Field Programmable Analog Array
A field programmable analog array ( FPAA, Eng. Field programmable analog array) is a fully programmable analog circuit, designed on the model of FPGAs. A FPAA consists of an array of configurable analog blocks. Such a block is called CAB (English for Configurable Analog Block).
With a CAB, various basic analog circuits such as filters or amplifiers realize. A logic circuit is used to establish the necessary connections within and between the CABs. Analog inputs and outputs make the connection to the outside world. The configuration is done via digital inputs and outputs that can be connected to memory chips or microprocessors.
Design and function
The core of an FPAA forms the matrix of programmable analog circuits. Herein are 2-20 CABs. These consist of one or more CAB operational amplifiers, and in particular transconductance amplifier and passive elements which can be linked to one another via electronic switches. Thus, it is possible to realize a variety of simple analog circuitry.
In the implementation of switchable components, two principles apply: the switched-capacitor technique and the use of conventional components (resistors and capacitors).
The switched capacitor (SC) can be both capacity and resistors produce. A resistor is produced by a capacitor at high frequency is first to switched to the negative and the positive pole of the signal path, it depends on the current signal voltage, the switching frequency and its capacity alternately partial charges and discharges. The advantage of this technique is its high flexibility and stability. Disadvantages are the low bandwidth as a function of the switching frequency (see Nyquist theorem ) and the parasitic effects that are also due to the switching frequency.
An alternative is the use of conventional components in the form of unit resistances and capacitances dar. Depending on the intended use of the FPAA are these wired partially fixed. This enables a higher bandwidth and avoids the artifacts, such as occur in sampled systems. A disadvantage is the low flexibility of the circuits.
In addition, the configurable analog blocks contain a digital memory (RAM) in which the desired configuration is charged. Some of FPAAs Anadigm have additional " shadow RAM ", which make it possible to load new configuration data during operation, without interruption of the analog signal.
The Analog matrix is surrounded by a number of auxiliary circuits. What is needed is a logic circuit which processes the configuration data, and provides for storage in the correct place. An internal digital timer coordinated the loading of the configuration. Frequently are configurable input and output cells are available that are equipped for example with switchable anti-aliasing filters or amplifiers. Are Switched - Capacitors used as analog timers are required.
Programming
Programming FPAAs done by a manufacturer-specific protocol, depending on the architecture of the chip. The manufacturers provide for their own development software products available, with the help of which may be the desired circuit developed and usually also simulated. The resulting configuration is then uploaded in the form of a bit stream in the chip.