input capacitance of CLK IN for PXI-6551

[ajog]

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.  

Thanks.

Anand

[Ryan_M]

Anand,

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.

[ajog]

Thanks Ryan,

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.

Anand

[David_Pratt]

Hey Anand,

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.