AD9772EB Analog Devices, AD9772EB Datasheet - Page 23

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

DIRECT IF
As discussed in the Digital Modes of Operation section, the
AD9772 can be configured to transform digital data represent-
ing baseband signals into IF signals appearing at odd multiples
of the input data rate (i.e., N f
is accomplished by configuring the MOD1 and MOD0 digital
inputs HIGH. Note, the maximum DAC update rate of 400 MSPS
limits the data input rate in this mode to 100 MSPS when the
“zero-stuffing operation” is enabled (i.e., MOD1 High). Appli-
cations requiring higher IFs (i.e., 140 MHz) using higher data
rates should disable the “zeros-stuffing” operation. Also, to
minimize the effects of the PLL Clock Multipliers phase noise
as shown in Figure 27, an external low jitter/phase noise clock
source equal to 4
Figure 51 shows the actual output spectrum of the AD9772
reconstructing a 16-QAM test vector with a symbol rate of
5 MSPS. The particular test vector was centered at f
f
tions, the pair of images appearing around f
attractive since they have the flattest passband and highest signal
power. Higher images can also be used with the realization that
these images will have reduced passband flatness, dynamic
range, and signal power, thus reducing the CNR and ACP per-
formance. Figure 52 shows a dual tone SFDR amplitude sweep
at the various IF images with f
400 MHz and the two tones centered around f
since an IF filter is assumed to precede the AD9772, the SFDR
was measured over a 25 MHz window around the images occur-
ring at 75 MHz, 125 MHz, 275 MHz and 325 MHz.
REV. 0
DATA
Figure 51. Spectral Plot of 16-QAM Signal in Direct IF
Mode at f
= 100 MSPS, and f
–100
–120
–20
–40
–60
–80
0
0
DATA
50
= 100 MSPS
f
DATA
100
DAC
is recommended.
150
FREQUENCY – MHz
= 400 MHz. For many applica-
DATA
DATA
200
= 100 MSPS and f
where N = 1, 3, . . .). This
250
DATA
300
DATA
will be more
350
/4. Note,
DATA
DAC
400
/4 with
=
–23–
Regardless of what image is selected for a given application, the
adjacent images must be sufficiently filtered. In most cases, a
SAW filter providing differential inputs represents the opti-
mum device for this purpose. For single-ended SAW filters, a
balanced-to-unbalanced RF transformer is recommended. The
AD9772’s high output impedance provides a certain amount of
flexibility in selecting the optimum resistive load, R
as any matching network.
For many applications, the data update rate to the DAC (i.e.,
f
reference clock (i.e., GSM – 13 MHz). Furthermore, these
applications prefer to use standard IF frequencies which offer a
large selection of SAW filter choices of varying passbands (i.e.,
70 MHz). These applications may still benefit from the AD9772’s
direct IF mode capabilities when used in conjunction with a
digital upconverter such as the AD6622. Since the AD6622 can
digitally synthesize and tune up to four modulated carriers, it is
possible to judiciously tune these carriers in a region which may
fall within an IF filter’s passband upon reconstruction by the
AD9772. Figure 53 shows an example in which four carriers
were tuned around 18 MHz with a digital upconverter operating
at 52 MSPS such that when reconstructed by the AD9772 in
the IF MODE, these carriers fall around a 70 MHz IF.
Figure 53. Spectral Plot of Four Carriers at 70 MHz IF with
f
DATA
DATA
Figure 52. Dual Tone “Windowed” SFDR vs. A
f
DATA
) must be some fixed integer multiple of some system
= 52 MSPS, PLLVDD = 0
–100
–10
–20
–30
–40
–50
–60
–70
–80
–90
85
80
75
70
65
60
55
50
45
40
= 100 MSPS
0
–15
65
–12
67.5
IF @ 125MHz
FREQUENCY – MHz
IF @ 325MHz
–9
A
OUT
IF @ 275MHz
70
– dBFS
IF @ 75MHz
–6
72.5
–3
AD9772
LOAD
OUT
, as well
75
0
at