EVAL-AD73311LEB View Datasheet(PDF) - Analog Devices
Part Name
Description
MFG CO.
'EVAL-AD73311LEB' PDF : 36 Pages View PDF
AD73311L
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Figure 20. FFT (DAC 64 kHz Sampling)
As the AD73311L can be operated at 8 kHz (see Figure 21) or
16 kHz sampling rates, which make it particularly suited for
voiceband processing, it is important to 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, per-
haps to reduce group delay, images of the original signal will be
generated at integer intervals of the sampling frequency. In this
case these images must be removed by external analog filtering.
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Figure 21. FFT (DAC 8 kHz Sampling)
Figure 22 shows the output spectrum of a 1 kHz tone being
generated at an 8 kHz sampling rate with the interpolator
bypassed.
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Figure 22. 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 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 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 AD73311L, 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.
REV. A
–21–
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