I'm struggling with the noise floor.
First tests where done with a 5nV/sqrt(Hz) opamp. Noise floor with
shorted mixer output at 10kHz was -140dBc/Hz. Then I tried with
1nV/sqrt(Hz) opamp, but that made no difference, noise floor at 10kHz
was still -140dBc/Hz
The setup was simplified to this schematic:
http://athome.kaashoek.com/time-nuts/PNA/SSPNA.JPG
The REF_buffer creates a virtual ground, the Audio_LNA amplifies into
the differential audio output .
Why did the lower noise opamp not make a difference?
Also the setup is acting like a nice microphone. Tapping the housing is
clearly audible. Which component may be causing the microphony?
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