AD73322LYRU Analog Devices Inc, AD73322LYRU Datasheet - Page 31

• Manufacturer Part Number: AD73322LYRU
• Description: IC ANALOG FRONT END DUAL 28TSSOP
• Manufacturer: Analog Devices Inc

Specifications of AD73322LYRU

• Rohs Status: RoHS non-compliant
• Number Of Bits: 16
• Number Of Channels: 4
• Power (watts): 73mW
• Voltage - Supply, Analog: 2.7 V ~ 5.5 V
• Voltage - Supply, Digital: 2.7 V ~ 5.5 V
• Package / Case: 28-TSSOP

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.
Figure 32 shows the output spectrum of a 1 kHz tone generated
at an 8 kHz sampling rate with the 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
–100
–100
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–30
–40
–50
–60
–70
–80
–90
–10
–20
–30
–40
–50
–60
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–90
Figure 32. FFT (DAC 8 kHz Sampling—Interpolator Bypassed)
0
0
0
0
500
0.5
Figure 31. FFT (DAC 8 kHz Sampling)
1000
1.0
1500
FREQUENCY (Hz)
FREQUENCY (Hz)
1.5
2000
2.0
2500
2.5
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3.0
3500
×10
4000
3.5
4
Rev. A | Page 31 of 48
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
where:
Order is the interpolator order (= 3).
L is the interpolation factor (= 32 @ 64 kHz, = 64 @ 32 kHz,
T
Consider a second example:
The analog section has a group delay of approximately 25 µs.
INT
= 128 @ 16 kHz, = 256 @ 8 kHz).
is the interpolation sample interval (= 1/2.048e6).
Group Delay (Interpolator) = Order × (L − 1)/2) × T
Group Delay (Interpolator @ 64 kHz) = 3 × (32 − 1)/2 ×
(1/2.048e6) = 22.7 µs
AD73322L
INT