Archive for October, 2007

You may be familiar with a famous quote often attributed to the U.S. Patent Office commissioner back in the late 1800s: “Everything that can be invented has been invented”. While research seems to conclude that this quote is actually a long-lived urban myth, could we say something similar about analog design? Has every analog circuit that can be designed … been designed?

Before all you analog designers take offense, I should explain my motivation for posing this provocative question. Firstly, it is to encourage you to review a little history of analog design by reading an excellent set of articles published in this month’s IEEE Solid State Circuits Society newsletter. You should be able to access the articles online even if you are not an IEEE member: Tales of the Continuum: A Subsampled History of Analog Circuits by Thomas H. Lee and A History of the Continuously Innovative Analog Integrated Circuit by Ian Young.

Some of my earliest design influences are recalled here: Bob Widlar, Paul Brokaw, Alan Grebene, Hans Camenzind, and many more. If you are doing analog design and are not already intimately familiar with the contributions of these individuals, this should be required reading. If, like me, you studied the designs of these icons of analog early in your career, you will enjoy the retrospective. You may also be surprised to learn that early demonstrations of how the world is analog can be traced back to the ancient Greeks. Lee begins the history of analog design with the discovery of the first analog computer, the Antikythera mechanism, which is traced to the wreck of a cargo vessel in the Aegean Sea more than 2000 years ago!

Secondly, my purpose is to encourage designers to look back at circuit techniques that may have been published long ago, perhaps in a different context, for inspiration on how to solve problems today. Perhaps a more appropriate quote or title for this post is that “everything old is new again”. There are many examples of innovation in analog designs that have demonstrated that is true. As an example, some may think that delta-sigma techniques for ADCs and DACs are new, but do you know that the delta-sigma modulator had its origin more than thirty years earlier, in the field of communications? CMOS technology and the requirement for small, high-resolution converters created the opportunity to apply the technique that is now used almost universally in consumer electronics.

What new-old analog circuit techniques have you seen lately?

-Mike

Analog design is Black Magic??? Recently there have been two occurrences where this description of analog design has been used; a panel discussion at the IEEE SOC Conference in Taiwan (09/27/07), and Mike Santarini’s blog (09/18/07) at EDN Magazine.

From the Cambridge Dictionary online:
black magic noun
a type of magic that is believed to use evil spirits (= beings which cannot be seen) to do harmful things

That’s a pretty derogatory description of analog design, in my opinion. Where does this misguided notion come from? Is analog design so hard that non-designers think it must be black magic?

Here’s my definition of analog design:
analog design noun

1. An art practiced by skilled engineers who possess the creativity and expertise to construct high performance circuits from individual transistors and passive components.
2. May appear to be magic compared to digital design

Yes… transistors and circuits, not gates and RTL. Take the simple case of a 2-transistor inverter. To a digital designer it’s quite simple; B = not A (or A’).

Now let’s take an analog view of the same two transistors. To an analog designer, we can make those two transistors into an amplifier: v_b = g_mv_ar_o, g_m = C_g·W/L· (v_a-vt₀)

In the digital case the designer is an expert in logic or writing RTL, and the circuit details are accounted for by a synthesizer. In the analog case the designer is a circuit expert, and the (increasingly complex and variable) device model details are accounted for in a simulator.

That doesn’t make either analog or digital design black magic! But both cases provide excellent examples of how advances in EDA tools have contributed to increased designer productivity and IC innovation. I have to address one of the EDN blog commenters who claimed “it will be a very unusual moment when someone actually comes up with an analog EDA tool that helps an analog designer”. Wow, and I thought the “black magic” myth was bad!

Unusual to make an analog EDA tool that helps a designer? Not hardly! The problem is that the first introduction of the analog designer’s primary EDA tool came before any of these folks were paying attention, in fact before there even was an EDA industry… and it was SPICE! Ten years from now, when logic synthesis has established as long a history as circuit simulation, I wouldn’t be surprised to hear the same type of comments about digital EDA.

SPICE created just as big a paradigm shift as Design Compiler did, it just came about 10-15 years earlier. If you look back at how IC design was done before SPICE simulation became so pervasive, you would see that breadboard prototyping with discrete transistors, paper & pencil, maybe even a slide rule calculation, were the only tools available. Can anyone imagine designing a 5GHz RF SoC in CMOS without modern analog EDA tools, starting with simulation? And that’s just one example of leading edge analog design from the most recent ISSCC.

The EDA industry certainly has not stood still on progress in analog tools. I could go on and on about layout tools, optimizers, waveform analyzers and many more, but let’s just look at analog circuit simulation. There have been more than thirty years of major innovations in circuit simulation that have powered the advances in analog design that some apparently seem to find so mysterious.

Here is a list of just a few of the significant advances in circuit simulation over the last 35 years:

• 1971: SPICE 1.0
• 1978: SPICE-2g6
• 1981: HSPICE – the 1st successful commercial SPICE. The Level-28 model
• 1989: SPICE-3
• 1995: 1st generation Fast-SPICE, SPICE-HDL co-simulation
• 1996: BSIM-3 model
• 1996: Introduction of Verilog-A behavioral modeling
• 2000: 2nd generation hierarchical Fast-SPICE, Verilog-AMS
• 2007: Multi-threading and parallel SPICE, new special-purpose Fast-SPICE engines

Analog design. An art. A science. A rare skill. But black magic it’s not!

I welcome your comments.

-Mike