AD9763/AD9765/AD9767
Data Sheet
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.
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.
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.
FERRITE
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
ELECTROLYTIC
CERAMIC
BEADS
AVDD
TTL/CMOS
LOGIC
CIRCUITS
10µF
TO
100µF
0.1µF
22µF
ACOM
TANTALUM
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,
,
5V
POWER SUPPLY
adjust the curve in Figure 77 by the scaling factor 20 × log(RLOAD).
For example, if RLOAD is 50 ꢀ, 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).
Figure 78. Differential LC Filter for Single 5 V and 3.3 V Applications
Rev. G | Page 30 of 44
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