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AD7730BRUZ-REEL Fiches technique(PDF) 28 Page - Analog Devices |
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AD7730BRUZ-REEL Fiches technique(HTML) 28 Page - Analog Devices |
28 / 53 page AD7730/AD7730L –28– Because of this effect, care should be taken in choosing an out- put rate that is close to the line frequency in the application. If the line frequency is 50 Hz, an output update rate of 50 Hz should not be chosen as it will significantly reduce the AD7730’s line frequency rejection (the 50 Hz will appear as a dc effect with only 6 dB attenuation). Choosing an output rate of 55 Hz will result in a 6 dB—attenuated aliased frequency of 5 Hz with only a further 25 dB attenuation based on the filter profile. This number is based on the filter roll-off and Figure 11 can be used as a reference by dividing the frequency scale by a factor of 4. Choosing 57 Hz as the output rate will give better than 90 dB attenuation of the aliased line frequency which appears as a 7 Hz signal. Similarly, multiples of the line frequency should be avoided as the output rate because harmonics of the line fre- quency will not be fully attenuated. The programmability of the AD7730’s output rate should allow the user to readily choose an output rate that overcomes this issue. An alternative is to use the part in nonchop mode. Figure 13 shows the frequency response for the AD7730 with the second stage filter set for normal FIR operation, chop mode disabled, the decimal equivalent of the word in the SF bits set to 1536 and a master clock frequency of 4.9152 MHz. The response is analogous to that of Figure 11, with the three-times-larger SF word producing the same 200 Hz output rate. Once again, the response will scale proportionally with master clock frequency. The response is shown from dc to 100 Hz. The rejection at 50 Hz ± 1 Hz, and 60 Hz ± 1 Hz is better than 88 dB. FREQUENCY – Hz 0 –60 –100 090 10 20 30 40 50 60 70 80 –10 –50 –70 –90 –30 –40 –80 –20 –120 –110 100 Figure 13. Detailed Full Frequency Response of AD7730 (Second Stage Filter as Normal FIR, Chop Disabled) The –3 dB frequency for the frequency response of the AD7730 with the second stage filter set for normal FIR operation and chop mode enabled, is determined by the following relationship: f 3dB = 0.039 × f CLK IN 16 × 1 SF In this case, f3 dB = 7.8 Hz and the stop band, where the attentua- tion is greater than 64.5 dB, is determined by: f STOP = 0.14 × f CLK IN 16 × 1 SF In this case, f3 dB = 28 Hz. Figure 14 shows the frequency response for the same set of conditions as for Figure 13, but in this case the response is shown out to 600 Hz. This plot is comparable to that of Figure 12. The most notable difference is the absence of the peaks in the response at 200 Hz and 400 Hz. As a result, interference at these frequencies will be effectively eliminated before being aliased back to dc. FREQUENCY – Hz 0 –60 –100 0450 50 100 150 200 250 300 350 400 –10 –50 –70 –90 –30 –40 –80 –20 –120 –110 500 550 600 Figure 14. Expanded Full Frequency Response of AD7730 (Second Stage Filter as Normal FIR, Chop Disabled) REV. B |
Numéro de pièce similaire - AD7730BRUZ-REEL |
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Description similaire - AD7730BRUZ-REEL |
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