Datasheet


SLAS353 − DECEMBER 2001
16
www.ti.com
THEORY OF OPERATION
power-down modes
The DAC8541 utilizes four modes of operation. These modes are programmable via two inputs (PD1 and PD0)
to the device. Table 3 shows how the state of these pins correspond to the mode of operation of the DAC8541.
Table 3. Modes of Operation for the DAC8541
PD1 PD0 OPERATING MODE
0 0 Normal operation
POWER-DOWN MODES
0 1 1 kΩ to AGND
1 0 100 kΩ to AGND
1 1 High impedance
When both pins are set to 0, the device works normally with its typical power consumption of 250 µA at
AV
DD
= 5 V. However, for the three power-down modes, the supply current falls to 200 nA at AV
DD
= 5 V (50 nA
at AV
DD
= 3 V). Not only does the supply current fall, but the V
OUT
terminal is internally switched from the output
of the amplifier to a resistor network of known values. This has the advantage that the output impedance of the
device is known while in power-down mode. There are three different options: The output is connected internally
to AGND through a 1-kΩ resistor, it is connected to AGND through a 100-kΩ resistor, or it is left open-circuited
(high impedance). The output stage is illustrated in Figure 31.
_
+
DAC
Powerdown
Circuitry
Amplifier
Resistor
Network
V
OUT
V
OUT
Sense
Figure 31. Output Stage During Power Down (High-Impedance)
All analog circuitry is shut down when a power-down mode is activated. However, the contents of the DAC
register are unaffected when in power-down. This allows the DAC’s output voltage to return to the previous level
when power-up resumes. The delay time required to exit power-down is typically 2.5 µs for AV
DD
= 5 V and 5
µs for AV
DD
= 3 V. (See the typical curves section for additional information.)
voltage reference inputs
Two voltage inputs provide the reference set points for the DAC architecture. These are V
REF
H and V
REF
L. For
typical rail-to-rail operation, V
REF
H should be equivalent to AV
DD
and V
REF
L tied to AGND. The output voltage
is given by:
V
OUT
+ V
REF
H * 2 V
REF
L
The use of the V
REF
L input allows minor adjustments to be made to the offset of the DAC output by applying
a small voltage to the V
REF
L input. The acceptable range is between −100 mV and 100 mV with respect to
AGND. A low output impedance source is needed, so that the accuracy of the DAC over its operating range is
not affected.