DAC121S101QML_10 NSC [National Semiconductor], DAC121S101QML_10 Datasheet - Page 19

• Manufacturer Part Number: DAC121S101QML_10
• Description: 12-Bit Micro Power Digital-to-Analog Converter with Rail-to-Rail Output
• Manufacturer: NSC [National Semiconductor]

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.
2.2.4 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 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.
2.3 BIPOLAR OPERATION
The DAC121S101 is designed for single supply operation and
thus has a unipolar output. However, a bipolar output may be
obtained with the circuit in
an output voltage range of ±5 Volts. A rail-to-rail amplifier
should be used if the amplifier supplies are limited to ±5V.
FIGURE 11. Using the LP3985 regulator
FIGURE 12. Using the LP2980 regulator
Figure
13. This circuit will provide
30018016
30018015
19
The output voltage of this circuit for any code is found to be
where D is the input code in decimal form. With VA = 5V and
R1 = R2,
A list of rail-to-rail amplifiers suitable for this application are
indicated in
2.4 LAYOUT, GROUNDING, AND BYPASSING
For best accuracy and minimum noise, the printed circuit
board containing the DAC121S101 should have separate
analog and digital areas. The areas are defined by the loca-
tions of the analog and digital power planes. Both of these
planes should be located in the same board layer. There
should be a single ground plane. A single ground plane is
preferred if digital return current does not flow through the
analog ground area. Frequently a single ground plane design
will utilize a "fencing" technique to prevent the mixing of ana-
log and digital ground current. Separate ground planes should
only be utilized when the fencing technique is inadequate.
The separate ground planes must be connected in one place,
preferably near the DAC121S101. Special care is required to
guarantee that digital signals with fast edge rates do not pass
over split ground planes. They must always have a continu-
ous return path below their traces.
The DAC121S101 power supply should be bypassed with a
10µF and a 0.1µF capacitor as close as possible to the device
with the 0.1µF right at the device supply pin. The 10µF ca-
pacitor should be a tantalum type and the 0.1µF capacitor
should be a low ESL, low ESR type. The power supply for the
DAC121S101 should only be used for analog circuits.
Avoid crossover of analog and digital signals and keep the
clock and data lines on the component side of the board. The
clock and data lines should have controlled impedances.
V
O
LMC7111
LM7301
LM8261
= (V
AMP
TABLE 2. Some Rail-to-Rail Amplifiers
A
x (D / 4096) x ((R1 + R2) / R1) - V
Table
FIGURE 13. Bipolar Operation
V
2.
O
SOT23-5
SOT23-5
SOT23-5
= (10 x D / 4096) - 5V
PKGS
DIP-8
SO-8
Typ V
0.03 mV
0.9 mV
0.7 mV
OS
A
Typ I
x R2 / R1)
www.national.com
620 µA
25 µA
1 mA
SUPPLY
30018017