AD1895AYRSRL Analog Devices Inc, AD1895AYRSRL Datasheet - Page 16

• Manufacturer Part Number: AD1895AYRSRL
• Description: IC SAMP-RATEHP/CONV 24BIT 28SSOP
• Manufacturer: Analog Devices Inc
• Type: Sample Rate Converterr

Specifications of AD1895AYRSRL

• Rohs Status: RoHS non-compliant
• Applications: Automotive Audio, receivers, set-top boxes
• Voltage - Supply, Digital: 3.13 V ~ 3.46 V
• Mounting Type: Surface Mount
• Package / Case: 28-SSOP
• For Use With: EVAL-AD1895EB - BOARD EVAL FOR AD1895
• Voltage - Supply, Analog: -
• Lead Free Status / RoHS Status: Not Compliant

AD1895
ASRC FUNCTIONAL OVERVIEW
THEORY OF OPERATION
Asynchronous sample rate conversion is converting data from one
clock source at some sample rate to another clock source at the
same or different sample rate. The simplest approach to asyn-
chronous sample rate conversion is the use of a zero-order hold
between two samplers as shown in Figure 4. 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
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
images from the sin(x)/x nature of the zero-order hold. The images
at f
attenuated. Since the ratio of T2 to T1 is an irrational number,
the error resulting from the resampling at f
eliminated. However, the error can be significantly reduced
through interpolation of the input data at f
conceptually interpolated by a factor of 2
THE CONCEPTUAL HIGH INTERPOLATION MODEL
Interpolation of the input data by a factor of 2
(2
both the time domain and the frequency domain of interpolation
by a factor of 2
20
S_IN
–1) samples between each f
Figure 4. Zero-Order Hold Being Used by f
Resample Data from f
FREQUENCY RESPONSE OF
HOLD SPECTRUM
, dc signal images, of the zero-order hold are infinitely
IN
f
S_IN
= 1/T1
20
SPECTRUM OF ZERO-ORDER HOLD OUTPUT
. Conceptually, interpolation by 2
SPECTRUM OF
SIN(X)/X OF ZERO-ORDER HOLD
ORIGINAL SIGNAL
f
ZERO-ORDER
SAMPLED AT
f
S_OUT
S_OUT
S_OUT
S_IN
HOLD
CONVOLVED WITH ZERO-ORDER
f
is convolved with the attenuated
S_OUT
S_IN
S_OUT
f
S_IN
sample. Figure 5 shows
SAMPLING
20
f
S_OUT
will be repeated or
S_IN
S_OUT
.
. The AD1895 is
20
= 1/T2
involves placing
can never be
OUT
2
S_OUT
20
f
S_OUT
would
to
–16–
involve the steps of zero-stuffing (2
between each f
with a digital low-pass filter to suppress the images. In the time
domain, it can be seen that f
sample from the zero-order hold as opposed to the nearest f
sample in the case of no interpolation. This significantly reduces
the resampling error.
Figure 5. Time Domain of the Interpolation and Resampling
In the frequency domain shown in Figure 6, 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
of the zero-order hold, which is f
zero-order hold are the determining factor for the fidelity of the
output at f
the zero-order hold frequency response, maximum image =
sin (π × F/f
worst-case image, which would be 2
f
The following worst-case images would appear for f
192 kHz:
S_INTERP
IN
Image at f
Image at f
f
S_IN
is f
S_INTERP
S_OUT
S_INTERP
S_INTERP
S_IN
INTERPOLATE
TIME DOMAIN OF
TIME DOMAIN OUTPUT OF THE LOW-PASS FILTER
TIME DOMAIN OF
TIME DOMAIN OF THE ZERO-ORDER HOLD OUTPUT
S_IN
BY N
. The worst-case images can be computed from
× 2
)/(π × F/f
sample and convolving this interpolated signal
20
– 96 kHz = –125.1 dB
+ 96 kHz = –125.1 dB
.
20
closer to the infinite attenuation point
f
f
S_INTERP
S_IN
S_OUT
S_OUT
LOW-PASS
FILTER
SAMPLES
RESAMPLING
S_IN
selects the closest f
). F is the frequency of the
20
× 2
20
–1) a number of samples
× f
20
. The images at the
S_IN
ZERO-ORDER
HOLD
± f
S_IN
/2 , and
S_IN
S_IN
f
REV. B
S_OUT
× 2
=
S_IN
OUT
20