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Why is the amplitude of the NI 54xx's output signal twice as large as the setting in the NI-FGEN?

Hardware: Signal Sources (AWG/FG/AO)>>Arbitrary Waveform and Function Generators>>PXI-5422

Problem:
I am using a TDS1002B oscilloscope to measure the output signal of a NI PXI-54xx. I found the measured signal was twice as large as the setting in the NI-FGEN. Why is the amplitude twice the setting and how do I correct for this?

Solution:
All the NI Signal Generators (NI 54xx) modules are signal sources with 50 ohm output impedance. It can be viewed as an ideal voltage source connected to a 50 ohm resistor in serial connection. When measuring a signal from this type of source you should match the output impedance. When matching, the source resistor and the load have the same voltage individually. In this case, you can acquire the expected signal.

The TDS1002B oscilloscope has a 1M ohm input impedance, so it absorbs almost all the power from the source. This results in an observed ampltude twice as much as the voltage. To solve the problem, you can set the load impedance of the NI Signal Generators (NI 54xx) as >1 Mohm.

1. In the FGEN-SFP, click Edit >> Device configuration. In the Device configuration window, set the load impedance as >1M ohm.
   
   
   
   
2. When using NI-FGEN with LabVIEW, there is a property node "Load Impedance". You can set the value for this property node as "1M." An example program is attached below.
   

   

   
   

Related Links: KnowledgeBase 42AJ3HJQ: Simple Troubleshooting Steps to Improve Scope Accuracy


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Report Date: 10/24/2006
Last Updated: 03/06/2008
Document ID: 42N6R6BF

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