AD9772EB Analog Devices, AD9772EB Datasheet - Page 12

• Manufacturer Part Number: AD9772EB
• Description: 14-Bit/ 160 MSPS TxDAC+ with 2x Interpolation Filter
• Manufacturer: Analog Devices

AD9772
Referring to Figure 23, the “new” 1st image associated with the
DAC’s higher data rate after interpolation is “pushed” out fur-
ther relative to the input signal, since it now occurs at 2 f
– f
DAC data rate before interpolation is suppressed by the digital
filter. As a result, the transition band for the analog reconstruc-
tion filter is increased, thus reducing the complexity of the ana-
log filter. Furthermore, the sin(x)/x roll-off over the original
input data passband (i.e., dc to f
As previously mentioned, the 2 interpolation filter can be con-
verted into a high pass response, thus suppressing the “funda-
mental” while passing the “original” 1st image occurring at
f
representation for a high pass response of a discrete time sine
wave. This action can also be modeled as a “1/2 wave” digital
mixing process in which the impulse response of the low-pass
filter is digitally mixed with a square wave having a frequency of
DATA
FUNDAMENTAL
– f
FUNDAMENTAL
. The “old” first image associated with the lower
Figure 24. Time and Frequency Domain Example of High-Pass 2 Digital Interpolation Filter
Figure 23. Time and Frequency Domain Example of Low-Pass 2 Digital Interpolation Filter
. Figure 24 shows the time and frequency
FREQUENCY
FREQUENCY
f
f
FUNDAMENTAL
DOMAIN
FUNDAMENTAL
DOMAIN
DOMAIN
DOMAIN
DATA
TIME
TIME
/2) is significantly reduced.
INPUT DATA LATCH
1/
INPUT DATA LATCH
1/
f
f
DATA
DATA
f
f
DATA
DATA
f
f
DATA
1
DATA
1
ST
ST
IMAGE
IMAGE
2
2
f
f
DATA
DATA
f
FUNDAMENTAL
SUPPRESSED
f
FUNDAMENTAL
SUPPRESSED
DATA
1
ST
IMAGE
2
2
–12–
LOWER IMAGE
UPPER AND
INTERPOLATION FILTER
RESPONSE
INTERPOLATION FILTER
DIGITAL
FILTER
f
f
DATA
exactly f
(refer to Table I), this process simplifies into inverting the cen-
ter coefficient of the low-pass filter (i.e., invert H(18)). Note,
this also corresponds into inverting the peak of the impulse
response shown in Figure 2a. The resulting high pass frequency
response becomes the frequency inverted mirror image of the
low-pass filter response shown in Figure 2b.
It is worth noting that the “new” 1st image now occurs at
f
f
the f
ing purposes in direct IF applications. Also, the “lower sideband
images” occurring at f
(i.e., N
sion while the “upper sideband images” occurring at f
f
do not.
DATA
2
DATA
FUNDAMENTAL
FUNDAMENTAL
2
2
1/ 2
1
2
f
/
f
DATA
2
DATA
f
f
FUNDAMENTAL
DATA
DATA
+ f
1
2
RESPONSE
2
ST
f
DATA
f
DATA
DIGITAL
NEW
DATA
FUNDAMENTAL
FILTER
IMAGE
f
DATA
/2. Since the even coefficients have a zero value
exists for image selection, thus mandating that
and its multiples (i.e., N
– f
FUNDAMENTAL
be placed sufficiently high for practical filter-
DATA
. A reduced transition region of 2
f
f
DATA
DAC'S SIN (X)/X
DATA
DAC'S SIN (X)/X
RESPONSE
RESPONSE
– f
DAC
DAC
FUNDAMENTAL
2
2
f
f
DATA
) experience a frequency inver-
DATA
f
DATA
and its multiples
+ f
FUNDAMENTAL
DATA
REV. 0
+
)