dac121s101wgmpr National Semiconductor Corporation, dac121s101wgmpr Datasheet - Page 18

• Manufacturer Part Number: dac121s101wgmpr
• Description: Dac121s101qml 12-bit Micro Power Digital-to-analog Converter With Rail-to-rail Output
• Manufacturer: National Semiconductor Corporation

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2.2 USING REFERENCES AS POWER SUPPLIES
Recall the need for a quiet supply source for devices that use
their power supply voltage as a reference voltage.
Since the DAC121S101 consumes very little power, a refer-
ence source may be used as the supply voltage. The advan-
tages of using a reference source over a voltage regulator are
accuracy and stability. Some low noise regulators can also be
used for the power supply of the DAC121S101. Listed below
are a few power supply options for the DAC121S101.
2.2.1 LM4130
The LM4130 reference, with its 0.05% accuracy over tem-
perature, is a good choice as a power source for the
DAC121S101. Its primary disadvantage is the lack of 3 V and
5 V versions. However, the 4.096 V version is useful if a 0 to
4.095 V output range is desirable or acceptable. Bypassing
the LM4130 VIN pin with a 0.1 µF capacitor and the VOUT
pin with a 2.2 µF capacitor will improve stability and reduce
output noise. The LM4130 comes in a space-saving 5-pin
SOT23.
2.2.2 LM4050
Available with accuracy of 0.44%, the LM4050 shunt refer-
ence is also a good choice as a power regulator for the
DAC121S101. It does not come in a 3 Volt version, but 4.096
V and 5 V versions are available. It comes in a space-saving
3-pin SOT23.
FIGURE 9. The LM4130 as a power supply
FIGURE 8. Microwire Interface
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18
The minimum resistor value in the circuit of
be chosen such that the maximum current through the
LM4050 does not exceed its 15 mA rating. The conditions for
maximum current include the input voltage at its maximum,
the LM4050 voltage at its minimum, the resistor value at its
minimum due to tolerance, and the DAC121S101 draws zero
current. The maximum resistor value must allow the LM4050
to draw more than its minimum current for regulation plus the
maximum DAC121S101 current in full operation. The condi-
tions for minimum current include the input voltage at its
minimum, the LM4050 voltage at its maximum, the resistor
value at its maximum due to tolerance, and the DAC121S101
draws its maximum current. These conditions can be sum-
marized as
and
where V
voltages ± the LM4050 output tolerance over temperature, I
(max) is the maximum allowable current through the LM4050,
I
proper regulation, I
ply current, and I
current.
2.2.3 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
DAC121S101. It comes in 3.0V, 3.3V and 5V versions, among
others, and sports a low 30 µV noise specification at low fre-
quencies. Since low frequency noise is relatively difficult to
filter, this specification could be important for some applica-
tions. The LP3985 comes in a space-saving 5-pin SOT23 and
5-bump micro SMD packages.
Z
(min) is the minimum current required by the LM4050 for
R(max) = ( V
R(min) = ( V
FIGURE 10. The LM4050 as a power supply
Z
(min) and V
A
IN
IN
(min) is the minimum DAC121S101 supply
A
(max) − V
(min) − V
(max) is the maximum DAC121S101 sup-
Z
(max) are the nominal LM4050 output
Z
Z
(max) / (I
(min) / (I
A
A
(min) + I
(max) + I
Figure 10
Z
Z
(max))
(min) )
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should
Z