AD9742ARURL7 datasheet(18/32 Pages) AD

18 / 32 page

AD9742

Data Sheet

Rev. C | Page 18 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 (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 36

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 36 by the scaling factor 20 Ω log (RLOAD). For instance, if

RLOAD is 50 Ω, the PSRR is reduced by 34 dB (i.e., PSRR of the DAC

at 250 kHz, which is 85 dB in Figure 36, becomes 51 dB VOUT/VIN).

Proper grounding and decoupling should be a primary objective in

any high speed, high resolution system. The AD9742 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 37. 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.

100

µF

ELECT.

0.1

µF

CER.

TTL/CMOS

LOGIC

CIRCUITS

3.3V

POWER SUPPLY

FERRITE

BEADS

AVDD

ACOM

µF–22µF

TANT.

Figure 37. Differential LC Filter for Single 3.3 V Applications

AD9742ARURL7 Datasheet(PDF) 18 Page - Analog Devices

AD9742ARURL7 Datasheet(HTML) 18 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Datasheet Download

Link URL