AD9744ARURL7 datasheet(19/32 Pages) AD

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Data Sheet

AD9744

Rev. C | Page 19 of 32

these switches, the PSRR is very code dependent. This can produce

a mixing effect that can modulate low frequency power supply

noise to higher frequencies. Worst-case PSRR for either one of

the differential DAC outputs will occur when the full-scale current

is directed toward that output. As a result, the PSRR measurement

in Figure 39 represents a worst-case condition in which the

digital inputs remain static and the full-scale output current of

20 mA is directed to the DAC output being measured.

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 (ignoring harmonics), all of this noise is

concentrated at 250 kHz. To calculate how much of this

undesired noise will appear as current noise superimposed on

the DAC’s full-scale current, IOUTFS, one must determine the

PSRR in dB using Figure 39 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 39 by the scaling

factor 20 Ω log (RLOAD). For instance, if RLOAD is 50 Ω, the PSRR

is reduced by 34 dB (that is, PSRR of the DAC at 250 kHz,

which is 85 dB in Figure 39, becomes 51 dB VOUT/VIN).

Proper grounding and decoupling should be a primary

objective in any high speed, high resolution system. The

AD9744 features separate analog and digital supplies 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. Similarly, DVDD, the

digital supply, should be decoupled to DCOM as close to the

chip as physically possible.

For those applications that require a single 3.3 V supply for both

the analog and digital supplies, a clean analog supply may be

generated using the circuit shown in Figure 40. 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.

Figure 40. Differential LC Filter for Single 3.3 V Applications

100

µF

ELECT.

0.1

µF

CER.

TTL/CMOS

LOGIC

CIRCUITS

3.3V

POWER SUPPLY

FERRITE

BEADS

AVDD

ACOM

µF–22µF

TANT.

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