCR meter, you'll be measuring this blocking cap.
That helps. So the load seen would be either 50Ohms or 1kOhms. The oscillator module can drive 50pF at 8.192MHz which is ~ 388 Ohms. So I should configure CLK IN with 1kOhm input impedance as adequate.
I would recommend seeing which setting works best for you. Regardless you will see slight amounts of reflection either way so you can just see which results in the best measurement.