AD9557SLASHPCBZ datasheet(53/92 Pages) AD

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Data Sheet

AD9557

Rev. A | Page 53 of 92

POWER SUPPLY PARTITIONS

The AD9557 power supplies are divided into four groups:

DVDD3, DVDD, AVDD3, and AVDD. All power and ground

pins should be connected, even if certain blocks of the chip

are powered down.

RECOMMENDED CONFIGURATION FOR 3.3 V

SWITCHING SUPPLY

A popular power supply arrangement is to power the AD9557

with the output of a 3.3 V switching power supply.

When the AD9557 is powered using 3.3 V switching power

supplies, all of the 3.3 V supplies can be connected to the

3.3 V switcher output, and a 0.1 μF bypass capacitor should

be placed adjacent to each 3.3 V power supply pin.

CONFIGURATION FOR 1.8 V SUPPLY

When 1.8 V supplies are preferred, it is recommended that

an LDO regulator, such as the ADP222, be used to generate

the 1.8 V supply from the 3.3 V supply.

The ADP222 offers excellent power supply rejection in

a small (2 mm × 2 mm) package. It has two 1.8 V outputs.

One output can be used for the DVDD pins (Pin 6, Pin 34,

and Pin 35), and the other output can drive the AVDD pins.

The ADP7104 is another good choice for converting 3.3 V to

1.8 V. The close-in noise of the ADP7104 is lower than that of

the ADP222; therefore, it may be better suited for applications

where close-in phase noise is critical and the AD9557 DPLL loop

bandwidth is <50 Hz. In such cases, all 1.8 V supplies can be

connected to one ADP7104.

Use of Ferrite Beads on 1.8 V Supplies

To ensure the very best output-to-output isolation, one ferrite

bead should be used instead of a bypass capacitor for each of

the following AVDD pins: Pin 11, Pin 17, and Pin 18. The ferrite

beads should be placed in between the 1.8 V LDO output and

each pin listed above. Ferrite beads that have low (<0.7 Ω) dc

resistance and approximately 600 Ω impedance at 100 MHz are

suitable for this application.

See Table 2 for the current consumed by each group. Refer to

Figure 20, Figure 21, and Figure 22 for information on the

power consumption vs. output frequency.

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