Digitizing analog circuits reduces chip area
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Because analog circuits refuse to follow Moore's Law, logic circuit designers are taking steps to take advantage of CMOS technology. By replacing analog functions with digital circuits, even if the digital circuits use more transistors than the analog versions, you get more predictable performance, easier testing, and reduced chip area. This digitization of analog circuits is not just about using a digital signal processor (DSP) to implement complex analog functions. In fact, a big concept today is the digitization of relatively simple analog functions. Digital hysteresis circuits are an excellent example of this technique because they transform an impossible customer requirement into a low-cost product that is being manufactured today. Traditionally, analog blocks have to be large enough to get a reasonable yield, making the entire chip very expensive. If we rethink the application from a digital point of view, we can cut the chip size in half and make the product viable. This success is encouraging a new way of thinking about analog functions. As the cost of digital transistors in SoCs drops to near zero, the choice between analog and digital implementation has clearly tilted toward digital. Now you can replace analog circuits with more complex but digital versions. This groundbreaking technology shift has already happened, but few analog designers really understand it. The following example of a digital hysteresis circuit illustrates what this means. Digital hysteresis circuits are not new, of course. What is new is the use of digital versions even when the application would seem to require analog functionality. Normally, both analog and digital circuit designers think of hysteresis as an analog function. We can use a mechanical system as an analogy: the threshold point for movement in one direction is different from the threshold point for movement in the other direction, and hysteresis is the difference between these two thresholds. Mechanical engineers work hard to reduce hysteresis, but you need some amount of hysteresis to ensure stability in any system that contains feedback. Therefore, analog designers are used to designing with hysteresis in mind. Digital designers don’t tend to think about hysteresis unless they need to control an analog/mechanical subsystem that must avoid jittering about a single set point. In this case, the customer's SoC application involved multiple analog signals that had to bounce. Instead of transitioning smoothly from one voltage to another, these signals rose and then bounced between the two voltages. The analog designer responsible for this part of the application knew that he could use hysteresis to filter out this jitter. The designer developed an obvious solution: the analog circuitry would respond immediately to the initial transition from a low voltage to a high voltage, but would not respond immediately if the voltage bounced at intervals on the order of 10 to 20 microseconds. This circuit worked fine in simulation and required only 14 analog transistors, but the matching resistors required by the circuit caused manufacturing problems. When these resistors were physically made large enough to achieve adequate matching, the die became so large that the SoC was no longer economically viable. As a result, the product could not be shipped. Fortunately, some digital designers heard analog designers talking about this unbuildable product and proposed a solution. An initial input transition resets all flip-flops, causing the circuit output to change immediately. At the same time, a 32kHz clock begins a series of toggles to synchronize the input transitions. The feedback signal prevents the circuit output from changing state until the initial input transition has cascaded through the clock toggles. Therefore, the clock frequency and the number of toggles determine the amount of hysteresis in the circuit. When used with a single analog input comparator, this digital hysteresis circuit can perform as accurately as its analog counterpart. However, the digital version has a fatal drawback for analog designers: it requires about 1,000 transistors, compared with the analog version's 14 transistors. But as it turned out, the digital version took up far less silicon area than the analog circuit. The digital transistors were much smaller, eliminating the need for large matching resistors. As a result, the SoC became a viable product. In addition to reducing chip area, a digital hysteresis circuit provides more predictable performance because its behavior does not vary with temperature or process-related tolerances. Similarly, the digital version is easier to test because the test equipment does not have to consider the various ways that the analog version can go wrong. Built-in self-test was not an option for the original analog version. For all these reasons, more analog functions must go digital. In fact, this transition has been going on for many years, with examples such as Class D amplifiers, delta-sigma converters, and programmable I/O in FPGAs. But some analog functions will successfully resist the trend toward digitization. The example of current-controlled D/A converters shows that large analog transistor counts work well when the transistors are used in a dense "digital" manner. For many other analog functions, the trend toward digitization will continue until the analog portion is reduced to just a few transistors. By Richard Tobias Vice President
Harry Peterson
Director of Design Engineering, ASIC and Foundry Business Unit
Toshiba America Electronic Devices
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