DAC104S085 Datasheet, PDF (16/18 Pages) National Semiconductor (TI) – 10-Bit Micro Power QUAD Digital-to-Analog Converter with Rail-to-Rail Output

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 DAC104S085 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 DAC104S085. 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.
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 DAC104S085 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.
20195315
FIGURE 7. Using the LP3985 regulator
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 5mΩ 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.
20195317
FIGURE 9. Bipolar Operation
The output voltage of this circuit for any code is found to be
V_O = (V_A x (D / 1024) x ((R1 + R2) / R1) - V_A x R2 / R1)
where D is the input code in decimal form. With V_A = 5V and
R1 = R2,
V_O = (10 x D / 1024) - 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
LMC7111
LM7301
LM8261
PKGS
DIP-8
SOT23-5
SO-8
SOT23-5
SOT23-5
Typ V_OS Typ I_SUPPLY
0.9 mV
25 µA
0.03 mV
0.7 mV
620 µA
1 mA
20195316
FIGURE 8. Using the LP2980 regulator
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-
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