AD73322AST datasheet(29/43 Pages) AD

29 / 43 page

AD73322

–29–

REV. B

As the AD73322 can be operated at 8 kHz (see Figure 26) 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 en-

coder 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 origi-

nal signal will be generated at integer intervals of the sampling

frequency. In this case these images must be removed by exter-

nal analog filtering.

FREQUENCY – Hz

500

–10

–20

–30

–40

–50

–60

1000

1500

2000

2500

3000

3500

4000

–70

–80

–90

–100

Figure 26. FFT (DAC 8 kHz Sampling)

Figure 27 shows the output spectrum of a 1 kHz tone being

generated at an 8 kHz sampling rate with the interpolator

bypassed.

FREQUENCY – Hz

0.5

–20

1.0

1.5

2.0

2.5

3.0

3.5

–100

–10

–30

–40

–50

–60

–70

–80

–90

104

Figure 27. 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) × T

INT

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 AD73322, 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 wave-

form which were created by the down-sampling process, there-

fore 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 rolloff and provide

a sharper transition band and greater stopband attenuation.

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