Why Is the Noise Worse With a 1 MΩ Input Impedance Compared to a 50Ω Input Impedance on the PXIe-5164?

Updated Jan 21, 2026

Reported In

Hardware

  • PXIe-5164

Issue Details

When the PXIe-5164 is left unterminated, the observed noise level with a 1 MΩ input impedance is higher than that with a 50 Ω input impedance, and a hump appears in the frequency-domain plot.

Is this behavior expected, and what is the underlying cause?

Solution

Yes, it is expected.

The reason is the 1MΩ path has some separate amplifier conditioning before it rejoins the common path in PXIe-5164. 

The 50Ω path gets passed straight to that final amplifier. The block diagram shows the basic idea:

 

 

Since the 1MΩ conditioning path has that extra conditioning amplifier, it's always adding some more noise relative to the base noise of the 50Ω path. You can see this in the plots below, when scaled for dBFs.
 

Additional Information

The 1MΩ path amplifier is not really a single amplifier, but a combination of amplifiers working together.

Each has their own 1/f and broadband noise, and loop interactions. 

Summed together, that can give you the NSD plot that has some humps or plateaus.

Related Links

PXIe-5164 Signal Conditioning - NI

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