PXI-5124 impedance issue

[gt77]

Hi! I'm using an NI PXI-5422 Arbitrary Waveform Generator to send a communications signal through an experimental setup (with transducers) to an NI PXI-5124 Digitizer. When I set the input impedance of the digitizer to 50Ω, which is recommended here for "better" communications signals, I don't see the signal at the digitizer. When I set it to 1MΩ, I see the signal but with an additional 60 Hz component. I could set it to 1MΩ and filter out 60 Hz, but I don't want to possibly affect the quality of the signal with the reflections from the 1MΩ circuit path components. So, what could be the reason that I do not see the signal through the 50Ω path and how can I fix it?

[Henrik_Volkers]

There is a (small) chance that your 50 Ohm resistor in the 5124 has changed his value .. (to short).

EDIT: First test:  Use the 5422 (and the NI-FGEN Frontpanel) , directly connect it to the 5124 and use the NI-Scope frontpanel to read the values (and switch input impedances).

Another test: Use a DC voltage source (1.5V battery) and connect it to the input in question.

Use NI-Scope Frontpanel (or MAX test ) to read the DC voltage. (you can add a measure mean voltage 😉 ) .

Switch the input impedances ... no or only minor change in voltage should be observed.

Now put a  1M ohm or a 50 Ohm (2x100Ohm) resistor in series to the voltage source. if series resistor and input impedance match, you should read about 1/2 the voltage.(And you can calculate the exact input impedance, if you know ypour resistors and the voltage source)

If this test runs fine, I would expect a buggy setup in your vi. 

[gt77]

Thanks Henrik! I tried the first test and I sent 1MHz 1Vpk-pk sine wave from the generator to the digitizer directly.

From CH1 of the digitizer, I got

• 1Vpk-pk with 1M Ohm AC Coupling (No 50 Ohm option with AC)
• 1Vpk-pk with 1M Ohm DC Coupling
• Almost nothing with 50 Ohm DC Coupling

and from CH0, I got

• .22 Vpk-pk with 1M Ohm AC Coupling (No 50 Ohm option with AC)
• .22 Vpk-pk with 1M Ohm DC Coupling
• A warning message that says "The input voltage is too high for the 50 Ohm input impedance." with 50 Ohm DC Coupling

I don't have the necessary equipment to perform the second test at the moment, but I believe from this test alone, something seems sketchy. Do you think it's 50 Ohm being changed or something else?

[Henrik_Volkers]

I assume the unit has passed a selftest and autocalibration... .. looks like a defective unit to me.

We just had a 5922 scopecard with (ESD?) blown input repaired...