Key Components to Analog CMOS

Published:2011/9/14 1:37:00 Author:Phyllis From:SeekIC

Higher and higher levels of performance and precision are always the pursuit of electronic products ranging from mobile handsets, audio systems, HDTVs, to CT scanners and industrial control systems. Most of these systems rely on some form of digital microprocessor or DSP as the computational engine. However, each of these products derives their own characters from the high-performance analog chips inside.

There is usually a specially-crafted IC process technology behind these analog ICs which can optimize performance and precision. Such specialty processes are designed for performance and generally not intended for routine, cost-sensitive applications, and they typically drive the creation of standalone devices with unique characteristics. Such standalone designs have become widely used within System-on-Chip implementations.

Thanks to the continuing stream of new electronic products striving for differentiation, the analog IC segment of the industry growing at a rate higher than the industry as a whole.

Combining the engineering talent and fine-tuning the key components can make a high-precision analog chip design tick. Key analog CMOS components include MOS transistors which are critical in every signal chain IC, resistors which are particularly important in DACs, and capacitors that are the key to ADCs. Resistors and capacitors also play an important role in amplifiers, but they stand front and center in their respective converter applications.

For MOS transistors, noise is particularly important since high-precision products often must maintain high signal-to-noise ratio (SNR) to pick a weak signal out of the background noise level. Noise must be measured early and often, and engineered, not simply recorded.

Parasitic capacitances in MOS transistors must be minimized wherever possible, as these can lead to SNR issues, since this capacitance creates voltage divider networks that can reduce the voltage across the intended capacitance. MOS transistors are used as gain stages.

For resistors, the major considerations are sheet resistance and resistor tolerance as well as voltage and temperature coefficients. A thin-film resistor can be used instead due to its better overall behavior and capability to be laser-trimmed if needed. While the TFR is more difficult to process, requiring more masking steps, its added complexity can often mean the difference between a good product and a great product. For specialty, leading-edge products, this is often an easy decision.

For capacitors, the key concerns are capacitor density, tolerance, voltage coefficient, and dielectric absorption (DA), sometimes called hysteresis. This latter effect is a function of charge-trapping in the capacitor dielectric which can allow residual charge to reappear on the plates after discharging the component.

For every component listed above, one parameter that is fundamentally important in analog design is component mismatch. Mismatch refers to the percentage ratio of the difference between two identically designed components to their average value. Matching generally improves with larger component size (to a limit). The smaller the mismatch, the smaller the size of the components needed in the design which, in turn, leads to a smaller die and a lower die cost for a given design. This is a critical parameter that can used to weed out inferior processes.

For other product applications, specialty components may be needed such as junction field effect transistors (JFETs), for low-noise inputs, or drain-extended CMOS (DECMOS) devices for extended-voltage capability. These components require their own specialized optimization efforts and must be blended into the overall high-precision process in a manner that does not degrade the critical core components.

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