Hello Jack,
The "update rate" parameter in Measurement and Automation Explorer determines how often the 4451 board updates the voltage on its analog output line. This value is not equal to the physical frequency of the analog sine output, but it is directly proportional to it.
I tried a wide range of update rates using the 4451 attached to an analog oscilloscope and I did not see any change in amplitude at lower update rates. It sounds like very low-frequency signals may be experiencing some filtering in your circuit. You can expect this if you are feeding into an oscilloscope set for AC coupling.
There are several examples available in LabVIEW, ComponentWorks, and LabWINDOW/CVI for programminmg analog output. In LabVIEW and ComponentWorks, you can use most hardwar
e-timed analog output examples for your DSA board. If you are programming in CVI, I would suggest taking a look at our example program available in the Developer Zone (search under "DSA" and "output").
If you are still running into problems, please call in for technical support or submit an email support request at:
www.ni.com/support/ask.
Good luck!
Bryan