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Hi @attila Thank you for your response. I just realised I should have started my question by explaining why I was trying to do what I was trying to do in the first place 🙂 I am not concerned with transfer characteristics of my audio device. I trust it works correctly as advertised. What I am actually trying to do is to design a circuit that taps off of a line output of a device connected to line input of another device. Basically connecting my to-be circuit in series (topologically) or in parallel (electrically) between the two devices like a line-out source and an amplifier. Like a Y-splitter cable, basically. My circuit will involve DC-biasing the audio signal and then feeding it into a single-ended, unipolar ADC built into a microcontroller. The design goal for my circuit is not to affect the audio signal in any perceivable way, that is not to load the line-out excessively, not to affect the frequency response (e.g. by introducing unwanted resonance), etc. Basically, my in-between device should be as close to being "transparent" as practically possible. That's where my initial idea of comparing the original response of the line input from the source's point of view and then comparing it to the response of the input with my circuit connected in parallel came from. My reasoning being that if the impedance graph of the line input with- and without my added circuit looks reasonably similar, I'm not introducing any significant distortion to the signal.

Hello I'm trying to measure my audio interface's line input impedance using AD2 with impedance analyzer PCB but don't seem to have a clue what I'm doing... I've got Focusrite Scarlett 2i2 3rd Gen USB audio interface. Its manufacturer specifies device's line input impedance at 60kΩ (https://focusrite.com/products/scarlett-2i2-3rd-gen). I have connected one end of an audio cable to one of the Scarlett's (mono) line inputs and the other end into the block terminal of the impedance analyzer (GND/jack sleeve to "-" and signal/jack tip to "+"). I have configured WaveForms instrument as per screenshot and got a below result. Does it make any kind of sense? I realize I'm not going to get a smooth linear impedance plot over the frequency range but does this look even remotely correct? Is the phase supposed to jump around all over the place like that? Also, I keep getting the "Resistor too low!?" message in the corner of the plot area. When I'm changing the load resistor value I can hear the relays clicking but the supposed impedance's average value just increases ten-fold with each increase of R by factor of ten and the warning stays there even at 1MΩ. I can hear the frequency sweeps just fine in the speakers, the Focusrite's input level is also set correctly (solidly registering but not clipping). Am I even using the right tool for the job or the analyzer is not meant for active inputs like that? I am a simple guy with basic digital logic skills and steady-state electronics understanding, any help in making heads or tails of this will be greatly appreciated 😉