2 docs tagged with "dac"

A successive-approximation ADC is, physically, a capacitor with a switch in front of it. Converting a voltage happens in two completely different phases: first the switch closes and the capacitor is allowed to charge towards your signal through the source impedance, and then the switch opens and a comparator plays twelve rounds of twenty questions against the trapped charge. The second phase is fixed by the hardware and takes exactly twelve clocks. The first phase is the one you configure, the one every tutorial leaves at its reset value, and the one that decides whether your reading means anything at all.

An analog-to-digital converter looks, from firmware, like a register you read. That framing hides the two things that actually determine whether the number is right. First, the conversion is a comparison against a reference, so the answer is a ratio, not a voltage — and a wrong or noisy reference is invisible in the result. Second, before any comparing happens the converter must charge a small capacitor through your circuit, and if you did not give it long enough, it will confidently report the voltage it managed to reach rather than the voltage that was there.