DAC121C081 Datasheet, PDF (19/24 Pages) National Semiconductor (TI) – 12-Bit Micro Power Digital-to-Analog Converter with an I2C-Compatible Interface

2.0 Applications Information
2.1 USING REFERENCES AS POWER SUPPLIES
While the simplicity of the DAC121C081 implies ease of use,
it is important to recognize that the path from the reference
input (V_A for the DAC121C081 & V_REF for the DAC121C085)
to V_OUT will have essentially zero Power Supply Rejection
Ratio (PSRR). Therefore, it is necessary to provide a noise-
free supply voltage to the reference. In order to use the full
dynamic range of the DAC121C085, the supply pin (V_A) and
V_REF can be connected together and share the same supply
voltage. Since the DAC121C081 consumes very little power,
a reference source may be used as the supply voltage. The
advantages of using a reference source over a voltage regu-
lator are accuracy and stability. Some low noise regulators
can also be used. Listed below are a few reference and power
supply options for the DAC121C081. When using the
DAC121C081, it is important to treat the analog supply (V_A)
as the reference.
2.1.1 LM4132
The LM4132, with its 0.05% accuracy over temperature, is a
good choice as a reference source for the DAC121C081. The
4.096V version is useful if a 0 to 4.095V output range is de-
sirable or acceptable. Bypassing the LM4132 V_IN pin with a
0.1µF capacitor and the V_OUT pin with a 2.2µF capacitor will
improve stability and reduce output noise. The LM4132
comes in a space-saving 5-pin SOT23.
30004913
FIGURE 10. The LM4132 as a power supply
2.1.2 LM4050
Available with accuracy of 0.44%, the LM4050 shunt refer-
ence is also a good choice as a reference for the
DAC121C081. It is available in 4.096V and 5V versions and
comes in a space-saving 3-pin SOT23.
30004914
FIGURE 11. The LM4050 as a power supply
The minimum resistor value in the circuit of Figure 11 must
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, and the DAC121C081
drawing zero current. The maximum resistor value must allow
the LM4050 to draw more than its minimum current for regu-
lation plus the maximum DAC121C081 current in full opera-
tion. The conditions 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
DAC121C081 draws its maximum current. These conditions
can be summarized as
R(min) = ( V_IN(max) − V_Z(min) ) /I_Z(max)
and
R(max) = ( V_IN(min) − V_Z(max) ) / ( (I_DAC(max) + I_Z(min) )
where V_Z(min) and V_Z(max) are the nominal LM4050 output
voltages ± the LM4050 output tolerance over temperature, I_Z
(max) is the maximum allowable current through the LM4050,
I_Z(min) is the minimum current required by the LM4050 for
proper regulation, and I_DAC(max) is the maximum
DAC121C081 supply current.
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