AD9708

–10–

REV. B

POWER DISSIPATION

several factors, including: (1) AVDD and DVDD, the power

waveform. The power dissipation is directly proportional to the

than a factor of 2 when DVDD is reduced from 5 V to 3 V.

APPLYING THE AD9708

Power and Grounding Considerations

In systems seeking to simultaneously achieve high speed and

high performance, the implementation and construction of the

printed circuit board design is often as important as the circuit

design. Proper RF techniques must be used in device selection

placement and routing and supply bypassing and grounding.

The evaluation board for the AD9708, which uses a four layer

PC board, serves as a good example for the above mentioned

considerations. The evaluation board provides an illustration of

the recommended printed circuit board ground, power and

signal plane layouts.

Proper grounding and decoupling should be a primary objective

in any high speed system. The AD9708 features separate analog

and digital supply and ground pins to optimize the management

of analog and digital ground currents in a system. In general,

AVDD, the analog supply, should be decoupled to ACOM, the

analog common, as close to the chip as physically possible. Simi-

larly, DVDD, the digital supply, should be decoupled to DCOM

as close as physically as possible.

For those applications requiring a single +5 V or +3 V supply

for both the analog and digital supply, a clean analog supply

may be generated using the circuit shown in Figure 22. The

circuit consists of a differential LC filter with separate power

supply and return lines. Lower noise can be attained using low

ESR type electrolytic and tantalum capacitors.

30

0

220

46

8

10

12

14

16

18

25

20

15

10

5

18

16

0

0.01

1

0.1

8

6

4

2

12

10

14

5MSPS

25MSPS

50MSPS

100MSPS

125MSPS

@ DVDD = 5 V

100 F

ELECT.

10-22 F

TANT.

0.1 F

CER.

TTL/CMOS

LOGIC

CIRCUITS

+5V OR +3V

POWER SUPPLY

FERRITE

BEADS

AVDD

ACOM

Figure 22. Differential LC Filter for Single +5 V or +3 V

Applications

Maintaining low noise on power supplies and ground is critical

to obtaining optimum results from the AD9708. 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 as well as any clock signals. 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

voltage drops in the signal ground paths. It is recommended that

all connections be short, direct and as physically close to the

package as possible in order to minimize the sharing of conduc-

tion paths between different currents. When runs exceed an inch

in length, strip line techniques with proper termination resistor

8

0

0.01

1

0.1

6

4

2

5MSPS

25MSPS

50MSPS

100MSPS

125MSPS

@ DVDD = 3 V