ADAV803ASTZ datasheet(19/56 Pages) AD

19 / 56 page

ADAV803

Rev. 0 | Page 19 of 56

VERVIEW

le rate conversion, data can be

conv

or at

t sam

rates.

The sim

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

m, T2

tional.

at fS_OUT are repeated or dropped, producing

pling process.

t r

S_OUT

ated images from the SIN(x)/x nature of the zero-

orde

es) of the zero-

order h

nfinite

he ratio of T2 to

T1 i

rom the

n never be eliminated. The error can be

tually interpolated by a factor of 220.

SAMPLE RATE CONVERTER (SRC) FUNCTIONAL O

During asynchronous samp

erted at the same sample rate

differen

ple

plest approach to an asynchronous sample rate

conversi

samplers, as shown in Figure 29. In an asynchronous syste

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

As a result, samples

an error in the resam

The frequency domain shows the wide side lobes tha esult

from this error when the sampling of f

is convolved with

the attenu

r hold. The images at fS_IN (dc signal imag

old are i

ly attenuated. Because t

s an irrational number, the error resulting f

resampling at fS_OUT ca

significantly reduced, however, through interpolation of t

input data at fS_IN. Therefore, the sample rate converter in t

ADAV803 is concep

SPECTRUM OF

fS_OUT SAMPLING

fS_OUT

2×

fS_OUT

FREQUENCY RESPONSE OF

fS_OUT CONVOLVED

WITH ZERO-ORDER HOLD SPECTRUM

ZERO-ORDER

HOLD

fS_IN =1/T1

fS_OUT = 1/T2

ORIGINAL SIGNAL

SAMPLED AT

fS_IN

SIN(X)/X OF ZERO-ORDER HOLD

SPECTRUM OF ZERO-ORDER HOLD OUTPUT

OUT

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

Conceptual High Interpolation Model

to suppress the

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

(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 b

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

images. In the time domain, it can be een

at 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

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.

Figure 30. SRC Time Domain

In the frequency domain shown in Figure 31, the interpolation

expands the frequency axis

fS_IN

fS_OUT

OUT

INTERPOLATE

BY N

LOW-PASS

FILTER

ZERO-ORDER

HOLD

FREQUENCY DOMAIN OF SAMPLES AT

fS_IN

220 ×

fS_IN

220 ×

fS_IN

220 ×

fS_IN

FREQUENCY DOMAIN OF THE INTERPOLATION

FREQUENCY DOMAIN OF

fS_OUT RESAMPLING

FREQUENCY DOMAIN

AFTER RESAMPLING

SIN(X)/X OF ZERO-ORDER HOLD

ling

Figure 31. Frequency Domain of the Interpolation and Resamp

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