Datasheet
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 f
S_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 f
S_OUT
is convolved with
the attenuated images from the sin(x)/x nature of the zero-order
hold. The images at f
S_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
f
S_OUT
can never be eliminated. The error can be significantly
reduced, however, through interpolation of the input data at
f
S_IN
. Therefore, the sample rate converter in the ADAV801 is
conceptually interpolated by a factor of 2
20
.
04577-009
ZERO-ORDER
HOLD
f
S_IN
= 1/T1
f
S_OUT
= 1/T2
OUT
IN
ORIGINAL SIGNAL
SAMPLED AT
f
S_IN
SIN(X)/X OF ZERO-ORDER HOLD
SPECTRUM OF ZERO-ORDER HOLD OUTPUT
SPECTRUM OF
f
S_OUT
SAMPLING
f
S_OUT
2 ×
f
S_OUT
FREQUENCY RESPONSE OF
f
S_OUT
CONVOLVED
WITH ZERO-ORDER HOLD SPECTRUM
Figure 29. Zero-Order Hold Used by f
S_ OUT
to Resample Data from f
S_IN
Conceptual High Interpolation Model
Interpolation of the input data by a factor of 2
20
involves placing
(2
20
− 1) samples between each f
S_IN
sample. Figure 30 shows
both the time domain and the frequency domain of interpolation
by a factor of 2
20
. Conceptually, interpolation by 2
20
involves the
steps of zero-stuffing (2
20
− 1) number of samples between each
f
S_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 f
S_OUT
selects the closest f
S_IN
× 2
20
sample from the zero-order hold, as opposed to the nearest f
S_IN
sample in the case of no interpolation. This significantly
reduces the resampling error.
04577-010
f
S_IN
f
S_OUT
OUT
IN
INTERPOLATE
BY N
LOW-PASS
FILTER
ZERO-ORDER
HOLD
TIME DOMAIN OF
f
S_IN
SAMPLES
TIME DOMAIN OUTPUT OF THE LOW-PASS FILTER
TIME DOMAIN OF
f
S_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 2
20
closer to the infinite attenuation point
of the zero-order hold, which is f
S_IN
× 2
20
. The images at the
zero-order hold are the determining factor for the fidelity of the
output at f
S_OUT
.
04577-011
f
S_IN
f
S_IN
2
20
×
f
S_IN
2
20
×
f
S_IN
2
20
×
f
S_IN
f
S_OUT
OUT
IN
INTERPOLATE
BY N
LOW-PASS
FILTER
ZERO-ORDER
HOLD
FREQUENCY DOMAIN OF SAMPLES AT
f
S_IN
FREQUENCY DOMAIN OF THE INTERPOLATION
FREQUENCY DOMAIN OF
f
S_OUT
RESAMPLING
FREQUENCY DOMAIN
AFTER RESAMPLING
SIN(X)/X OF ZERO-ORDER HOLD
Figure 31. Frequency Domain of the Interpolation and Resampling