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AD73322L 데이터시트(PDF) 32 Page - Analog Devices |
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AD73322L 데이터시트(HTML) 32 Page - Analog Devices |
32 / 49 page AD73322L Rev. A | Page 31 of 48 Because the AD73322L can be operated at 8 kHz (see Figure 31) or 16 kHz sampling rates, which make it particularly suited for voice-band processing, the user must understand the action of the interpolator’s sinc3 response. As was the case with the encoder section, if the output signal’s frequency response is not bounded by the Nyquist frequency, it may be necessary to perform some initial digital filtering to eliminate signal energy above Nyquist to ensure that it is not imaged at the integer multiples of the sampling frequency. If the user chooses to bypass the interpolator, perhaps to reduce group delay, images of the original signal are generated at integer intervals of the sampling frequency. In this case these images must be removed by external analog filtering. FREQUENCY (Hz) 4000 0 500 1000 1500 2000 2500 3000 3500 0 –10 –20 –30 –40 –50 –60 –70 –80 –90 –100 Figure 31. FFT (DAC 8 kHz Sampling) Figure 32 shows the output spectrum of a 1 kHz tone generated at an 8 kHz sampling rate with the interpolator bypassed. FREQUENCY (Hz) ×104 3.5 0 0.5 1.0 1.5 2.0 2.5 3.0 0 –10 –30 –20 –40 –50 –70 –60 –90 –80 –100 Figure 32. FFT (DAC 8 kHz Sampling—Interpolator Bypassed) ON-CHIP FILTERING The primary function of the system filtering’s sinc-cubed (Sinc3) response is to eliminate aliases or images of the ADCs or DAC’s resampling, respectively. Both modulators are sampled at a nominal rate of DMCLK/8 (which is 2.048 MHz for a DMCLK of 16.384 MHz), and the simple, external RC antialias filter is sufficient to provide the required stop-band rejection above the Nyquist frequency for this sample rate. In the case of the ADC section, the decimating filter is required to both decrease sample rate and increase sample resolution. The process of changing sample rate (resampling) leads to aliases of the original sampled waveform appearing at integer multiples of the new sample rate. These aliases would get mapped into the required signal pass band without the application of some further antialias filtering. In the AD73322L, the sinc-cubed response of the decimating filter creates spectral nulls at integer multiples of the new sample rate. These nulls coincide with the aliases of the original waveform, which were created by the down-sampling process, therefore reducing or eliminating the aliasing due to sample rate reduction. In the DAC section, increasing the sampling rate by interpolation creates images of the original waveform at intervals of the original sampling frequency. These images may be sufficiently rejected by external circuitry but the sinc-cubed filter in the interpolator again nulls the output spectrum at integer intervals of the original sampling rate, which corresponds with the images due to the interpolation process. The spectral response of a sinc-cubed filter shows the charac- teristic nulls at integer intervals of the sampling frequency. Its pass-band characteristic (up to Nyquist frequency) features a roll-off that continues up to the sampling frequency, where the first null occurs. In many applications this smooth response does not give sufficient attenuation of frequencies outside the band of interest; therefore, it may be necessary to implement a final filter in the DSP to equalize the pass-band roll-off and provide a sharper transition band and greater stop-band attenuation. DECODER GROUP DELAY The interpolator roll-off is mainly due to its sinc-cubed function characteristic, which has an inherent group delay given by the equation Group Delay (Interpolator) = Order × (L − 1)/2) × TINT where: Order is the interpolator order (= 3). L is the interpolation factor (= 32 @ 64 kHz, = 64 @ 32 kHz, = 128 @ 16 kHz, = 256 @ 8 kHz). TINT is the interpolation sample interval (= 1/2.048e6). Consider a second example: Group Delay (Interpolator @ 64 kHz) = 3 × (32 − 1)/2 × (1/2.048e6) = 22.7 µs The analog section has a group delay of approximately 25 µs. |
유사한 부품 번호 - AD73322L_17 |
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유사한 설명 - AD73322L_17 |
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