AD73311LARS datasheet(21/36 Pages) AD

21 / 36 page

REV. A

AD73311L

–21–

FREQUENCY – Hz

0.5

–20

1.0

1.5

2.0

2.5

3.0

3.5

–100

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104

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.

FREQUENCY – Hz

500

–10

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–40

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1000

1500

2000

2500

3000

3500

4000

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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.

FREQUENCY – Hz

0.5

–20

1.0

1.5

2.0

2.5

3.0

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104

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.

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