Datasheet

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, I
OUTFS
, one must determine the PSRR in dB using Figure 36
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 36 by the scaling factor 20 Ω log (R
LOAD
). For instance, if
R
LOAD
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 V
OUT
/V
IN
).
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.
100µF
ELECT.
0.1µF
CER.
TTL/CMOS
LOGIC
CIRCUITS
3.3V
POWER SUPPLY
FERRITE
BEADS
AVDD
ACOM
10µF–22µF
TANT.
02912-B-036
Figure 37. Differential LC Filter for Single 3.3 V Applications