-110db noise floor. Is this normal?

With the left input shorted with a 75ohm BNC connector.

I’m seeing about -110db as a noise floor. Unweighted.

But, I’ve seen many others post measurements with the noise floor at -140

This is with the laptop unplugged and all AC power nearby unplugged.

I have tried different USB cables and FFT length.

The problem is. I’m testing some high quality amplifiers. And some of the amplifiers must have a noise floor below or around -110. Because it doesn’t increase when the amplifier is connected.

Any ideas?

Hi @mitchwolos, if this is for the QA401, with both L+ and L- shorted, it should be around -115 dBV, 20 to 20K, unweighted.

You can replicated this by shorting the L+ and L- inputs with 75 or 50 BNC shorting blocks, do a file-Reset Startup Defaults, make sure atten is off (+6 dBV) and then look at reported noise as shown below.

Where it might get confusing is the trace is around -155 dBV, but that depends on the FFT size. The RMS noise measurement takes the FFT size into account and reports the total noise in the measurement bandwidth (20 to 20 kHz) and the -115 figure will be returned no matter what size FFT.

Can you see if you can replicate this on your hardware?

Thanks Matt. Yes I can replicate this with shorting BNC connectors.

The reason I was inquiring is that most amplifiers I’ve measured are nowhere near the rated Snr/Thd.