DAC121C081 Datasheet, PDF (20/24 Pages) National Semiconductor (TI) – 12-Bit Micro Power Digital-to-Analog Converter with an I2C-Compatible Interface

2.1.3 LP3985
The LP3985 is a low noise, ultra low dropout voltage regulator
with a 3% accuracy over temperature. It is a good choice for
applications that do not require a precision reference for the
DAC121C081. It comes in 3.0V, 3.3V and 5V versions,
among others, and sports a low 30 µV noise specification at
low frequencies. Since low frequency noise is relatively diffi-
cult 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.
2.2 BIPOLAR OPERATION
The DAC121C081 is designed for single supply operation and
thus has a unipolar output. However, a bipolar output may be
obtained with the circuit in Figure 14. 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.
30004915
FIGURE 12. 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 ca-
pacitor with an ESR requirement of 5mΩ to 500mΩ is required
at the output. Careful interpretation and understanding 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.
FIGURE 14. Bipolar Operation
30004917
The output voltage of this circuit for any code is found to be
V_O = (V_A x (D / 4096) 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 / 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
LMC7111
PKGS
DIP-8
SOT23-5
 Typ V_OS Typ I_SUPPLY
0.9 mV
25 µA
LM7301
SO-8
SOT23-5
0.03 mV
620 µA
LM8261
SOT23-5
0.7 mV
1 mA
30004916
FIGURE 13. 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 combi-
nation of small size and ESR. Ceramic capacitors are attrac-
tive due to their small size but generally have ESR values that
are too low for use with the LP2980. Aluminum electrolytic
capacitors are typically not a good choice due to their large
size and have ESR values that may be too high at low tem-
peratures.
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