Hi @Bartosz, these are excellent curves. So, to generalize from these curves, there are a few observations:
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For DUTs with THD worse than -110, the input range doesn’t matter much as long as the atten is off. 0, 12 and 18 will get you close.
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For DUTs with THD better than -110, then input range matters more and you need to pay more attention to the setup.
Similar statements can be made for THDN.
Now, the loopback is a bit pessimistic, because the ADC on its own can deliver THDN around -114 dB when driven from an extremely high quality source such as the Topping E70 Velvet DAC.
It is interesting how there is a big dip when you go up from an input of 18 dBV to 24 dBV.
Yes, that the due to the front-end attenuator clicking in. That attenuator provides 24 dB of attenuation. So, that raises your noise floor by about 24 dB on paper but it also increases your maximum input level considerably so that the next range of gain adjustments can do their work. If you look at the SNR of each range, you see it’s roughly the same. So, noise floor at 0 dBV is -117 RMS (20k, no weighting). And the full scale input is 0 dBV, so that suggests a dynamic range around 117 dB. And at 6 dBV full scale input, the noise floor is -113 and the max input is 6, so 119
In a table, you get:
| Full Scale Input (dBV) | Noise 20 kHz (dBV) | DR |
|---|---|---|
| 0 | -117 | 117 |
| 6 | -113 | 119 |
| 12 | -108 | 120 |
| 18 | -103 | 121 |
| 24 | -82 | 106 |
| 30 | -82 | 112 |
| 36 | -81 | 117 |
| 42 | -77 | 119 |
Note that when the switch from 18 to 24 occurs, the dynamic range shifts a lot, and doesn’t really come back until you get to the 36 and 42 dB ranges. This is due to the input opamp (OPA1612) objecting to the size of the attenuator resistor (roughly 6K–a divider made from a 93K resistor on top, and 6K resistor on bottom gives you 24 dB of attenuation with an output impedance out of the divider of ~6K. The OPA1612 has a great noise voltage, but a middling noise current. A JFET amp would do much better there, but the distortion would suffer.
In this case, how would you accurately measure an amplifier with a higher output if you were forced to attenuate it when moved to an input between 24 and 42 dBV?
Once you have learned the sweet spot for the measurement you want to make, you can then use an external resistor attenuator to get your amp back into that region. That is why on the QA451 there’s an internal 12 dB attenuator. Higher power amps would want a bit more, and lower power amps would want a bit less. There’s a post HERE on using attenuators, and if you search the forum for attenuator you’ll find a lot more there too.
And in the realm between -114 dB THDN and -123 dB or so, that brings you into the world of notch filters. The aim with a notch filter is to knock down the signal a known amount, while leaving the noise in tact. That reduced signal level means the “contaminating” harmonics that form in the ADC don’t materialize, and that let’s you push to the next level.
More on notch filters HERE
PS. What amp are you measuring? The numbers you are measuring are quite good.