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AD6643 Datasheet(PDF) 24 Page - Analog Devices |
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AD6643 Datasheet(HTML) 24 Page - Analog Devices |
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24 / 36 page  AD6643 Rev. 0 | Page 24 of 36 NOISE SHAPING REQUANTIZER (NSR) The AD6643 features a noise shaping requantizer (NSR) to allow higher than 11-bit SNR to be maintained in a subset of the Nyquist band. The harmonic performance of the receiver is unaffected by the NSR feature. When enabled, the NSR contributes an additional 0.6 dB of loss to the input signal, such that a 0 dBFS input is reduced to −0.6 dBFS at the output pins. The NSR feature can be independently controlled per channel via the SPI. Two different bandwidth modes are provided; the mode can be selected from the SPI port. In each of the two modes, the center frequency of the band can be tuned such that IFs can be placed anywhere in the Nyquist band. 22% BW MODE (>40 MHz at 184.32 MSPS) The first bandwidth mode offers excellent noise performance over 22% of the ADC sample rate (44% of the Nyquist band) and can be centered by setting the NSR mode bits in the NSR control register (Address 0x3C) to 000. In this mode, the useful frequency range can be set using the 6-bit tuning word in the NSR tuning register (Address 0x3E). There are 57 possible tuning words (TW); each step is 0.5% of the ADC sample rate. The following three equations describe the left band edge (f0), the channel center (fCENTER), and the right band edge (f1), respectively: f0 = fADC × .005 × TW fCENTER = f0 + 0.11 × fADC f1 = f0 + 0.22 × fADC Figure 39 to Figure 41 show the typical spectrum that can be expected from the AD6643 in the 22% BW mode for three different tuning words. 0 –20 –40 –60 –80 –100 –120 –140 10 0 2030 40 50 60 708090 FREQUENCY (Hz) 200MSPS 140.1MHz @ –1.6dBFS SNR = 73.6dB (75.2dBFS) SFDR = 86dBc (IN BAND) Figure 39. 22% BW Mode, Tuning Word = 13 0 –20 –40 –60 –80 –100 –120 –140 10 0 2030 40 50 60 708090 FREQUENCY (Hz) 200MSPS 140.1MHz @ –1.6dBFS SNR = 73.4dB (75dBFS) SFDR = 86dBc (IN BAND) Figure 40. 22% BW Mode, Tuning Word = 28 (fS/4 Tuning) 0 –20 –40 –60 –80 –100 –120 –140 10 0 2030 40 50 60 708090 FREQUENCY (Hz) 200MSPS 140.1MHz @ –1.6dBFS SNR = 73.5dB (75.1dBFS) SFDR = 86dBc (IN BAND) Figure 41. 22% BW Mode, Tuning Word = 41 |
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