DAC081S101 Datasheet, PDF (17/19 Pages) National Semiconductor (TI) – 8-Bit Micro Power Digital-to-Analog Converter with Rail-to-Rail Output

2.0 Applications Information
(Continued)
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.
2.3 BIPOLAR OPERATION
The DAC081S101 is designed for single supply operation
and thus has a unipolar output. However, a bipolar output
may be obtained with the circuit in Figure 13. 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.
20155117
FIGURE 13. Bipolar Operation
The output voltage of this circuit for any code is found to be
V_O = (V_A x (D / 256) 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 / 256) - 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
2.4 LAYOUT, GROUNDING, AND BYPASSING
For best accuracy and minimum noise, the printed circuit
board containing the DAC081S101 should have separate
analog and digital areas. The areas are defined by the
locations 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 analog 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 DAC081S101. Special care
is required to guarantee that digital signals with fast edge
rates do not pass over split ground planes. They must al-
ways have a continuous return path below their traces.
The DAC081S101 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 capacitor should be a tantalum type and the 0.1µF
capacitor should be a low ESL, low ESR type. The power
supply for the DAC081S101 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.
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