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AD73311L 数据表(PDF) 21 Page - Analog Devices |
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AD73311L 数据表(HTML) 21 Page - Analog Devices |
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21 / 37 page  REV. A AD73311L –20– The AD73311L also features direct sampling at the lower rate of 8 kHz. This is achieved by the use of extended decimation registers within the decimator block, which allows for the increased word growth associated with the higher effective oversampling ratio. Figure 17 details the spectrum of a 1 kHz test tone converted at an 8 kHz rate. FREQUENCY – Hz 0 500 0 50 100 1000 1500 2000 2500 3000 3500 4000 150 Figure 17. FFT (ADC 8 kHz Direct Sampling) The device features an on-chip master clock divider circuit that allows the sample rate to be reduced as the sampling rate of the sigma-delta converter is proportional to the output of the MCLK Divider (whose default state is divide by 1). The decimator’s frequency response (Sinc3) gives some pass- band attenuation (up to FS/2) which continues to roll off above the Nyquist frequency. If it is required to implement a digital filter to create a sharper cutoff characteristic, it may be prudent to use an initial sample rate of greater than twice the Nyquist rate in order to avoid aliasing due to the smooth roll-off of the Sinc3 filter response. In the case of voiceband processing where 4 kHz represents the Nyquist frequency, if the signal to be measured were externally bandlimited, an 8 kHz sampling rate would suffice. However, if it is required to limit the bandwidth using a digital filter, it may be more appropriate to use an initial sampling rate of 16 kHz and to process this sample stream with a filtering and decimat- ing algorithm to achieve a 4 kHz bandlimited signal at an 8 kHz rate. Figure 18 details the initial 16 kHz sampled tone. FREQUENCY – Hz 0 1000 0 –20 –40 –60 –80 –100 –120 2000 3000 4000 5000 6000 7000 8000 –140 Figure 18. FFT (ADC 16 kHz Direct Sampling) Figure 19 details the spectrum of the final 8 kHz sampled filtered tone. FREQUENCY Hz 0 500 0 –20 –40 –60 –80 –100 –120 1000 1500 2000 2500 3000 3500 4000 –140 Figure 19. FFT (ADC 8 kHz Filtered and Decimated from 16 kHz) Encoder Group Delay When programmed for high sampling rates, the AD73311L offers a very low level of group delay, which is given by the following relationship: Group Delay (Decimator) = Order × ((M – 1)/2) × TDEC where: Order is the order of the decimator (= 3), M is the decimation factor (= 32 @ 64 kHz, = 64 @ 32 kHz, = 128 @ 16 kHz , = 256 @ 8 kHz) and TDEC is the decimation sample interval (= 1/2.048e6) (based on DMCLK = 16.384 MHz) => Group Delay (Decimator @ 64 kHz) = 3 × (32 – 1)/2 × (1/2.048e6) = 22.7 µs If final filtering is implemented in the DSP, the final filter’s group delay must be taken into account when calculating overall group delay. Decoder Section The decoder section updates (samples) at the same rate as the encoder section. This rate is programmable as 64 kHz, 32 kHz, 16 kHz or 8 kHz (from a 16.384 MHz MCLK). The decoder section represents a reverse of the process that was described in the encoder section. In the case of the decoder section, signals are applied in the form of samples at an initial low rate. This sample rate is then increased to the final digital sigma-delta modulator rate of DMCLK/8 by interpolating new samples between the original samples. The interpolating filter also has the action of canceling images due to the interpolation process using spectral nulls that exist at integer multiples of the initial sam- pling rate. Figure 20 shows the spectral response of the decoder section sampling at 64 kHz. Again, its sigma-delta modulator shapes the noise so it is reduced in the voice bandwidth dc–4kHz. For improved voiceband SNR, the user can implement an initial anti-imaging filter, preceded by 8 kHz to 64 kHz interpolation, in the DSP. |
类似零件编号 - AD73311L_17 |
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类似说明 - AD73311L_17 |
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