5401 Amplitude description error?

[schelgr]

I am controlling the 5401 PXI Fgen through Delphi and noticed that the documentation for the 5401 function call ConfigureStandardWaveform located in the help file calls for a peak to peak voltage. I am sure that it is actually peak voltage. The NI soft panel also shows the setting to be peak to peak, but measurements (rms) show that it is only peak voltage. Is this a known error in the documentation and the soft panel?

[RossC]

schelgr,

This behavior actually occurs due to improper settings for load impedance of the arb. If the device you are sending the signal to is high impedence (such as a scope probe), you need to set this in the Soft Front Panel in the Edit » Device Configuration menu under the "Output" tab. The setting to change is "Load Impedance", and it should reflect the impedence of the device connected to the arb. If that device is a scope probe or another high impedance device, the impedance needs to be set to ">1MOhm".

RossC

[ScotH]

I changed the Device Configuration/load Impedance on the SFP to > 1Mohm and this causes the Output of the FGEN PXI-5401 to set a voltage that represents what I ask it to with a scope as a load.

My question is how do I set the load impedance programatically using the FGEN commands.  The only options I can find are to set the output impedance to 50 or 75 ohms.

Scot

[RossC]

ScotH,

The load impedance and output impedance are separate settings, but both can be manipulated in software.  If you are using LabVIEW, you would use a NI-FGEN Property Node.  To select the load impedance property, click on the lower white box of the property node and go to Output Attributes » Load Impedance.  You can specify your load impedance there and the board will compensate appropriately (like you mentioned in the SFP).  If you are using C or CVI, use the "niFgen_SetAttributeViReal64" function to set the "NIFGEN_ATTR_LOAD_IMPEDANCE" attribute to your load impedance.

The output impedance property/attribute just switches between the available output impedances, it does not compensate for load impedance.

Ross C

[ScotH]

Ross,

Works perfect. thanks,

I was usinging VB.Net with the FGEN wrappers and i used the following code.

' Describes high impedance load assumes load is > 1M ohm

my_niFgen.SetDouble(niFgenProperties.LoadImpedance, 1000000)

bResult =

True

Thanks,

Scot

[RossC]

Scot,

Great to hear it is working now!  I forgot to post the calls for VB.NET, its good to hear there was enough info for you to derive the solution.  Thanks for taking the time to document your .NET solution in case other run across the same issue!

RossC