PXI-5422 ConfigureArbWaveform().

[A Wooten]

I am using C++ code to control the PXI-5422 Function Generator via the IVI interface.

I am able to load, configure and generate an arbitrary waveform, however the gain that is applied to the waveform is always double the gain that I supply to the

IviFgen_ConfigureArbWaveform() function.

IVI waveforms are normalized from -1 to 1.  So if I supply a gain of 3 I expect to see a waveform with voltages from -3 to 3.

Currently, if I supply a gain of 3 to the IviFgen_ConfigureArbWaveform() function I am seeing a waveform with voltages from -6 to 6.

I've used two different scope view the waveform with identical results ( NI PXI-5922 digitizer and a desktop scope from another vender ).

Any ideas why I would be getting this behavior.

Thanks,

Anson Wooten

[Drew Creel]

You may have an impedance mis-match.  The PXI-5422 assumes a 50 Ohm load impedance by default; if your scope has a high input impedance, like 1MOhm, you'll see double the voltage you expect on the scope's input.

If you can check and verify that your scope does have a high input impedance, you can either switch the scope's input impedance to 50 Ohms (available on the PXI-5922) or set a load impedance attribute on the PXI-5422 to change its assumed load impedance to match your scope's input impedance.  To do this, you can call Ivi_SetAttributeViReal(vi, "0", NIFGEN_ATTR_LOAD_IMPEDANCE, 0, 1000000) on the PXI-5422.  You will probably have to include the niFgen.h header file in your program to get that attribute constant.

Drew Creel
NI Software Engineer
Signal Generators Group

[Yardov]

Hi A Wooten,

Here is a knowledgebase about what Drew is talking about.