AD9859

Rev. A | Page 22 of 24

When the CFR1<3> bit is 0 and the PWRDWNCTL input pin is

high, the AD9859 is put into a fast recovery power-down mode.

In this mode, the digital logic and the DAC digital logic are

powered down. The DAC bias circuitry, PLL, oscillator, and

clock input circuitry is NOT powered down.

When the CFR1<3> bit is high and the PWRDWNCTL input

pin is high, the AD9859 is put into the full power-down mode.

In this mode, all functions are powered down. This includes

the DAC and PLL, which take a significant amount of time to

power up.

When the PWRDWNCTL input pin is high, the individual

power-down bits (CFR1<7>, <5:4>) are invalid (Don’t Care)

and unused. When the PWRDWNCTL input pin is low, the

individual power-down bits control the power-down modes of

operation.

Note that the power-down signals are all designed such that a

Logic 1 indicates the low power mode and a Logic 0 indicates

the active or power-up mode.

Table 8 indicates the logic level for each power-down bit that

drives out of the AD9859 core logic to the analog section and

the digital clock generation section of the chip for the external

power-down operation.

Layout Considerations

For the best performance, these layout guidelines should be

observed. Always provide the analog power supply (AVDD) and

the digital power supply (DVDD) on separate supplies, even if

just from two different voltage regulators driven by a common

supply. Likewise, the ground connections (AGND, DGND)

should be kept separate as far back to the source as possible (i.e.,

separate the ground planes on a localized board, even if the

grounds connect to a common point in the system). Bypass

capacitors should be placed as close to the device pin as possi-

ble. Usually, a multitiered bypassing scheme consisting of a

small high frequency capacitor (100 pF) placed close to the

supply pin and progressively larger capacitors (0.1 µF, 10 µF)

further away from the actual supply source works best.

Table 8. Power-Down Control Functions

Control

Mode Active

Description

PWRDWNCTL = 0 CFR1<3> Don’t Care

Software Control

Digital Power-Down = CFR1<7>

DAC Power-Down = CFR1<5>

Input Clock Power-Down = CFR1<4>

PWRDWNCTL = 1 CFR1<3> = 0

External Control,

Fast Recovery Power-Down Mode

Digital Power-Down = 1’b1

DAC Power-Down = 1’b0

Input Clock Power-Down = 1’b0

PWRDWNCTL = 1 CFR1<3> = 1

External Control,

Full Power-Down Mode

Digital Power-Down = 1’b1

DAC Power-Down = 1’b1

Input Clock Power-Down = 1’b1