AD9744ARURL7 Analog Devices Inc, AD9744ARURL7 Datasheet - Page 19

• Manufacturer Part Number: AD9744ARURL7
• Description: IC DAC 14BIT 210MSPS 28-TSSOP
• Manufacturer: Analog Devices Inc
• Series: TxDAC®r

Specifications of AD9744ARURL7

• Rohs Status: RoHS non-compliant
• Settling Time: 11ns
• Number Of Bits: 14
• Data Interface: Parallel
• Number Of Converters: 1
• Voltage Supply Source: Analog and Digital
• Power Dissipation (max): 145mW
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-TSSOP
• For Use With: AD9744ACP-PCBZ - BOARD EVAL FOR AD9744ACP

Note that the ratio in Figure 39 is calculated as amps out/volts
in. Noise on the analog power supply has the effect of modulat-
ing the internal switches, and therefore the output current. The
voltage noise on AVDD, therefore, will be added in a nonlinear
manner to the desired IOUT. Due to the relative different size
of these switches, the PSRR is very code dependent. This can
produce a mixing effect that can modulate low frequency power
supply noise to higher frequencies. Worst-case PSRR for either
one of the differential DAC outputs will occur when the full-
scale current is directed toward that output. As a result, the
PSRR measurement in Figure 39 represents a worst-case condi-
tion in which the digital inputs remain static and the full-scale
output current of 20 mA is directed to the DAC output being
measured.
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 con-
centrated 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
using Figure 39 at 250 kHz. To calculate the PSRR for a given
R
V/V, adjust the curve in Figure 39 by the scaling factor 20 Ω log
(R
by 34 dB (that is, PSRR of the DAC at 250 kHz, which is 85 dB
in Figure 39, becomes 51 dB V
LOAD
LOAD
, such that the units of PSRR are converted from A/V to
). For instance, if R
OUTFS
, one must determine the PSRR in dB
LOAD
is 50 Ω, the PSRR is reduced
OUT
/V
IN
).
Rev. B | Page 19 of 32
Proper grounding and decoupling should be a primary objec-
tive in any high speed, high resolution system. The AD9744
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 40. The circuit con-
sists 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.
TTL/CMOS
CIRCUITS
LOGIC
POWER SUPPLY
Figure 40. Differential LC Filter for Single 3.3 V Applications
3.3V
FERRITE
BEADS
100µF
ELECT.
10µF–22µF
TANT.
0.1µF
CER.
AD9744
AVDD
ACOM