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[Crystal Semiconductor Corporation]

The CS5322 Digital Filter

Delta-Sigma A/D converters have become widely accepted by data acquisition designers because of their high performance. One of the highest dynamic range Delta-Sigma converters is the Crystal Semiconductor CS5321/CS5322 chip set. The CS5321 is a fourth order modulator. The CS5322 is a linear phase FIR decimation filter. The purpose of this Technical Brief is to compare the architecture of a general purpose DSP chip with that of the CS5322.

General purpose DSPs are available which support either fixed-point arithmetic or floating-point arithmetic. Fixed-point processors are lower cost but floating-point processors have more horsepower. Fixed point processors work with either 16, 20, or 24-bit data words, while floating-point processors operate with 32-bit data words. The size of the data word greatly influences the cost of the chip and the cost of the overall system. This is because data word length determines the size of the registers used in the processor, the number of pins required on the package, and the word-width of the external memory. If low cost is the number one goal in a DSP design, the designer will specify a DSP which has the smallest word size that the application can tolerate.