A bit about analog,and a bit about bits

Prof. Tom LeeStanford University

Matrix Semiconductortomlee@ee.stanford.edu

http://www-smirc.stanford.edu

• Analog quantities – such as voltage, temperature, and pressure – span a continuum of values.

– Signals can range from, say, WiFi microvolts to power line megavolts.

– Analog systems typically evince high functionality per unit power consumed (e.g., the human ear consumes tens of microwatts), but

– Difficult to process and store analog signals faithfully.– Analog functions are usually highly specialized;

typically can’t readily convert one analog circuit into another one.

• “Nature is analog.”

What is analog, anyway?

• Digital signals are discrete in amplitude and in time– Signals are of uniform amplitude, say.– Digital systems typically consume high power per function, but– Storage and replication may be performed essentially without

error.– Digital systems can be quite flexible.

• Paradoxically, this property has killed off circuit design innovation.

• “Digital is unnatural.”

What, then, is digital?

• Analog processing of some kind is unavoidable at interfaces with the physical world.– No such thing as an “all-digital” radio, for example

(despite the buzz about “software” radios).• Digital processing is a powerful and flexible way

of transforming, storing, conveying and regenerating information.– Can’t expect a 100th-generation photocopy to resemble

the original, but a 100th-generation CD-ROM can be just as good as the master.

• Analog and digital are good at quite different things.– Explains why the arc of history has traced an increasing

synthesis of these two.

Analog vs. digital: A false dichotomy

Increasing symbiosis through the ages

Increasing symbiosis through the ages

Increasing symbiosis through the ages

Bottom electrode (silicon)

Silicon nitride membrane

Top electrode

Vacuum

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Oxide passivation

Insulating layer

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100 m

20 m100 m100 m

Another: 3-D imaging with 2-D CMUTs

What you can do with a CMUT

Mechanically scanned transducerAnalog processing

2-D electronically scanned arrayMassive digital processing

• Purely digital paradigms are limited.– The Pentium 4 consumes ~100W, the human brain but

25W.

So, (why) is analog ascending?

Biology has thrown down the gauntlet

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• One view: Analog will serve as merely another rationale for Intel to build ever-more powerful CPUs. This co-dependency will continue until market forces dictate otherwise.

• Another view: Insights gained from studying diverse fields will inform, and drive, a convergent evolution of seemingly disparate technologies.– Hard to predict where this will go, but my guess is that

ethicists and social scientists will have plenty to chew on.

Whither analog?

Man is the only computer that can be mass-produced by unskilled labor.

-Wernher von Braun, rocket scientist