Demystifying Analog & Mixed-Signal ASICs

Unlike multi-function standard product ICs such as a micro-controller that can find its way into a wide variety of applications, ASICs are designed for one specific application and generally for one specific product or product family. To better understand the role and applicability of ASICs, it is important to briefly review their historical origins. The first Integrated Circuits from the early '60's contained just a few transistors and performed simple digital logic functions such as "and", "or", "nor", etc. These were called SSI devices, meaning Small-Scale Integration. As photolithography techniques improved, more and more transistors could be built on a single sliver of silicon. Soon, chip companies were developing Medium Scale "MSI" logic function like flip-flops, buffers, latches, etc (10-100 transistors). Large Scale "LSI" (100-1'000 transistors) and eventually VLSI (up to 100'000 transistors) ICs followed, providing lower system costs and higher levels of performance. Today of course, we have digital chips in excess of a billion transistors thanks to advanced sub-micron lithography and the low voltage, high speed processes upon which they are built. The first digital ASICs were built using a standard cell library consisting of fixed-height, variable-width 'tiles' containing the digital logic functions discussed above. The ability to reuse these blocks over and over saved time and money when designing a custom logic IC. Analog ICs were initially comprised of a pair of matched transistors and soon expanded to include rudimentary Op Amps, Voltage Regulators, Comparators, Timers and much more. Analog applications typically involve much higher voltages so these ICs needed their own unique set of manufacturing processes. More recently, market demands for smaller size, higher speeds and lower power consumption have forced a merging of analog and digital functionality on a single silicon chip. Cells consisting of the basic analog building blocks discussed above were created and added to the digital libraries. These Analog cells were restricted to the digital fab processes developed for predominately logic applications. Today, most ASIC companies offer some degree of analog functionality as a part of their services. In many cases, the analog functions are mimicked with digital design techniques. In others, compromises to the analog functionality must be made to facilitate the use of standard library cells that are designed to yield well in the fab processes developed for high speed, high density, low power digital designs. Often, these chips are referred to as Mixed-signal ASICs or as big "D", little "A" ASICs, meaning high digital content and minimal analog content. Analog ASICs play a critical role in our lives. Without them, none of the portable electronic devices we use in our daily lives would exist. Imagine a world without Cell Phones, MP3 players and Navigation Systems. Building them with standard products would make them prohibitively expensive and physically impossible to carry in our purses or pockets. Every automobile contains dozens of ASIC chips for everything from climate control to airbag deployment; suspension control to entertainment systems. ASICs also play important roles in applications for hospital medical equipment, eMeters, home appliances such as washers and dryers, scuba gear, hearing aids, and much more. The Analog ASIC market is huge. In fact, research firm, IC Insights reports that almost 60% of the nearly $37B of Analog ICs sold in 2010 were ASICs. Yet very few mixed-signal ASIC design houses fully understand the implications of custom Analog design and its applicability to Analog centric ASICs. ASICs requiring high Analog content should be directed to those design houses that specialize in Analog circuit design rather than those who simply select Analog IP blocks from a library. Analog ASIC companies have large staffs of competent, well-experienced, Analog engineers with expertise in a wide range of Analog functions. The following table offers a range of these required design skills. Reviewing an ASIC house's patent portfolio as a quick guide as to the creativity of its engineering team will serve as a first order measure of its Analog expertise. Clearly, the large Analog IC houses (like ADI, Linear Tech, Maxim, National, TI) have patent portfolios a mile deep. Those that also engage in Analog ASIC development set high bars regarding who can access this capability and impose high minimum order requirements. For example, TI reports that their application-specific analog business focuses on a small number of large customers like Seagate, Sony, Samsung, Hitachi Global Storage Technology, Toshiba and a few others that require custom application-specific products. Minimum annual unit and or dollar volumes force the majority of the smaller customers to seek out independent Analog or