SBASAP7 December 2024 ADC3664-SP
PRODUCTION DATA
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The complex decimation operation is illustrated with an example in Figure 7-18. First, the input signal (and the negative image) are frequency shifted by the NCO frequency as shown on the left side of the figure. Next, a digital filter is applied (centered around 0Hz) and the output data rate is decimated by 8 complex, In this example, the output data rate is complex of FOUT = FS/8 complex with a spectrum from -FS/16 to FS/16. During the complex mixing operation, the spectrum (signal and noise) is split into real and complex parts, and the amplitude is reduced by 6dB. To compensate for the amplitude reduction, there is a 6dB digital gain option in the decimation filter block that can be enabled via SPI.
The real decimation operation is shown in Figure 7-19. There is no mixing in this mode, only the real portion of the complex digital filter is exercised, and the output data rate is reduced. A real decimation by 8 results in an output data rate of FOUT = FS/8 with a spectrum from 0 to FS/16.