It used to be easy to pin down analog and mixed-signal electronics. The basic building blocks included amplifiers and data converters. Chip companies specialized in continually improving those blocks. And, very special engineers could put them together into useful designs.
Today, new chips tend to combine multiple functions with an eye toward specific applications, fabless startups regularly challenge mainline chip companies, and analog-circuit wizards like Bob Pease are growing scarce.
To get a line on where analog is heading, we spoke with three gurus: Linear Technology’s VP and chief technical officer, Robert Dobkin; Robert Hum, the VP and general manager of Mentor Graphics’ deep-submicron division; and analog industry analyst Stephen Ohr of Gartner Inc.
DOBKIN ON PROCESS TECHNOLOGIES
Q: What do you think about building analog and mixed-signal chips on a foundry’s state-of-the-art CMOS process tech-nology versus an analog-specialty company’s dedicated in-house technologies?
A: It depends on what you’re looking for. Almost all logic is made from CMOS because of its improved density and its ability to handle the ones and zeros at low power. With analog, that is not the case! Bipolar and CMOS are different tools, and bipolar transistors are still the best transistors in many types of analog circuits.
RF ICs, precision amplifiers, references, and low-noise signal processing all require bipolar transistors for highest performance. Many of these bipolar transistors and various CMOS types are unique to the analog IC manufacturers and not readily available at foundries. This yields analog ICs that are unique to the manufacturers and to their internal semiconductor fabs.
Also, bipolar transistors are not subject to the same lithographic shrinking that MOS devices enjoy, so the density of bipolar transistor circuits remains relatively constant with time. Some bipolar ICs introduced 25 years ago have the same chip size today and cannot be reduced.
Still, the continuing push for smaller line widths in digital ICs has had its effect on analog as well. These processes continually get faster and enhance the speed of analog ICs made on those processes. Analog-to-digital (A/D) converters are a good example where smaller line widths have led to faster A/D converters. Circuit advances improve the resolution as well as the speed, and new devices convert at 200 Mbits/s with 16 bits of resolution. There’s no reason to think that they will not continue to accelerate in speed and precision as techniques are developed to go faster and more accurately.
These smaller line widths make practical some digital functions that were previously impractical in analog ICs. Pow-er-supply controllers with digital readback of voltage, current, and status are achieved on the same chip as the power functions. A side benefit to the smaller line widths is the inclusion of DMOS transistors on the small-feature digital process. These DMOS devices can handle high voltage as well as high current and allow the complete integration of power converters, switching regulators with both power and control on the same device.
Q: How do you increase performance when you’re using mature processes?
A: Analog advances are usually the result of new circuit concepts that are developed in-house. These new circuits are often closely mated to the process, giving a synergistic result. Very few of these types of circuits cross manufacturer boundaries and rarely end up sourced by multiple companies in multiple products. This is unlike digital advances in process technology, which tend to be proliferated across the industry.
Accumulated knowledge about making complex functions in different processes continues to advance. This knowledge allows still higher-functionality mixed-signal ICs to be generated each year.
Circuit advances in bipolar technology with some process enhancements yield incrementally better references, amplifiers, and RF ICs. These enhancements are due to refinement in the circuitry and optimization rather then any type of breakthrough due to process. The competitive landscape for analog ICs keeps all manufacturers on their toes in trying to improve their products.
However, it should be noted that since analog IC performance is based on real-world parameters, the ICs can end up at theoretical limitations of performance. Once the theoretical limitation is approached, further improvements are just not possible. That’s why some of today’s bestselling and most widely used ICs were designed over 20 years ago and haven’t changed in the interim.
Q: Will analog circuits follow the price curve that digital circuits follow?
A: No! No! Buyers always want analog pricing to follow the same path as digital. That can never be the case! Analog IC die sizes and costs are dictated by the voltages and currents they manipulate. The cost is based on real-world parameters that are inherent to the IC. By the time digital ICs are four years old, they are obsolete and are being replaced by denser, smaller chips. After four years, the analog IC has only just been optimized and all production-cost reductions have been realized. Some 25-year-old analog ICs are still in high-volume production, and expecting 25 years of continuing price reductions is unreasonable.