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For power-constrained applications, power-down mode
can be used to reduce the supply current whenever the
DAC output is not needed. When in power-down, the buf-
fer amplifier, bias circuit, and reference circuit are dis-
abled and draw essentially zero current. The DAC output
is put into a high-impedance state, and the output pin
is passively pulled to ground through a 200kΩ resistor.
Input and DAC register contents are not disturbed during
The DAC can be put into power-down mode by using
command 0100. The supply current is reduced to 1.8µA
maximum when the DAC is powered down.
Normal operation resumes after executing any command
that includes a DAC update, as shown in Table 1. The DAC
is powered up and its voltage output is updated. Normal
settling is delayed while the bias, reference, and amplifier
circuits are re-enabled. The power-up delay time is 18µs
for settling to 12-bits.
For applications where an accurate external reference is
not available, the LTC2630 has a user-selectable, inte-
grated reference. The LTC2630-LM and LTC2630-LZ pro-
vide a full-scale output of 2.5V. The LTC2630-HM and
LTC2630-HZ provide a full-scale output of 4.096V.
The internal reference can be useful in applications where
the supply voltage is poorly regulated. Internal Reference
mode can be selected by using command 0110, and is
the power-on default.
The DAC can also operate in supply as reference mode
using command 0111. In this mode, V
DAC’s reference voltage and the supply current is reduced.
The LTC2630’s integrated rail-to-rail amplifier has guar-
anteed load regulation when sourcing or sinking up to
10mA at 5V, and 5mA at 3V.
Load regulation is a measure of the amplifier’s ability to
maintain the rated voltage accuracy over a wide range of
load current. The measured change in output voltage per
change in forced load current is expressed in LSB/mA.
DC output impedance is equivalent to load regulation, and
may be derived from it by simply calculating a change in
units from LSB/mA to ohms. The amplifier’s DC output
impedance is 0.1Ω when driving a load well away from
When drawing a load current from either rail, the output
voltage headroom with respect to that rail is limited by
the 50Ω typical channel resistance of the output devices
(e.g., when sinking 1mA, the minimum output voltage
is 50Ω • 1mA, or 50mV). See the graph “Headroom at
Rails vs. Output Current” in the Typical Performance
The amplifier is stable driving capacitive loads of up to
Rail-to-Rail Output Considerations
In any rail-to-rail voltage output device, the output is lim-
ited to voltages within the supply range.
Since the analog output of the DAC cannot go below
ground, it may limit for the lowest codes as shown in
Figure 4b. Similarly, limiting can occur near full scale
when using the supply as reference. If VFS = VCC and the
DAC full-scale error (FSE) is positive, the output for the
highest codes limits at V
, as shown in Figure 4. No
full-scale limiting can occur if VFS is less than VCC–FSE.
Offset and linearity are defined and tested over the region
of the DAC transfer function where no output limiting can