AD8303
POWER SUPPLY
The very low power consumption of the AD8303 is a direct
result of a circuit design optimizing the use of a CBCMOS
process. By using the low power characteristics of CMOS for
the logic, and the low noise, tight matching of the complementary
bipolar transistors, excellent analog accuracy is achieved.
One advantage of the rail-to-rail output amplifiers used in the
AD8303 is the wide range of usable supply voltage. The part is
fully specified and tested for operation from +2.7 V to +5.5 V.
If reduced linearity and source current capability near full scale
can be tolerated, operation of the AD8303 is possible down to
+2.7 V.
POWER SUPPLY BYPASSING AND GROUNDING
Whether or not a separate power supply trace is available,
however, generous supply bypassing will reduce supply-line
induced errors. Local supply bypassing consisting of a 10
µF
tantalum electrolytic in parallel with a 0.1
µF
ceramic capacitor
is recommended in all applications (Figure 25).
+2.7V TO +5.5V
13
V
DD
2
SDI
CLK
CS
LDA
LDB
RS
MSB
SHDN
7
6
5
8
10
9
11
12
0.1µF
10µF
V
OUTA
AD8303
14
V
OUTB
Precision analog products, such as the AD8303, require a well
filtered power source. Since the AD8303 operates from a single
+3 V to +5 V supply, it seems convenient to simply tap into the
digital logic power supply. Unfortunately, the logic supply is
often a switch-mode design, which generates noise in the
20 kHz to 1 MHz range. In addition, fast logic gates can
generate glitches hundred of millivolts in amplitude due to
wiring resistances and inductances. The power supply noise
generated thereby means that special care must be taken to
insure that the inherent precision of the DAC is maintained.
Good engineering judgment should be exercised when addressing
the power supply grounding and bypassing of the AD8303.
The AD8303 should be powered directly from the system power
supply. This arrangement, shown in Figure 24, employs an LC
filter and separate power and ground connections to isolate the
analog section from the logic switching transients. Analog and
digital ground pins of the AD8303 should be connected
together directly at the IC package.
FERRITE BEAD:
2 TURNS, FAIR-RITE
#2677006301
TTL/CMOS
LOGIC
CIRCUITS
+5V
100µF
ELECT.
10-22µF
TANT.
0.1µF
CER.
+5V
RETURN
AGND
1
DGND
4
TO ANALOG GROUND
Figure 25. Recommended Supply Bypassing for the
AD8303
INPUT LOGIC LEVELS
All digital inputs are protected with a Zener-type ESD protection
structure (Figure 26) that allows logic input voltages to exceed
the V
DD
supply voltage. This feature can be useful if the user is
driving one or more of the digital inputs with a 5 V CMOS logic
input voltage level while operating the AD8303 on a +3 V power
supply. If this mode of interface is used, make sure that the V
OL
of the 5 V CMOS meets the V
IL
input requirement of the
AD8303 operating at 3 V. See Figure 6 for a graph for digital
logic input threshold versus operating V
DD
supply voltage.
V
DD
LOGIC
IN
GND
Figure 26. Equivalent Digital Input ESD Protection
+5V
POWER SUPPLY
Figure 24. Use Separate Traces to Reduce Power Supply
Noise
For power consumption-sensitive applications, it is important to
note that the internal power consumption of the AD8303 is
strongly dependent on the actual logic input voltage levels
present in the SDI, CLK,
CS, LDA, LDB, SHDN, RS
and
MSB pins. Since these inputs are standard CMOS logic
structures, they contribute static power dissipation which
depends on the actual driving logic V
OH
and V
OL
voltage levels.
Consequently, using CMOS logic versus TTL will provide
minimal dissipation in the static state.
REV. 0
–9–
AD [ ANALOG DEVICES ]
