ADAV802_15 datasheet(16/53 Pages) AD

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ADAV802

Preliminary Technical Data

Rev. Pr G | Page 16 of 53

SRC FUNCTIONAL OVERVIEW

THEORY OF OPERATION

Asynchronous sample rate conversion is converting data from

at the same or different sample rate. The simplest approach to

an asynchronous sample rate conversion is the use of a zero-

order hold between the two samplers shown in Figure 9 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 will

be repeated or dropped producing an error in the re-sampling

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 holdare infinitely attenuated. Since the ratio of

T2 to T1 is an irrational number, the error resulting from the re-

sampling at fS_OUT can never be eliminated. However, the

error can be significantly reduced through interpolation of the

input data at fS_IN. The sample rate converter in the ADAV802

is conceptually interpolated by a factor of 220.

ZERO-ORDER

HOLD

OUT

fS_IN =1/T1

fS_OUT =1/T2

ORIGINAL SIGNAL

SAMPLED AT fS_IN

SIN(X)/X OF ZER0-ORDER HOLD

SPECTRUM OF ZERO-ORDER HOLD OUTPUT

SPECTRUM OF fS_OUT SAMPLING

× fS_OUT

FREQUENCY RESPONSE OF fS_OUT CONVOLVED WITH ZERO-ORDER

HOLD SPECTRUM

fS_OUT

801-0008

Figure 9. Zero Order Hold Being 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 10 shows

both the time domain and the frequency domain of

interpolation by a factor of 220. Conceptually, interpolation by

220 would involve 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 re-sampling error.

OUT

fS_IN

fS_OUT

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

INTERPOLATE

BY N

LOW-PASS

FILTER

ZERO-ORDER

HOLD

Figure 10. SRC Time Domain

In the frequency domain shown in Figure 11, 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. The worst-case images can be computed from

the zero-order hold frequency response, maximum image = sin

(× F/fS_INTERP)/(× F/fS_INTERP). F is the frequency of the worst-case

image that would be 220 × fS_IN ± fS_IN/2 , and fS_INTERP is fS_IN × 220.

The following worst-case images would appear for fS_IN = 192

kHz:

Image at fS_INTERP − 96 kHz = –125.1 dB

Image at fS_INTERP + 96 kHz = –125.1 dB

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