Digital layout generation is performed using a specific library of characterized layout
cells. These cells are pieced together and routed to form an overall block or design.
Because digital layout is generated from a fixed cell library and due to the regenerative
nature of digital signals, which alleviates some routing concerns, digital layout generators
have been available for the past decade. Analog circuits, on the other hand, do not draw
from a library of devices. Thus there can be a wide variety of layouts for the same
circuit, and finding the optimum layout for a given design is a very difficult task. It is the
intuition of the designer and/or layout engineer that determines which layout will give
optimal performance. Obviously there can and will be some error in this process, but
finding the truly optimal layout would require unreasonable design and computation time.
Analog layout can be broken down into three phases: device generation, device
placement and routing. In the device generation phase, three types of devices are
necessary: MOSFET’s, resistors, and capacitors. After creating the devices, they must
then be placed and routed. Because of the sensitivity of analog circuits to path
differences, thermal and processing gradients, and parasitic resistances and capacitances,
each stage of analog layout is crucial. The following sections discuss the creation of the
different types of devices, MOSFET’s, resistors and capacitors and the routing between
these devices. Within these sections, some of the placement issues found in analog
layout are also addressed.