AD9765AST datasheet(30/44 Pages) AD

30 / 44 page

AD9763/AD9765/AD9767

Data Sheet

Rev. G | Page 30 of 44

Proper grounding and decoupling are primary objectives in any

high speed, high resolution system. The AD9763/AD9765/AD9767

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 the digital supply (DVDD1/DVDD2)

to the digital common (DCOM1/DCOM2) as close to the chip

as possible.

Note that the data in Figure 77 is given in terms of current out

vs. voltage in. Noise on the analog power supply has the effect

of modulating the internal current sources and therefore the

output current. The voltage noise on AVDD, therefore, is added

in a nonlinear manner to the desired IOUT. PSRR is very code

dependent, thus producing mixing effects that can modulate

low frequency power supply noise to higher frequencies. Worst-

case PSRR for either one of the differential DAC outputs occurs

when the full-scale current is directed toward that output. As a

result, the PSRR measurement in Figure 77 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.

For those 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 78. 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.

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 77 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 77 by the scaling factor 20 × log(RLOAD).

For example, 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 77,

becomes 51 dB VOUT/VIN).

TTL/CMOS

LOGIC

CIRCUITS

100µF

0.1µF

AVDD

ACOM

ELECTROLYTIC

TANTALUM

CERAMIC

POWER SUPPLY

FERRITE

BEADS

10µF

22µF

Figure 78. Differential LC Filter for Single 5 V and 3.3 V Applications

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