AD9709_15 datasheet(21/32 Pages) AD

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AD9709

Rev. B | Page 21 of 32

An example serves to illustrate the effect of supply noise on the

analog supply. Suppose a switching regulator with a switching

frequency of 250 kHz produces 10 mV of noise and, for simplicity’s

sake, all of this noise is concentrated at 250 kHz (that is, ignore

harmonics). To calculate how much of this undesired noise will

appear as current noise superimposed on the DAC full-scale

current, IOUTFS, one must determine the PSRR in decibels using

Figure 41 at 250 kHz. To calculate the PSRR for a given RLOAD,

such that the units of PSRR are converted from A/V to V/V,

adjust the curve in Figure 41 by the scaling factor 20 × log(RLOAD).

For instance, if RLOAD is 50 Ω, the PSRR is reduced by 34 dB

(that is, the PSRR of the DAC at 250 kHz, which is 85 dB in

Figure 41, becomes 51 dB VOUT/VIN).

Proper grounding and decoupling should be a primary

objective in any high speed, high resolution system. The

AD9709 features separate analog and digital supply and ground

pins to optimize the management of analog and digital ground

currents in a system. In general, decouple the analog supply

(AVDD) to the analog common (ACOM) as close to the chip as

physically possible. Similarly, decouple DVDD1/DVDD2, the

digital supply (DVDD1/DVDD2) to the digital common

(DCOM1/DCOM2) as close to the chip as possible.

For applications that require a single 5 V or 3.3 V supply for

both the analog and digital supplies, a clean analog supply can

be generated using the circuit shown in Figure 42. The circuit

consists of a differential LC filter with separate power supply

and return lines. Lower noise can be attained by using low-ESR

type electrolytic and tantalum capacitors.

TTL/CMOS

LOGIC

CIRCUITS

100µF

0.1µF

AVDD

ACOM

ELECTROLYTIC

TANTALUM

CERAMIC

POWER SUPPLY

FERRITE

BEADS

10µF

22µF

Figure 42. Differential LC Filter for Single 5 V and 3.3 V Applications

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