Field programmable analog array

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A field programmable analog array ( FPAA , engl. Field-programmable analog array ) is a freely programmable analog circuit , designed in accordance with the model of the FPGAs . An FPAA consists of a matrix of configurable analog blocks. Such a block is called a CAB ( Configurable Analog Block ).

A CAB can be used to implement various basic analog circuits such as filters or amplifiers . A logic circuit is used to establish the necessary connections within and between the CABs. Analog inputs and outputs establish the connection to the outside world. The configuration takes place via digital inputs and outputs, which can be connected to memory modules or microprocessors .

Layout and function

Building an FPAA

The core of an FPAA is the matrix of programmable analog circuits. There are between 2 and 20 CABs in it. These CABs consist of one or more operational amplifiers , in particular transconductance amplifiers , and passive components that can be linked to one another via electronic switches. It is thus possible to implement a large number of simple analog circuits.

When implementing the switchable components, two principles are used: the switched capacitor technology and the use of conventional components (resistors and capacitors).

With the Switched Capacitor (SC) both capacitances and resistances can be generated. A resistor is created by first connecting a high-frequency capacitor to the negative and positive pole of the signal path, and depending on the current signal voltage, the switching frequency and its capacitance, it is partially charged and discharged alternately. The advantage of this technology lies in its high flexibility and stability. Disadvantages are the narrow bandwidth depending on the switching frequency (see Nyquist theorem ) and the parasitic effects that are also caused by the switching frequency.

An alternative is the use of conventional components in the form of unit resistances and capacitances. Depending on the intended use of the FPAA, some of these are hard-wired. This enables a higher bandwidth and avoids the artifacts that occur with scanned systems. A disadvantage is the lower flexibility of these circuits.

In addition, the configurable analog blocks contain a digital working memory (RAM) into which the desired configuration is loaded. Some Anadigm FPAAs have additional "shadow RAMs" that allow new configuration data to be loaded during operation without interrupting the analog signal.

The analog matrix is ​​surrounded by a number of auxiliary circuits. A logic circuit is required which processes the configuration data and ensures that it is stored in the right place. An internal digital timer coordinates the loading of the configuration. Configurable input and output cells are often available. B. are equipped with switchable anti-aliasing filters or amplifiers. If switched capacitors are used, analog timers are required.

programming

The FPAAs are programmed using a manufacturer-specific protocol , depending on the architecture of the chip. The manufacturers provide their own development software for their products , with the help of which the desired circuit can be developed and usually also simulated. The resulting configuration is then uploaded to the chip in the form of a bitstream. This configures the chip in the desired way.

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