I needed an oscilloscope that is able to record minutes of signal and export that data to a computer. An oscilloscope is, in fact, nothing more than a analogue-to-digital convertor that samples the input signal fast enough. Exactly what a sound card’s recorder does!
The line-in of my MacBook Pro (6,2) has an input impedance of 28kΩ, and is AC-coupled. So it’s not possible to measure DC voltages. The DC-decoupling capacitor has a 30ms half-time (τ=43.3ms), which makes it capable of measuring down to 3.6 Hz (@-3dB).
Test measurements have shown that 350mV results in 0.115 full-scale deflection, yielding an approximate full-scale voltage of 3V (i.e. ~2.1Vrms).
The signal that I wanted to measure was 12Vdc, with pulses down to 0V. Putting a 330kΩ resistor in series with the input resulted in a voltage division of ~12.7, bringing the 12V safely below 1V and limiting the maximum current draw to 33µA.