Analog Devices AD9763 Evaluation Board AD9763-EBZ AD9763-EBZ データシート

 

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 I

OUT

. 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, I

OUTFS

, one must determine the PSRR in decibels using 

Figure 77 at 250 kHz. To calculate the PSRR for a given R

LOAD

, 

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(R

LOAD

). 

For example, if R

LOAD

 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 V

OUT

/V

IN

).  

00

61

7-

0

78