Incorrect Measurement When Performing Waveform Acquisition Using NI FlexRIO 5785

Issue Details

I am using a PXIe-5785 to acquire a waveform generated externally. In this setup, a signal generator is used to generate a 100 mV (200 mVpp) sine wave. However, when capturing the signal with the PXIe-5785, I only measure about 78 mV (156 mVpp). Why is the acquired amplitude lower than the generated signal?

Solution

• Verify impedance matching:
  • Set the signal source to a 50 Ω impedance to match the PXIe‑5785 input impedance, which is also 50 Ω. For instance, when using an NI waveform generator, you can choose options such as “Match Output,” “High‑Z,” or a custom setting to define the impedance. Ensure the waveform generator is configured to 50 Ω so it properly matches the PXIe‑5785.
    
     
    
• Check signal frequency against device bandwidth:

  • Verify that the generated signal frequency is within the PXIe-5785 analog input bandwidth.
  • Note that the analog input bandwidth of PXIe-5785 is normalized to 10MHz. 
• Minimize signal path losses:
  • Use 50 Ω coaxial cables throughout the signal chain.
  • Avoid unnecessary adapters, filters, or attenuators unless intended.
  • Inspect cables and connectors for damage or improper seating.

Additional Information