Surveying the “high-performance” analog landscape is a lot like looking through a kaleidoscope. With each turn, you’ll see performance characteristics: precision, bandwidth, conversion rates, noise, power consumption, physical size, dynamic range, price, etc. Depending on the application, you may be happy to sacrifice some to optimize others.
Turn the kaleidoscope a little, and you’ll see basic topologies or input/output configurations. Turn again. Analog-todigital converter (ADC) “performance” characteristics overlap depending on whether they’re pipelines, successive approximation registers (SARs), folded-interpolated, or delta-sigmas. Lately, thanks to National Semiconductor, it also depends on whether the delta-sigmas have the old-fashioned discrete-time inputs or whether they’re continuous-time delta-sigmas.
Yet some of this view also depends on characteristics such as reference voltage and clock stability, what kinds of clever tricks designers use to split the input signal between multiple data converters, the voltage range of the input signal, and whether it’s double-ended or ground-referenced. What it boils down to is that one engineer’s high-performance needs differ from those of another engineer at a company down the road.
Then let’s have some folks in marketing shake the kaleidoscope. Some will tell you that they’re getting beat up by customers who simply can’t duplicate the chip company’s datasheet guaranteed performance specs on their production boards. Another group thinks its applications engineers should create bulletproof reference designs for its front-end amps and data converters.
Others think it’s best to customize the silicon totally so that analog comes in one end and data comes out the other, while still others take a modular route, with multiple die in a single package. Indeed, it’s not unheard of to find combinations of these approaches in a single company, depending on the sales volumes of the potential enduses of the products.
THE FINAL TWIST OF THE WRIST With another turn of the kaleidoscope, one can look at analog performance in terms of the absolute physical limits of thermal noise or clock jitter. Is there some kind of absolute Heisenberg limit you can’t beat? This is where the question of high performance gets really interesting.
I talked to a number of people in preparing this article, and I confess the most stimulating conversations occurred in the lab at Linear Technology, where I got to pull the legendary Jim Williams away from his test bench and engage him on the topic of high performance.
When the discussion turned to fundamental physical limitations, Williams surprised me by insisting that no one—not Boltzmann, not Shannon, but no one—had the power to say “Thus far, and no further.” He thought it wasn’t so much a matter of cheating. Rather, it involved renogotiating the physical issues that we engineers have to face and overcome.
He presented me with a number of examples, most of which I should have seen coming. The chopper-stabilized amplifier and the sigma-delta modulator, he pointed out, were two examples from long, long ago that overcame “fundamental physical limitations” by redefining the problems.
More recently, Williams said, Intel had at least temporarily put the gate leakage problem aside and gained several more generations for Moore’s law by resurrecting the multicore processor approach attempted by Sun with SPARC in 1992. It didn’t work for Scott McNealy, but had for Paul Otellini.
Curiously, since we were talking about parallel processing, Williams’ real hero on the digital side is Gene Amdahl, which surprised me. Williams started talking about oscilloscope probes, which seemed to be a change in topic. But at least I was on firm ground since I’d spent several years working at Tektronix and figured I knew a 465 from a 7912. He then described how an oscilloscope probe and vertical amplifier “broke the rules” about transmission lines.
Essentially, the probe doesn’t cause a significant perturbation on the point in the circuit it’s measuring. Also, the probe reactance is adjusted to “compensate” for transmission- line effects. Therefore, what the vertical amplifier presents to the scope display circuitry is an accurate representation of what’s going on down at the probe tip.
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