Analog Simulation Insights

 

Analog design is NOT black magic… but it is VERY hard

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’).

inverter2.gif

The Digital View

  • It’s an inverter
  • Easy to determine state
  • Design with 1 equation
  • Only 2 logical variables

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: vb = gmvaro, gm = Cg·W/L· (va-vt0)

amplifier2.gif

The Analog View

  • It’s an amplifier
  • Hard to determine state
  • A large number of equations
  • Numerous variables (physical, electrical, etc.

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

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