AD9772A Datasheet, PDF (23/32 Pages) Analog Devices – 14-Bit, 160 MSPS TxDAC+ with 2x Interpolation Filter

AD9772A
BASEBAND SINGLE-CARRIER
The AD9772A is also well suited for wideband single-carrier
applications such as WCDMA and multilevel QAM whose
modulation scheme requires wide dynamic range from the
reconstruction DAC to achieve the out-of-band spectral mask as
well as the in-band CNR performance. Many of these applica-
tions strategically place the carrier frequency at one quarter of
the DAC’s input data rate (i.e., f_DATA/4) to simplify the digital
modulator design. Since this constitutes the first fixed IF fre-
quency, the frequency tuning is accomplished at a later IF stage.
To enhance the modulation accuracy as well as reduce the shape
factor of the second IF SAW filter, many applications will often
specify the passband of the IF SAW filter be greater than the
channel bandwidth. The trade-off is that the TxDAC must now
meet the particular application’s spectral mask requirements
within the extended passband of the 2nd IF, which may include
two or more adjacent channels.
Figure 33 shows a spectral plot of the AD9772A reconstructing
a test vector similar to those encountered in WCDMA applica-
tions with the following exception. WCDMA applications
prescribe a root raised cosine filter with an alpha = 0.22, which
limits the theoretical ACPR of the TxDAC to about 70 dB. This
particular test vector represents white noise that has been band-
limited by a “brickwall” bandpass filter with the same passband
such that its maximum ACPR performance is theoretically
83 dB and its peak-to-rms ratio is 12.4 dB. As Figure 33 reveals,
the AD9772A is capable of approximately 78 dB ACPR per-
formance when one accounts for the additive noise/distortion
contributed by the FSEA30 spectrum analyzer.
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C0
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C11
C11
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CENTER 16.25MHz
C0
Cu1
600kHz
Cu1
SPAN 6MHz
Figure 33. AD9772A Achieves 78 dB ACPR Performance
Reconstructing a “WCDMA-Like” Test Vector with f_DATA =
65.536 MSPS and PLLVDD = 0
DIRECT IF
As discussed in the Digital Modes of Operation section, the
AD9772A 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 Ï« f_DATA where N = 1, 3, . . .). This
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 9, an external low jitter/phase noise clock
source equal to 4× f_DATA is recommended.
Figure 34 shows the actual output spectrum of the AD9772A
reconstructing a 16-QAM test vector with a symbol rate of
5 MSPS. The particular test vector was centered at f_DATA/4 with
f_DATA = 100 MSPS, and f_DAC = 400 MHz. For many applica-
tions, the pair of images appearing around f_DATA will be more
attractive since they have the flattest passband and highest signal
power. Higher images can also be used with the understanding
that these images will have reduced passband flatness, dynamic
range, and signal power, thus reducing the CNR and ACP per-
formance. Figure 35 shows a dual tone SFDR amplitude sweep
at the various IF images with f_DATA = 100 MSPS and f_DAC =
400 MHz and the two tones centered around f_DATA/4. Note,
since an IF filter is assumed to precede the AD9772A, the
SFDR was measured over a 25 MHz window around the images
occurring at 75 MHz, 125 MHz, 275 MHz, and 325 MHz.
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 optimum
device for this purpose. For single-ended SAW filters, a balanced-
to-unbalanced RF transformer is recommended. The AD9772A’s
high output impedance provides a certain amount of flexibility
in selecting the optimum resistive load, R_LOAD, as well as any
matching network.
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0
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300
400
FREQUENCY – MHz
Figure 34. Spectral Plot of 16-QAM Signal in Direct IF
Mode at f_DATA = 100 MSPS
REV. A
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