Analog design

Analog design

Before the advent of the microprocessor and software based design tools, analog ICs were designed using hand calculations. These ICs were basic circuits, op-amps are one example, usually involving no more than ten transistors and few connections. An iterative trial-and-error process and "overengineering" of device size was often necessary to achieve a manufacturable IC. Reuse of proven designs allowed progressively more complicated ICs to be built upon prior knowledge. When inexpensive computer processing became available in the 1970s, computer programs were written to simulate circuit designs with greater accuracy than practical by hand calculation. The first circuit simulator for analog ICs was called SPICE (Simulation Program with Integrated Circuits Emphasis). Computerized circuit simulation tools enable greater IC design complexity than hand calculations can achieve, making the design of analog ASICspractical. The computerized circuit simulators also enable mistakes to be found early in the design cycle before a physical device is fabricated. Additionally, a computerized circuit simulator can implement more sophisticated device models and circuit analysis too tedious for hand calculations, permitting Monte Carlo analysis and process sensitivity analysis to be practical. The effects of parameters such as temperature variation, doping concentration variation and statistical process variations can be simulated easily to determine if an IC design is manufacturable. Overall, computerized circuit simulation enables a higher degree of confidence that the circuit will work as expected upon manufacture.

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Coping with variability

A challenge most critical to analog IC design involves the variability of the individual devices built on the semiconductor chip. Unlike board-level circuit design which permits the designer to select devices that have each been tested and binned according to value, the device values on an IC can vary widely which are uncontrollable by the designer. For example, some IC resistors can vary ±20% and β of an integrated BJT can vary from 20 to 100. To add to the design challenge, device properties often vary between each processed semiconductor wafer. Device properties can even vary significantly across each individual IC due to doping gradients. The underlying cause of this variability is that many semiconductor devices are highly sensitive to uncontrollable random variances in the process. Slight changes to the amount of diffusion time, uneven doping levels, etc. can have large effects on device properties.

The design techniques necessary to reduce the effects of the device variation are:

• Using the ratios of resistors, which do match closely, rather than absolute resistor value.
• Using devices with matched geometrical shapes so they have matched variations.
• Making devices large so that statistical variations becomes an insignificant fraction of the overall device property.
• Segmenting large devices, such as resistors, into parts and interweaving them to cancel variations.
• Using common centroid device layout to cancel variations in devices which must match closely (such as the transistor differential pair of an op amp).

Fortunately for IC design, the absolute values of the devices are less critical than the identical matching of device performance. However, this fabrication variability forces certain design techniques and prevents the use of other design techniques familiar to the board-level designer.