Mixed-signal ASIC design houses. Myth #1. It is only economical to integrate Analog functions into an ASIC if the Analog content is minimal. The ASIC concept began as an integration tool to lower the costs of computationally heavy logic circuits. Today, after 30+ years, ASICs remain heavily digitally oriented. When we hear the terms like SoC (System on Chip) and Re-useable IP (Intellectual Property) associated with ASICs, we often think of the massively complicated, digital centric ASICs that may contain a few important analog functions. Historically, it is these products that have garnered the attention of the media and established a mind set among the user community that a little Analog can go a long ways. But what about the applications requiring Analog centric ASICs? These are SoCs as well, even though they may not contain a uP core or even memory. The medical/industrial world is rife with such requirements yet most ASIC companies are quite unprepared for the challenges of hand-crafting the unique Analog circuitry required for these important applications. The actual manufacturing cost of the ASIC chip may imply a huge savings when compared to the collective costs of the ICs it replaces. However, there are other costs associated with the ASIC that must be considered and amortized over the life of the product. Non-Recurring Engineering costs, based on the complexity of the design as well as hard tooling costs such as masks and test hardware, can add a few pennies or a few dollars to the ASIC chip cost, depending on the complexity and lifetime volume of the device. Incorporating elements into the chip that require more exotic processes for features like high currents or low noise or high frequency will increase the cost of all the elements in that chip. Therefore, it is as important to know what to incorporate into the ASIC as it is to know what should remain a discrete component. Interestingly, the use of multiple smaller, less complicated Analog ASICs, differentiated by their manufacturing processes, can offer surprisingly stunning cost reduction results. Most Analog applications use a collection of passive elements and discrete transistors in addition to the ICs involved. Integrating as many of these components as possible to the ASIC often comes for free and can have a dramatic effect in lowering the end product's total assembly cost. It is this potential total system cost saving that bolsters the justification to develop the Analog ASIC. Myth #2. Mixed-Signal ASIC means the same thing as Analog ASIC While the term "Mixed-Signal" implies a combination of Analog and Digital circuitry on a single chip, there is a distinct difference in the skill levels required to combine library cells (Analog and Digital) on a silicon chip versus actually creating an Analog design that uniquely satisfies all requirements of the specification. For many applications, Analog library cells offer sufficient performance to meet the system requirements. However, more and more frequently, the increased sophistication of the Analog application necessitates designs that are truly 'application specific' and not a compilation of general-purpose Analog cell blocks. Like the big Analog IC companies, true Analog ASIC companies employ experienced Analog designers who are artisans at Analog invention. Many of them have spent years at the big Analog companies, learning from the industry gurus. Be careful not to let a Mixed-signal design house negotiate you away from your ideal specification. Close isn't good enough…analog must be exact. Myth #3. Only Ultra-high volume applications can benefit from Analog ASICs. Myth #4. Using existing IP from Analog Cell Libraries lowers the chip cost. Myth #5. Cell based ASIC designs ensure product differentiation Myth #6. Handcrafted Analog is too expensive, compared to standard cells. Myth #7. The most cost effective solution is to pack as much as possible into the Mixed-signal ASIC chip. Conclusions The application will always determine the appropriate combinations of technologies that are best suited for the ASIC design. As our dependence on cognitive prosthesis devices (smart phones, Wii controllers, tablet PCs, etc.) increases, copper tethers disappear and Analog increases its dominance in ASIC designs. MEMS advances have placed Star Trek style sensors in our daily lives. Medical imaging, sensing and monitoring continues to improve our daily lives. All of these and more increasingly rely upon better, faster, denser Analog circuit content. When considering a new ASIC design, carefully consider the role Analog will play in its deployment. To minimize risk, choose your ASIC development partner carefully. Most of the time, Mixed-signal ASIC design skills will be sufficient. To minimize risk, seek out an Analog ASIC partner with the right Analog design skills and experience to match the application. ----- The full whitepaper can been read here.