DAC121S101EVAL National Semiconductor, DAC121S101EVAL Datasheet - Page 16

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DAC121S101EVAL

Manufacturer Part Number
DAC121S101EVAL
Description
BOARD EVALUATION DAC121S101
Manufacturer
National Semiconductor
Series
PowerWise®r
Datasheet

Specifications of DAC121S101EVAL

Number Of Dac's
1
Number Of Bits
12
Outputs And Type
1, Single Ended
Data Interface
Serial
Settling Time
12µs
Dac Type
Voltage
Voltage Supply Source
Single
Operating Temperature
-40°C ~ 125°C
Utilized Ic / Part
DAC121S101
Lead Free Status / RoHS Status
Lead free by exemption / RoHS compliant by exemption
www.national.com
2.1.4 Microwire Interface
Figure 8
device and the DAC121S101. Data is clocked out on the rising
edges of the SCLK signal.
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.
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 4.096V
and 5V versions are available. It comes in a space-saving 3-
pin SOT23.
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
FIGURE 10. The LM4050 as a power supply
shows an interface between a Microwire compatible
FIGURE 8. Microwire Interface
FIGURE 9. The LM4130 as a power supply
Figure 10
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20114914
should
16
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 3V and
5V versions. However, the 4.096V version is useful if a 0 to
4.095V output range is desirable or acceptable. Bypassing
the LM4130 VIN pin with a 0.1µF capacitor and the VOUT pin
with a 2.2µF capacitor will improve stability and reduce output
noise. The LM4130 comes in a space-saving 5-pin SOT23.
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
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
20114913
Z
Z
(max) / (I
(min) / (I
A
A
(min) + I
(max) + I
Z
Z
(max))
(min) )
Z

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