ADAV801ASTZ datasheet(20/61 Pages) AD

20 / 61 page

ADAV801

Rev. A | Page 19 of 60

SAMPLE RATE CONVERTER (SRC) FUNCTIONAL

OVERVIEW

During asynchronous sample rate conversion, data can be

converted at the same sample rate or at different sample rates.

The simplest approach to an asynchronous sample rate

conversion is to use a zero-order hold between the two

samplers, as shown in Figure 29. In an asynchronous system, T2

is never equal to T1, nor is the ratio between T2 and T1

rational. As a result, samples at fS_OUT are repeated or dropped,

producing an error in the resampling process.

The frequency domain shows the wide side lobes that result

from this error when the sampling of fS_OUT is convolved with

the attenuated images from the sin(x)/x nature of the zero-order

hold. The images at fS_IN (dc signal images) of the zero-order

hold are infinitely attenuated. Because the ratio of T2 to T1 is an

irrational number, the error resulting from the resampling at

fS_OUT can never be eliminated. The error can be significantly

reduced, however, through interpolation of the input data at

fS_IN. Therefore, the sample rate converter in the ADAV801 is

conceptually interpolated by a factor of 220.

ZERO-ORDER

HOLD

fS_IN = 1/T1

fS_OUT = 1/T2

OUT

ORIGINAL SIGNAL

SAMPLED AT

fS_IN

SIN(X)/X OF ZERO-ORDER HOLD

SPECTRUM OF ZERO-ORDER HOLD OUTPUT

SPECTRUM OF

fS_OUT SAMPLING

fS_OUT

2 ×

fS_OUT

FREQUENCY RESPONSE OF

fS_OUT CONVOLVED

WITH ZERO-ORDER HOLD SPECTRUM

Figure 29. Zero-Order Hold Used by fS_ OUT to Resample Data from fS_IN

Conceptual High Interpolation Model

Interpolation of the input data by a factor of 220 involves placing

(220 − 1) samples between each fS_IN sample. Figure 30 shows

both the time domain and the frequency domain of interpolation

by a factor of 220. Conceptually, interpolation by 220 involves the

steps of zero-stuffing (220 − 1) number of samples between each

fS_IN sample and convolving this interpolated signal with a

digital low-pass filter to suppress the images. In the time

domain, it can be seen that fS_OUT selects the closest fS_IN × 220

sample from the zero-order hold, as opposed to the nearest fS_IN

sample in the case of no interpolation. This significantly

reduces the resampling error.

fS_IN

fS_OUT

OUT

INTERPOLATE

BY N

LOW-PASS

FILTER

ZERO-ORDER

HOLD

TIME DOMAIN OF

fS_IN SAMPLES

TIME DOMAIN OUTPUT OF THE LOW-PASS FILTER

TIME DOMAIN OF

fS_OUT RESAMPLING

TIME DOMAIN OF THE ZERO-ORDER HOLD OUTPUT

Figure 30. SRC Time Domain

In the frequency domain shown in Figure 31, the interpolation

expands the frequency axis of the zero-order hold. The images

from the interpolation can be sufficiently attenuated by a good

low-pass filter. The images from the zero-order hold are now

pushed by a factor of 220 closer to the infinite attenuation point

of the zero-order hold, which is fS_IN × 220. The images at the

zero-order hold are the determining factor for the fidelity of the

output at fS_OUT.

fS_IN

220 ×

fS_IN

220 ×

fS_IN

220 ×

fS_IN

fS_OUT

OUT

INTERPOLATE

BY N

LOW-PASS

FILTER

ZERO-ORDER

HOLD

FREQUENCY DOMAIN OF SAMPLES AT

fS_IN

FREQUENCY DOMAIN OF THE INTERPOLATION

FREQUENCY DOMAIN OF

fS_OUT RESAMPLING

FREQUENCY DOMAIN

AFTER RESAMPLING

SIN(X)/X OF ZERO-ORDER HOLD

Figure 31. Frequency Domain of the Interpolation and Resampling

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