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E1DA Cosmos ADC

Yes, it was an unbalanced connection, so the input impedance of the Scaler to the DAC was 100k ohms as you pointed out. The output impedance of the Scaler was 10 ohms and the input impedance of the ADC was 1.66k ohms.
Good setup. On par with AP analyzer for a fraction of the cost.
 
I've been playing with the ADC and scaler for a couple of weeks now. Thoroughly impressed.

I see a couple of posts above about the input impedance of the ADC. Interestingly I was going to comment on that topic, too.

Although I see why one wants to have high input impedance on testing equipment---i.e., to cover all kinds of DUT by providing essentially no load---, I do notice a disadvantage. Such high input impedance causes interconnect cables to be vulnerable to noise! Of course, in most cases the noise is way below audible levels. But these days we hobbyists measure these things not just from audibility concerns. We measure devices whose noise floors are extremely low.

For example, today I found that my very short (2 ft) Canare L-4E6S star quad balanced cables, used to connect a DAC to the Cosmos Scaler, was very finicky with their position (literally posture) relative to other cables. The effects were clearly shown on measurements---not a beautiful picture I'd like to see! Took some time to troubleshoot and finally find the culprit: the scaler's 200 kOhm input impedance. That made sense.

For testing modern devices, 10k - 20k Ohm should be high enough. I wish the scaler had selectable input impedance :)

It may drive the golden ears crazy, but you could always use a precision resistor in parallel with the input. That is common in the RF world if a device's input impedance isn't 50Ω.

What is the freq. resolution of the FFT?
A 10kΩ resistor @ 24C looks to have ~2.7 µV rms (-111 dBV) thermal noise over 44.1 kHz.
 
What is the SOTA for a similarly priced signal generator to measure ADC and analog circuits?
Based on @IVX own measurements, the E1DA #9039S USB Balanced DAC has performance on par or better than the AP signal generator.

THD+N@1kHz@0dbFS -126db averaged over 10 samples 32b/96kHz@10k 49MHz
THD@1kHz@0dbFS -140db typical calibrated with Cosmos APU 32b/96kHz@10k 49MHz.
THD@1kHz@0dbFS -145db typical calibrated with Cosmos APU 32b/96kHz@10k 24MHz

He also mentioned in his blog that the latest results show that there is a way to get all harmonics <-150dB.
Moreover, with the yet to be released Cosmos LPF, the #9039S->LPF combination has a balanced output with THD@1kHz -155dB and all harmonics at -160dB or less.
 
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Based on @IVX own measurements, the E1DA #9039S USB Balanced DAC has performance on par or better than the AP signal generator.

THD+N@1kHz@0dbFS -126db averaged over 10 samples 32b/96kHz@10k 49MHz
THD@1kHz@0dbFS -140db typical calibrated with Cosmos APU 32b/96kHz@10k 49MHz.
THD@1kHz@0dbFS -145db typical calibrated with Cosmos APU 32b/96kHz@10k 24MHz

He also mentioned in his blog that the latest results show that there is a way to get all harmonics <-150dB.
Moreover, with the yet to be released Cosmos LPF, the #9039S->LPF combination has a balanced output with THD@1kHz -155dB and all harmonics at -160dB or less.

When I see such numbers my innate response is to call BS. Just shows how damn fast ADC/DAC technology has progressed in such a short period of time!

I'm going to have to explore these for other (non-audio equipment) purposes.

Can they be DC coupled? [pretty please]
 
I'm not sure what you mean DC coupled. Describe or draw a diagram of the connections.

Oh. Ignoring the technical aspects, AC coupled essentially means it rejects or filters out DC signals. Basically a high-pass filter.
DC coupled generally means it's flat/usable for DC signals are slow varying AC signals.

I would say the classic example are oscilloscope inputs which often can be switched between the two.
 
DC coupled generally means it's flat/usable for DC signals are slow varying AC signals.
Here's what the E1DA #9039S DAC specs say about it:
Frequency response: DC-130kHz +.05/-3db @32b/384kHz, -.5db at 45kHz. DC performance is limited by DC protection.
For more information about DC performance you should contact the author.
 
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