Is a digital signal actually analog at the physical level?

Yes — every digital signal is physically a continuous voltage, subject to the same rise times, ringing, and noise as any analog signal. What makes it "digital" is not the physics of the wire, but the discipline imposed at the receiving gate: that continuous voltage is deliberately interpreted through only two threshold zones (VIH/VIL) rather than measured for its exact value, and the output is re-driven from clean supply-referenced levels rather than passed through.

Why can't analog circuits just add more amplification to overcome noise the way digital regenerates signals?

Amplifying an analog signal amplifies its noise right along with it — gain has no way to distinguish signal from noise once they're combined on the same wire, so the signal-to-noise ratio doesn't improve. Digital regeneration works differently: it doesn't amplify the corrupted voltage, it discards it and generates a brand-new clean level based only on which side of a threshold the input landed on. That's a fundamentally different operation than gain, which is why it can actually remove accumulated noise instead of just scaling it up.

Does a higher bit-width ADC make a digital system as precise as an analog one?

It gets closer, but precision is still fundamentally quantized — a 16-bit ADC has 65,536 discrete codes across its input range, so any two analog voltages that fall within the same code are indistinguishable after conversion. Analog processing has no such floor; its precision is limited only by component tolerance, thermal noise, and drift, which is why very high-precision instrumentation and RF front ends are still done in the analog domain even in an otherwise all-digital system.

Why do digital ICs still need analog design skill at all?

Every digital gate is, at the transistor level, an analog circuit — its switching threshold, rise/fall time, and drive strength are all set by continuous transistor physics (Vgs, Vt, channel width/length, parasitic capacitance). At advanced process nodes and high clock speeds, effects that used to be safely ignorable — supply IR drop, crosstalk, jitter, process/voltage/temperature variation — become significant enough that digital design increasingly requires analog-level signal integrity analysis to close timing and functionality at all.

What is a "mixed-signal" IC?

A single die containing both analog circuitry (amplifiers, references, ADC/DAC front ends, PLLs) and digital logic (processors, memory, digital signal processing), typically because the chip needs to interface with a continuous real-world signal and also store, compute on, or transmit that information reliably. Microcontrollers with built-in ADCs, audio codecs, and RF transceiver SoCs are all common mixed-signal examples.

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Analog vs. Digital ICs

The real dividing line isn't "uses 1s and 0s." It's whether the circuit has to preserve a signal's exact continuous value, or only has to tell two ranges apart.

Every signal in the physical world — a microphone's output, a temperature sensor's voltage, a radio wave — is analog: continuous in both time and amplitude, able to take on infinitely many values. An analog IC (an op-amp, a linear voltage regulator, an RF amplifier) processes that signal directly, preserving its exact shape at every instant. A digital IC never sees that continuous signal at all. It only ever asks one question of any voltage on its input pins: is this closer to "high" or closer to "low"? Everything about how digital and analog design differ — precision, noise behavior, how errors accumulate, how hard the circuit is to design — falls out of that one difference in what each domain is actually trying to preserve.