Some info about SNR/SINAD

QA40x
1

Hello Matt,

I have made some measurements with QA403, which is great piece of hardware and software.
I tested the generator in loopback and observed poor SNR, then tested the QA403 genarator with Cosmos ADC and REW and got much better SNR. QA403 tested in SE mode, and SE to balanced with Cosmos (no active parts).

qa403-loop-SE
qa403-loop-SE1280×993 86.1 KB

QA403-cosmos-bal
QA403-cosmos-bal1280×995 135 KB

Note that REW report ~0.5V because balanced input.
I am doing something wrong, i did a mistake in my setup?
PS. i tested yesterday an DAC, Topping DX3Pro+ with QA403 and Cosmos ADC, with same difference in SNR.

Best regards,
Sorin Baicus

2

Hi @bmsorin, it’s a bit hard to compare the two plots because one is absolute while the other is relative. Also, I think % distortion at these levels is really hard to deal with and that dB are better. But as a quick look the 3 kHz harmonic in REW is shown at 0.00022%, which is -113 dBc, the THDN is 0.00046%, which is -106.7 dB. THDN is also SNR when the bandwidth are the same. Normally, SNR refers to a bandwidth all the way up to Nyquist (see first link below). But if SNR bandwidth is constrained, the SNR and THD are just the opposites of each other (eg -100 dB THDN is 100 dB SNR). And practically, with modern converters they are the same because there’s not much going on between the 20 kHz of the measurement limit and the 24 kHz that is Nyquist–all the action is below 10 kHz).

So, in the REW plot, your THDN measurement is mostly set by the noise and it is -106.7 dB, but if the 3rd harmonic were gone, I think the THDN would be -108.

In the QA40x plot, the 3 kHz is about -117 dBc. The THDN is reported at -105.62, which is considerably better than the -117, so we know the THDN measurement is dominated by the noise. We can also see the hum at 50 Hz is -112 dB or so, which might in fact be degrading the THDN by a dB (if the noise were at -107, and your hum was at -117, the RMS sum of those values would be -106.58 dB).

So, I think there are two things to observe:

First, the THDN looks very close (-106.7 versus -105.62), and second, the hum on the QA40x is probably degrading the measurement by a dB. For the hum, I’d look at other grounding options, or potentially orientation and/or proximity to nearby AC-powered devices. Does the 50 Hz change if the put the QA40x on end? Upside down? If you move it to the floor with longer cables? The aim there is just to see what might change it. That will tell you if it’s radiated or conducted.

In loopback, the THDN of the QA403 should be very close to what is shown on the page 16 of the product spec (link 2). Note in the lower part of that graph you can see the note "Autoset 2 dB adder, Hann, 32k. So, to repro, use Hann with 32k FFT. But the autoset 2 dB sometimes isn’t clear. That means you want your full scale input to be the nearest input that is at least 2 dB higher than your incoming signal. So, if your incoming signal is 2 dBV, you’d want a full scale input of 4 dBV, but since that isn’t an option, you’d pick 6 dBV.

In link 3, I made some measurements on a Topping D10B (balanced) which you might find interesting too. It’s challenging measuring at these levels for sure. In the link 3, I used a balanced notch to (I hope) confidently determine the DAC limit around -117 dB THDN.

  1. https://www.analog.com/media/en/training-seminars/tutorials/MT-003.pdf

  2. https://cdn.shopify.com/s/files/1/1631/5609/files/QA403_Product_Sheet_REVH_February_2022.pdf?v=1645842772

  3. Best low noise connection for balanced output into QA403 - #2 by matt

3

Thank you Matt for your response. Yes, my environment is not perfect for measuring, but i will duplicate the tests on laptop, and another room.
One more clarification if you may. On QA40x THD+N is reported at -106.5dB while in REW is -118.6dB. I know the fft on QA40x is more dirty, but even so, 12dB seems to much.
Here are 2 measurements on DX3Pro+, here the difference are much small.

DX3Pro+-FFT-rew
DX3Pro+-FFT-rew1920×1032 273 KB

DX3Pro+-FFT
DX3Pro+-FFT1919×1030 278 KB

Best regards,
Sorin Baicus

4

I could be wrong as I don’t use REW that often, but it clearly shows the THD+N is 0.00041%, which is -107.7 dB.

It also shows the N+D at -118.5 dBFS, but your signal isn’t full scale. Your signal is -8.67 dB below full scale.

Also, when REW says N+D is -118.5 DBFS A, that suggests A-weighting is applied, and A weighting will generally knock your noise measurement down another 2 dB. So, we might estimate the N+D is -118.5 when A-weighted (which it shows), and -116.5 dB when not weighted (which we estimate), and then if you adjust the 116.5 dB figure to account for the fact that you are -8.67 dB below full scale, that takes you to -107.83 dB for THDN, which is 0.00041%.

I’m pretty sure everything REW is telling you is that THDN for that measurement is -107.8 dB.

Also, the QA40x measurement in showing -110.72 dB for the THD+N in the new plot you sent.

5

Thank you Matt for clarification.
Best regards!

6

Sure thing! BTW, what do you think is the difference in the powerline hum in the first and second plots on the QA40x? It’s completely gone in the second plot, but very prominent in the first plot.

7

The e1da @ 1.7v inout is only 640 ohms input impedance and so 320 ohms in mono mode. When using SE input take another 30% off giving you 224 ohms input impedance. Your dac is providing more like .62 vrms input to the e1da and compared to the qa403 you are likely to get a worse snr.
@matt pls correct me if that is off track.
That is why the e1da needs a buffer in front of it which of course the qa403 has.

8

Second plot was taken on different environment (Laptop and another room). But in SE too, not balanced.

9

Thanks @moto, I didn’t remember that about E1DA. Most opamps will contemplate 600 ohm loads as a lower bound. The OPA1612 has some plots of THDN for various loads, but 600 ohms is as low as they go. So, it’s not completely clear what might happen to the harmonics at 200 ohms. The currents are still pretty low, but it’s an unknown until someone characterizes it.

10

I have an e1da and for example the smsl su-9 has close to 350 ohms output impedance. Measuring that dac with the e1da required a buffer which I built with an opa1612.
Makes you appreciate the QA403 for its flexibility.