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AD73322L View Datasheet(PDF) - Analog Devices

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AD73322L Datasheet PDF : 40 Pages
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AD73322L
As the AD73322L can be operated at 8 kHz (see Figure 22) or
16 kHz sampling rates, which make it particularly suited for voice-
band processing, it is important to understand the action of the
interpolators Sinc3 response. As was the case with the encoder
section, if the output signals 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 interpo-
lator, perhaps to reduce group delay, images of the original
signal will be generated at integer intervals of the sampling fre-
quency. In this case these images must be removed by external
analog filtering.
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500 1000 1500 2000 2500 3000 3500 4000
FREQUENCY Hz
Figure 22. FFT (DAC 8 kHz Sampling)
Figure 23 shows the output spectrum of a 1 kHz tone being gener-
ated at an 8 kHz sampling rate with the interpolator bypassed.
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0.5
1.0
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FREQUENCY Hz
؋ 104
Figure 23. FFT (DAC 8 kHz Sampling—Interpolator
Bypassed)
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) and
TINT is the interpolation sample interval (= 1/2.048e6)
=> 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.
On-Chip Filtering
The primary function of the system filterings sinc-cubed (Sinc3)
response is to eliminate aliases or images of the ADCs or DACs
resampling, respectively. Both modulators are sampled at a nomi-
nal 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 stopband 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 interpola-
tion 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 character-
istic nulls at integer intervals of the sampling frequency. Its
passband 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 will
not give sufficient attenuation of frequencies outside the band of
interest; therefore, it may be necessary to implement a final filter
in the DSP which will equalize the passband roll-off and provide
a sharper transition band and greater stopband attenuation.
–26–
REV. 0
 

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