Analog vs digital chips: what's the difference?

Digital and analog chips differ in the signals they handle: a digital chip only knows two states, 0 and 1; an analog chip handles signals that change smoothly, like how loud a sound is, how bright the light is, how hot it is, or how much voltage a battery has left. An analog chip is also called an analog IC; a digital chip, a digital IC. Neither is more advanced than the other. Digital chips compute and remember; analog chips deal with the real world. Many chips hold both kinds, and that's called mixed-signal.

For what a chip is in the first place, see What is a chip?.

How is a digital signal different from an analog one?

Think of the lights at home:

• Digital is an ordinary light switch: on or off, nothing else. On is 1, off is 0.
• Analog is a dimmer knob: anywhere from fully dark to fully bright, and every level in between.

The most basic part in a chip is the transistor, a tiny switch controlled by electricity (see What is a transistor?); a digital circuit uses many switches, each one on or off, to stand for a string of 0s and 1s.

A digital signal is really a voltage too. The circuit only checks whether it's high enough or low enough: high counts as 1, low counts as 0, and a band in the middle is left unused. So if the signal wobbles a little, it still isn't misread.

That's digital's biggest advantage: a little noise doesn't matter, and a copy of a copy of a copy is still exactly the same. An analog signal can't do that. Once some noise gets mixed in, there's no telling which part was there to begin with. It's like photocopying a photocopy: each one comes out blurrier.

The real world is analog, so what do chips do about it?

Sound is air being squeezed and relaxed, over and over. How bright the light is, how hot it is, a battery's voltage: they all change smoothly too, and none of them turns itself into 0s and 1s.

For a chip to know about any of this, the first step is turning it into electricity. A microphone turns sound into a voltage that rises and falls with it; a camera's image sensor turns light into electricity; a temperature sensor turns hot and cold into a higher or lower voltage. These electrical signals all start out analog, and they're often very weak.

That's where analog circuits come in:

• Pick it up and amplify it: boost a tiny signal until it's big enough to use. A circuit that does this is called an amplifier.
• Filter it: take out the noise you don't want.
• Convert it: turn analog into numbers, or numbers back into analog.

What do an ADC and a DAC do?

An ADC is an analog-to-digital converter. It does two things:

1. It measures again and again, a tiny moment apart: like walking along a road that rises and dips and checking the height at every step. This is called sampling.
2. It records each reading as the nearest step: a ruler has only so many marks, so each reading can only be written down as the nearest mark, then stored as a string of 0s and 1s.

The more often you measure and the finer the marks, the closer the record is to the original. Take the standard for music CDs: 44,100 readings every second, each one stored as 16 0s and 1s, which splits the range from low to high into 65,536 steps. Why so many readings? People can roughly hear sounds that vibrate anywhere from 20 to 20,000 times a second, and to record a vibration, you have to measure at least twice as often as it vibrates.

A DAC goes the other way: a digital-to-analog converter turns a string of numbers back into a smoothly changing voltage, which is what it takes to drive a speaker or earphones.

So inside a device, a sound often travels like this: microphone → amplifier → ADC → digital processing → DAC → speaker.

Once the sound is numbers, a lot of things get much easier: boost some pitches more than others, push the background noise down, store it, send it somewhere.

Why do power chips count as analog too?

Inside a phone, the processor, the memory, the screen and the circuits that send and receive wireless signals all need different voltages, and a battery's voltage slowly drops as it's used up. A power management chip takes the electricity from the battery or the wall socket and turns it into the steady voltage each part needs.

It keeps a constant eye on the voltage it puts out, topping it up when it dips and pulling it back when it rises. What it handles are voltages and currents that change smoothly, so it's usually counted as an analog chip. There are two common ways to do it:

• Like a faucet: it holds back the extra voltage. It's simple and quiet, but the part it holds back turns into heat.
• Like flicking a switch very fast: the power goes on, off, on, off, and inductors and capacitors smooth it out. This wastes less, and it's common where the current is large.

Why do analog chips often use mature nodes and stay in production for years?

When digital circuits shrink, the benefit is direct. When analog circuits shrink, the benefit is small, and sometimes they get harder to make (the article The news says "mature nodes" covers this):

• The newest generations run on very low voltages, so a signal has less room to swing up and down, and a little noise looks big.
• How big a resistor, capacitor or inductor has to be depends on the value it needs. When the switches shrink, these don't.
• Analog circuits care a lot about every part having exactly the right value. A change in temperature, or a tiny difference from manufacturing, and the result drifts.

So many analog chips are made the way chips were made several generations ago, and plenty of them still use 8-inch wafers (see What do 12-inch and 8-inch wafers mean?).

They often stay in production for years, too. Cars and factory machines are used for many years, and the chips inside have to stay available. Some analog chips designed long ago are still being made today.

Why do digital chips keep chasing smaller sizes?

What a digital chip can do comes down mostly to two things: how many switches it holds, and how fast and how efficiently they switch. So every generation that shrinks makes it stronger right away: the line width gets smaller, and the same area holds more switches. Processors, memory and AI chips need this most (see What is Moore's law? and The news says "2 nm". What exactly is 2 nm?).

	Digital chips	Analog chips
Signals they handle	Only 0 and 1	Smoothly changing
Common examples	Processors, memory	Amplifiers, power management, circuits that send and receive wireless signals
What shrinking gives	A lot: more, faster, lower power	Not much, and sometimes it makes them harder
Usual process	The newest few generations	Mature nodes

Can one chip have both?

Yes. That's called mixed-signal. An ADC or a DAC is mixed-signal by nature: analog on one side, numbers on the other.

Plenty of chips are like this. The microcontrollers common in home appliances and remote controls often have an ADC built in, so they can read a temperature sensor's voltage directly. A phone's SoC has blocks that compute, and also blocks that talk to the outside world and handle analog signals (see What is an SoC?).

Putting them together has a catch. Digital circuits are always switching on and off, which makes noise, and the noise travels through the silicon the whole chip shares underneath. Analog circuits are sensitive, so they have to be kept apart and shielded. It's like recording sound next to a building site: you shut the windows first.

In fab24hr

fab24hr's main story, The life of a chip, follows a fictional hearing-aid chip, "Glowmere". A hearing aid is a small device that helps people hear sounds clearly. Devices like this generally pick up sound with a microphone, amplify it, turn it into numbers to process, then turn it back into sound for the ear; they also have to make a tiny battery last (hearing aids often use a zinc-air battery). The game doesn't say what blocks Glowmere is divided into, and it doesn't show any of these conversions.

Here is what you do see in the game. In chapter 11, "The Ball Lifted", Shu-Hui, the assembly line's line leader, tells you that Glowmere goes into SiP modules: a small board with the chip on it, plus a few resistors and capacitors (see What is SiP?). In the finale, chapter 13, "Into His Hands", you're Mr. Shen. Your daughter is talking at the door; you can see her lips moving, but the sound is far away and blurry. Once the battery is in, the hearing aid gives a short chime, and she asks, "Dad, can you hear me?" Later your wife says "Dinner's ready" in the kitchen, and you hear it, through a wall.

Tap "Clock in" and choose "The life of a chip" to follow Glowmere from the R&D line into Mr. Shen's hands. The companies, the chip and the people in the game are fictional; the parts about analog and digital chips come from public, general sources. For other jargon, see Chip fab jargon in plain English.