It doesn't matter what product we want to think about. Every product, when put in an environment in which it does something useful, is an analog system. That is because this is an analog world and digital is an artificial construct that our human brains created to attempt to make life simpler. We like binaries, be they for computation purposes or socially defining things, such as male, female, black, white, etc. It is much easier to talk about binaries rather than try and define something in between.
So what makes analog and digital so different? Let's start with analog. Analog entities are continuous in time and value. Things such as sound, heat, and pressure can vary over time, over distance, and can never be defined with exact precision. A sensor can convert between these environmental factors and electrical signals, creating a relationship between them, hopefully with some degree of accuracy. There are equally an infinite number of possible voltages and when formed into a circuit, currents that flow around a circuit. Those current flows create magnetic fields that can induce currents in other signal wires. We rely on this effect to create radios and transformers although within a chip they are usually undesirable effects and referred to as electromagnetic interference or EMI. But they exist and they can influence an analog circuit — a subject we will probably return to several times in the future.
In the digital world there are only three possible values — '1,' '0,' and don't know or don't care. Most of the time we don't even care about what voltage is associated with those logic values. Transitions between logic values are considered to happen at a particular instant in time and we are not interested in what changes they may make except at specific points that we call a sampling point. This is just one of the abstractions that have been applied in the digital domain over the past 20 years. If we consider the analog domain we find that little has changed over that period. Design is still done at the primitive device schematic-level and verified using a SPICE-like simulator.
This has created an ever widening abstraction and complexity gap between the two domains. Those digital abstractions have enabled automated synthesis solutions. Analog remains a manual process.
What happens when the two come together? Most of the time, they may as well be talking different languages and we can blame Gene Amdahl for what happens when they try to work together on a problem. For example, a software engineer may want to simulate for minutes or hours of real time, the digital designer want to simulate for seconds or minutes of real time, while the analog guys might be happy with a millisecond. Digital designers will generally ignore chip to chip variations, temperature, or voltage; instead just rely on their designs being impervious to them within specified limits so long as they follow a number of design rules. For the analog designer, no such things can be ignored and their problems are being made worse with every process improvement made for the digital folks.
But it is important for the two sides to work together on a single chip. Increasing levels of integration are necessary and it is no longer possible to design and verify the analog and digital parts separately. I will be exploring the problems and some solutions to these problems, especially with respect to integrated analog functionality, in the blogs that follow. Watch for further thoughts here on Planet Analog and on the Integration Nation site.