Meanwhile, on the other side of the railroad tracks...Hello fellow buy-once, cry-once heretics!
In case you were looking for an end-game for a two channel audio interface for measurement (and recording), the RME Babyface Pro is a bargain IMO.
Build quality is Apple computer, Lectrosonics, Mercedes, etc. It's just wonderful. The Windows WDM and ASIO drivers are the best and most flexible I've ever seen. The included routing software includes basic, but excellent EQ and effects. This ain't Behringer - everything that comes with this device is something someone is proud of, not something birthed in marketing.
SummarySample rates from 32k to 192k show near theoretical noise floors on both input and output. Because USB is an asynchronous animal, SR's can be adjusted in 0.001% increments around the primary frequency for syncing with other stuff that's not playing by the rules (common).
Its channels have identical transfer functions, so dual channel FFT is dead accurate at any SR. Its USB bus powered or externally powered in stand-alone mode (mic preamp, D/A converter, that sort of thing). You can also use it as an interface with an iOS gizmo in class compliant mode.
Unlike the inexpensive Scarlett 2i2 Gen 2, this thing is a solid recording interface and the included EQ and effects in the routing software probably make it the world's best Podcast interface. That's what I'm going to use it for along with D/A conversion at my desk when it's not out making measurements.
Odds and EndsMeasurement gizmos used were the Audio Precision APx515, FIR Capture, Keysight U1282A and U1733C. The APx515 allows you to send or receive to/from a DUT through ASIO, thus allowing closed loop measurement of the analog inputs and outputs independently.
Phantom power is excellent, 47.3V unloaded and 13.4V when both channels have full 10mA loads. The spec for phantom is 48V +/-4V unloaded and 14V +/-1.2V loaded with 10mA. Most phantom powered mics I'm aware of draw 3mA to 5mA. The XL outputs are unaffected by the presence of phantom power. XL input impedance is 2,053Ω from 100Hz to 10kHz, sloping down to 1,850Ω at 100kHz. This impedance is the same with or without phantom power engaged as it should be. Optical digital I/O can do SPDIF or ADAT, MIDI I/O with an adapter cable is included as well. Metering is used for display feedback of operating adjustments as well as level indication. They are nicely visible, and like everything else on this unit, accurate.
MeasurementsThis unit has analog gain adjustments for both input and output, thus you should adjust both so that you can keep the digital signal as close to full scale as possible. I chose -3dBFS for these measurements.
Flat lines from a dual channel FFT measurement program called FIR Capture at all sample rates:
Phase input and output:
Magnitude input and output:
Noise input and output:
Distortion at -3dBFS:
Finally, my Pin 1 torture test that few devices have ever passed. Passing is when the 60Hz square wave fundamental and harmonics stay below about -100dBFS. The Babyface Pro (why, oh why did they name it that?!), doesn't show any evidence that the square wave is on! The is a best ever result for me. No response whatsoever to a nasty 10V square wave shoved down Pin 1's throat. RME obviously got the memo and connected Pin 1 directly to chassis shared only with the negative power supply rail:
