Why be bound by hardwired, fixed-function, fixed-parameter analog circuits, when you can have the signal handling of the analog world combined with the flexibility of the programmable one? Or perhaps an analog-laden version of a system-on-chip?
Good questions, but they have complex answers, depending both on the definition of programmable as well as on the type of design effort and application priorities involved. The term "programmable" can refer to basic fixed-function ICs like filters, which allow the user to set, via a resistor, jumper or dc signal, an operating point, bandwidth or other parameter. It also means devices with fixed functions but with one-time or reprogrammable software-settable parameters, which are retained internally after power-off. And finally, it means devices that can be set to establish the topology and interconnect, similar to an FPGA, and even be reconfigured in the field for maximum flexibility.
Of course, configurable analog components are not the answer to every analog challenge. They offer advantages but also have limitations in performance, precision and speed that restrict them to the lower end of the spectrum and such applications as power circuits, motor control and sensor I/O. These, however, encompass a fairly big part of the analog application world, where 12 or 14 bits is sufficient.
Available programmable analog components range from small-scale devices such as the electrically programmable analog device (EPAD) MOSFETs from Advanced Linear Devices Inc., to complex, multifaceted topologies such as the PSoC family from Cypress Semiconductor Corp., with many waypoints in between.
The driving factor for programmable analog devices is their flexibility. With engineers pressured by short design cycles on one side, and cost and time of board design, layout and fabrication on the other, an analog block that adds an extra channel, changes bandwidth or sampling rate, provides extra I/O, or redefines itself in the field is very attractive. It's even better if the same part can provide different functions in actual product use, reconfiguring and redefining itself, for example, to shift from wideband, fast data acquisition to narrowband, lower-noise mode with a longer time base as appropriate.
There's also a parallel trend toward application- and function-specific devices that are optimized for a specific signal-processing operation or role such as power-supply management. Further, some of the applications for programmable analog ICs are less exciting than classic new-product development. An example might be such tasks as providing functional replacements for obsolete or unavailable devices in a form-factor-compatible design.