AD9772AST datasheet(21/30 Pages) AD

21 / 30 page

REV. 0

AD9772

–21–

–

100 F

ELECTROLYTIC

–

10 F–22 F

TANTALUM

0.1 F

CERAMIC

AVDD

ACOM

TTL/CMOS

LOGIC

CIRCUITS

+3.0V OR +3.3V

POWER SUPPLY

FERRITE

BEADS

Figure 46. Differential LC Filter for +3 V or 3.3 V

Applications

Maintaining low noise on power supplies and ground is critical

to obtain optimum results from the AD9772. If properly

implemented, ground planes can perform a host of functions on

high speed circuit boards: bypassing, shielding current trans-

port, etc. In mixed signal design, the analog and digital portions

of the board should be distinct from each other, with the analog

ground plane confined to the areas covering the analog signal

traces, and the digital ground plane confined to areas covering

the digital interconnects.

All analog ground pins of the DAC, reference and other analog

components should be tied directly to the analog ground plane.

The two ground planes should be connected by a path 1/8 to

1/4 inch wide underneath or within 1/2 inch of the DAC to

maintain optimum performance. Care should be taken to ensure

that the ground plane is uninterrupted over crucial signal paths.

On the digital side, this includes the digital input lines running

to the DAC. On the analog side, this includes the DAC output

signal, reference signal and the supply feeders.

The use of wide runs or planes in the routing of power lines is

also recommended. This serves the dual role of providing a low

series impedance power supply to the part, as well as providing

some “free” capacitive decoupling to the appropriate ground

plane. It is essential that care be taken in the layout of signal and

power ground interconnects to avoid inducing extraneous volt-

age drops in the signal ground paths. It is recommended that all

connections be short, direct and as physically close to the pack-

age as possible in order to minimize the sharing of conduction

paths between different currents. When runs exceed an inch in

length, strip line techniques with proper termination resistors

should be considered. The necessity and value of this resistor

will be dependent upon the logic family used.

For a more detailed discussion of the implementation and con-

struction of high speed, mixed signal printed circuit boards,

refer to Analog Devices’ application notes AN-280 and AN-333.

APPLICATIONS

MULTICARRIER

The AD9772’s wide dynamic range performance makes it well

suited for next generation base station applications in which it

reconstructs multiple modulated carriers over a designated

frequency band. Cellular multicarrier and multimode radios are

often referred to as software radios since the carrier tuning and

modulation scheme is software programmable and performed

digitally. The AD9772 is the recommended TxDAC in Analog

Device’s Softcell chipset which comprises the AD6622, Quadra-

ture Digital Upconverter IC, along with its companion Rx Digi-

tal Downconverter IC, the AD6624, and 14-bit, 65 MSPS

ADC, the AD6644. Figure 47 shows a generic software radio

Tx signal chain based on the AD9772/AD6622.

OTHER AD6622's FOR

INCREASED CHANNEL

CAPACITY

AD9772

PLLLOCK

CLK

SUMMATION

SPORT

RCF

CIC

FILTER

NCO

QAM

SPORT

RCF

CIC

FILTER

NCO

QAM

SPORT

RCF

CIC

FILTER

NCO

QAM

SPORT

RCF

CIC

FILTER

NCO

QAM

CLK

JTAG

PORT

AD6622

Figure 47. Generic Multicarrier Signal Chain Using the

AD6622 and AD9772

Figure 48 shows a spectral plot of the AD9772 operating at

64.54 MSPS reconstructing eight IS-136 modulated carriers

spread over a 25 MHz band. For this particular test scenario,

the AD9772 exhibited 74 dBc SFDR performance along with a

carrier-to-noise ratio (CNR) of 73 dB. Figure 49 shows a spec-

tral plot of the AD9772 operating at 52 MSPS reconstructing

four equal GSM carriers spread over a 15 MHz band. The

SFDR and CNR (in 100 kHz BW) measured to be 76 dBc and

83.4 dB respectively along with a channel power of –13.5 dBFS.

Note, the test vectors were generated using Rohde & Schwarz’s

WinIQSIM software.

FREQUENCY – MHz

–10

–20

–30

–40

–50

–60

510

–70

–80

–90

–100

Figure 48. Spectral Plot of AD9772 Reconstructing

Eight IS-136 Modulated Carriers @ fDATA= 64.54 MSPS,

PLLVDD = 0

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