CS5531/32/33/34 - Eshop-Rychle.cz

CS5531/32/33/342.9. Digital FilterThe CS5531/32/33/34 have linear phase digital filterswhich are programmed to achieve a range ofoutput word rates (OWRs) as stated in the Channel-Setup Register Descriptions section. The ADCs useaSinc 5 digital filter to output word rates at 3200Sps and 3840 Sps (MCLK = 4.9152 MHz). Otheroutput word rates are achieved by using the Sinc 5filter followed by a Sinc 3 filter with a programmabledecimation rate.Figure 16 shows the magnituderesponse of the 60 Sps filter, while Figures 17 and18 show the magnitude and phase response of thefilter at 120 Sps. The Sinc 3 is active for all output0word rates except for the 3200 Sps and 3840 Sps(MCLK = 4.9152 MHz) rate. The Z-transforms ofthe two filters are shown in Figure 19. For the Sinc 3filter, “D” is the programmable decimation ratio,which is equal to 3840/OWR when FRS = 0 and3200/OWR when FRS = 1.The converter’s digital filters scale with MCLK.For example, with an output word rate of 120 Sps,the filter’s corner frequency is at 31 Hz. If MCLKis increased to 5.0 MHz, the OWR increases by1.0175 percent and the filter’s corner frequencymoves to 31.54 Hz. Note that the converter is notspecified to run at MCLK clock frequencies greaterthan 5 MHz.Gain (dB)-40-80-1200 60 120 180 240 300Frequency (Hz)Figure 16. Digital Filter Response (Word Rate = 60 Sps)Gain (dB)0-40-80-120FlatnessFrequency dB2 -0.014 -0.056 -0.118 -0.1910 -0.3012 -0.4314 -0.5916 -0.7719 -1.0932 -3.130 40 80 120Frequency (Hz)Figure 17. 120 Sps Filter Magnitude Plot to 120 HzPhase (Degrees)Note:180900-90-1800 30 60 90 120Frequency (Hz)Figure 18. 120 Sps Filter Phase Plot to 120 HzSinc 5 (-------------------------1 – z – 80 ) 5 ( 1 – z – 16 ) 3( 1 – z – 16 ) 5 -------------------------( 1 – z – 4 ) 2( 1 – z – 4 ) 3 -----------------------( 1 – z – 2 )=× ×3( 1 – z – 2 ) 2 × -----------------------( 1 – z – 1 ) 3Sinc 3 =(------------------------1 – z – D ) 3( 1 – z – 1 ) 3See the text regarding the Sinc 3 filter’sdecimation ratio “D”.Figure 19. Z-Transforms of Digital Filters40 DS289PP5