dac124s085eb

Manufacturer Part Numberdac124s085eb
Description12-bit Micro Power Quad Digital-to-analog Converter With Rail-to-rail Output
ManufacturerNational Semiconductor Corporation
dac124s085eb datasheet
 
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1.7 POWER-DOWN MODES
The DAC124S085 has four power-down modes, two of which
are identical. In power-down mode, the supply current drops
to 20 µA at 3V and 30 µA at 5V. The DAC124S085 is set in
power-down mode by setting OP1 and OP0 to 11. Since this
mode powers down all four DACs, the address bits, A1 and
A0, are used to select different output terminations for the
DAC outputs. Setting A1 and A0 to 00 or 11 causes the out-
puts to be tri-stated (a high impedance state). While setting
A1 and A0 to 01 or 10 causes the outputs to be terminated by
2.5 kΩ or 100 kΩ to ground respectively (see Table 1).
TABLE 1. Power-Down Modes
 Operating Mode
A1
A0
OP1
OP0
0
0
1
1
0
1
1
1
1
0
1
1
100 kΩ to GND
1
1
1
1
The bias generator, output amplifiers, resistor strings, and
other linear circuitry are all shut down in any of the power-
down modes. However, the contents of the DAC registers are
unaffected when in power-down. Each DAC register main-
tains its value prior to the ADC124S085 being powered down
unless it is changed during the write sequence which instruct-
ed it to recover from power down. Minimum power consump-
tion is achieved in the power-down mode with SYNC and
D
idled low and SCLK disabled. The time to exit power-down
IN
(Wake-Up Time) is typically 0.8 µsec at 3V and 0.5 µsec at
5V.
2.0 Applications Information
2.1 USING REFERENCES AS POWER SUPPLIES
While the simplicity of the DAC124S085 implies ease of use,
it is important to recognize that the path from the reference
input (V
) to the VOUTs will have essentially zero Power
REFIN
Supply Rejection Ratio (PSRR). Therefore, it is necessary to
provide a noise-free supply voltage to V
lize the full dynamic range of the DAC124S085, the supply
pin (V
) and V
can be connected together and share the
A
REFIN
same supply voltage. Since the DAC124S085 consumes very
little power, a reference source may be used as the reference
input and/or the supply voltage. The advantages of using a
reference source over a voltage regulator are accuracy and
stability. Some low noise regulators can also be used. Listed
below are a few reference and power supply options for the
DAC124S085.
2.1.1 LM4130
The LM4130, with its 0.05% accuracy over temperature, is a
good choice as a reference source for the DAC124S085. The
4.096V version is useful if a 0 to 4.095V output range is de-
sirable 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.
FIGURE 5. The LM4130 as a power supply
High-Z outputs
2.1.2 LM4050
2.5 kΩ to GND
Available with accuracy of 0.44%, the LM4050 shunt refer-
ence is also a good choice as a reference for the
DAC124S085. It is available in 4.096V and 5V versions and
High-Z outputs
comes in a space-saving 3-pin SOT23.
. In order to uti-
REFIN
FIGURE 6. The LM4050 as a power supply
The minimum resistor value in the circuit of Figure 6 must be
chosen such that the maximum current through the LM4050
does not exceed its 15 mA rating. The conditions for maxi-
mum current include the input voltage at its maximum, the
LM4050 voltage at its minimum, and the DAC124S085 draw-
ing zero current. The maximum resistor value must allow the
LM4050 to draw more than its minimum current for regulation
plus the maximum DAC124S085 current in full operation. 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 DAC124S085
draws its maximum current. These conditions can be sum-
marized as
and
R(max) = ( V
where V
(min) and V
Z
voltages ± the LM4050 output tolerance over temperature, I
(max) is the maximum allowable current through the LM4050,
I
(min) is the minimum current required by the LM4050 for
Z
proper
regulation,
DAC124S085 supply current.
15
R(min) = ( V
(max) − V
(min) ) /I
(max)
IN
Z
Z
(min) − V
(max) ) / ( (I
(max) + I
IN
Z
DAC
(max) are the nominal LM4050 output
Z
and
I
(max)
is
the
DAC
www.national.com
20173213
20173214
(min) )
Z
Z
maximum