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DAC122S085 Datasheet(PDF) 16 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor. Click here to check the latest version.
Part No. DAC122S085
Description  12-Bit Micro Power DUAL Digital-to-Analog Converter with Rail-to-Rail Output
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Maker  NSC [National Semiconductor (TI)]
Homepage  http://www.national.com
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DAC122S085 Datasheet(HTML) 16 Page - National Semiconductor (TI)

 
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2.0 Applications Information
(Continued)
LM4050, I
Z(min) is the minimum current required by the
LM4050 for proper regulation, and I
DAC(max) is the maxi-
mum DAC122S085 supply current.
2.1.3 LP3985
The LP3985 is a low noise, ultra low dropout voltage regu-
lator with a 3% accuracy over temperature. It is a good
choice for applications that do not require a precision refer-
ence for the DAC122S085. It comes in 3.0V, 3.3V and 5V
versions, among others, and sports a low 30 µV noise speci-
fication at low frequencies. Since low frequency noise is
relatively difficult to filter, this specification could be important
for some applications. The LP3985 comes in a space-saving
5-pin SOT23 and 5-bump micro SMD packages.
An input capacitance of 1.0µF without any ESR requirement
is required at the LP3985 input, while a 1.0µF ceramic
capacitor with an ESR requirement of 5m
Ω to 500mΩ is
required at the output. Careful interpretation and under-
standing of the capacitor specification is required to ensure
correct device operation.
2.1.4 LP2980
The LP2980 is an ultra low dropout regulator with a 0.5% or
1.0% accuracy over temperature, depending upon grade. It
is available in 3.0V, 3.3V and 5V versions, among others.
Like any low dropout regulator, the LP2980 requires an
output capacitor for loop stability. This output capacitor must
be at least 1.0µF over temperature, but values of 2.2µF or
more will provide even better performance. The ESR of this
capacitor should be within the range specified in the LP2980
data sheet. Surface-mount solid tantalum capacitors offer a
good combination of small size and ESR. Ceramic capaci-
tors are attractive due to their small size but generally have
ESR values that are too low for use with the LP2980. Alumi-
num electrolytic capacitors are typically not a good choice
due to their large size and have ESR values that may be too
high at low temperatures.
2.2 BIPOLAR OPERATION
The DAC122S085 is designed for single supply operation
and thus has a unipolar output. However, a bipolar output
may be obtained with the circuit in Figure 9. This circuit will
provide an output voltage range of ±5 Volts. A rail-to-rail
amplifier should be used if the amplifier supplies are limited
to ±5V.
The output voltage of this circuit for any code is found to be
V
O =(VA x (D / 4096) x ((R1 + R2) / R1) - VA xR2/R1)
where D is the input code in decimal form. With V
A =5Vand
R1 = R2,
V
O =(10xD/ 4096) - 5V
A list of rail-to-rail amplifiers suitable for this application are
indicated in Table 2.
TABLE 2. Some Rail-to-Rail Amplifiers
AMP
PKGS
Typ V
OS
Typ I
SUPPLY
LMC7111
DIP-8
SOT23-5
0.9 mV
25 µA
LM7301
SO-8
SOT23-5
0.03 mV
620 µA
LM8261
SOT23-5
0.7 mV
1 mA
20195215
FIGURE 7. Using the LP3985 regulator
20195216
FIGURE 8. Using the LP2980 regulator
20195217
FIGURE 9. Bipolar Operation
www.national.com
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