Hi @Gallantus, @mhuth1776 is correct here.

Here’s a bit more on wny the 10 dB margin is important. First, let’s do an experiment on the QA403.

Set up single-ended loopback (L+ OUT to L+ IN), pick a 32K FFT, dial in some averaging, set the full scale to +42 dBV. What this setup does is measure the self-noise of the QA403 in +42 dBV mode. as the plot below shows, the noise is about -78.6 dBV (20 kHz BW). With the full scale being _42, this means the noise is about -120 (42+78.6) below the full scale input. So far so good.

Now, enable white noise and set the white noise level to -78.6 dBV. Now we have two sources of noise at the same level and uncorrelated. We see the RMS noise is -76, which is 2.61 dB higher.

When you have two uncorrelated noise sources and you wish to add them, you must do as follows:

N=SQRT(X^2+Y^2+Z^2…)

So, if we have noise source one which is -100 dBV = 10uV rms, and noise source 2 which is the same, then the total noise is SQRT(10uV*10uV + 10uV*10uV) = 14.16uV = 96.98 dBV, which is 3.01 dBV. This is pretty close to what we saw above.

This is a very important property to understand: Uncorrelated sources (noise) added together increase the signal by 3 dB = sqrt(2) = 1.41. And correlated sources (think of two identical sine waves) added together increase the signal by 6 dB = 2X.

This is how you can parallel two chip DACs together and improve the SNR. The sine waves add by 6 dB, but the noise only adds by 3 dB. So, every time you double the numbers of DACs, you improve the SNR by 3 dB

OK, so back to the 10 dB rule I’d first mentioned. Much of what we want to measure is defined by the noise level. So, if your instrument has -120 dBV of noise, and your DUT has -120 dBV of noise, you’d measure -117 dBV of noise. That is a 3 dB error.

You can run through the math and you get a table like that below. And that shows that if your analyzer noise is 10 dB better than your DUT noise, your measurement error is 0.41 dB. That is, the DUT noise measures about a half dB worse than it actually is. This is because of the analyzer noise.

Now, the good news is that if you are certain of the analyzer noise, you can back that out. That is, if you know your analyzer noise is -120, and you are measuring DAC noise at -117.5, it’s probably a safe bet that your DAC noise is actually -121.2 dBV.