PXIe-5785 Analog Input Signal Is Distorted When Performing Loopback Connection

Updated Jun 16, 2026

Reported In

Hardware

  • PXIe-5785

Software

  • LabVIEW

Issue Details

I am performing a loopback connection from PXIe‑5785 AO0 to PXIe‑5785 AI0, and the measured signal is accurate at lower amplitudes. For example, when AO0 outputs 200 mV, AI0 correctly reads back 200 mV.
However, when I increase the output amplitude above approximately 440 mV, the waveform becomes clipped. This clipping is also observed when routing the PXIe‑5785 AO signal to an external oscilloscope.
Image_2026-04-30_15-26-18.png
I am using the default bitfile and FlexRIO Getting Started for Integrated IO Getting Started Basic.vi from the LabVIEW shipping examples.
Could you help explain what might be causing this behavior?

Solution

This behavior is expected and is related to the output power limitations of the PXIe‑5785 depending on the number of active channels.
  • Verify the configured channel mode:
    • Single Channel Mode
    • Dual Channel Mode
  • Compare the output amplitude with the PXIe‑5785 specifications:
    • In Single Channel Mode, the maximum output is approximately -3.33 dBm (431 mVpp).
    • In Dual Channel Mode, the maximum output increases to approximately 2.85 dBm (878 mVpp).
  • Confirm whether the configured amplitude is specified in:
    • mV peak (mVp) or
    • mV peak-to-peak (mVpp)
  • Ensure the configured waveform amplitude does not exceed the allowed range for the active channel configuration.
  • Reduce the output amplitude or switch to Dual Channel Mode if a higher amplitude is required.

Additional Information

  • The PXIe‑5785 output stage is limited by its power capabilities and the number of active channels, which defines the maximum achievable amplitude. When this limit is exceeded, the output signal will clip, and this behavior can be observed both in internal loopback measurements and on external instruments such as oscilloscopes.
  • Additionally, the PXIe‑5785 analog input full-scale range is 1.25 Vpk‑pk (equivalent to 0.625 V amplitude). When using an external waveform generator, any input signal exceeding 0.625 V amplitude cannot be accurately measured and may result in distortion or clipping at the input stage.

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