ltc2631-z Linear Technology Corporation, ltc2631-z Datasheet - Page 24

• Manufacturer Part Number: ltc2631-z
• Description: Single 12-/10-/8-bit I 2c Vout Dacs With Bidirectional Reference
• Manufacturer: Linear Technology Corporation

LTC2631
OPERATION
The DAC can be put into power-down mode by using
command 0100. The supply current is reduced to 1.8μA
maximum (C and I grades) and the REF pin becomes high
impedance (typically > 1GΩ).
Normal operation resumes after executing any command
that includes a DAC update, as shown in Table 3. The DAC
is powered up and its voltage output is updated. Normal
settling is delayed while the bias, reference, and ampli-
fi er circuits are re-enabled. When the REF pin output is
bypassed to GND with 1nF or less, the power up delay
time is 20μs for settling to 12 bits. This delay increases
to 200μs for 0.33μF , and 10ms for 10μF .
Voltage Output
The LTC2631’s integrated rail-to-rail amplifi er 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 amplifi er’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 amplifi er’s DC output
impedance is 0.1Ω when driving a load well away from
the rails.
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 Charac-
teristics section.
The amplifi er is stable driving capacitive loads of up to
500pF .
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 the lowest codes, as shown in Figure 5b.
24
Similarly, limiting can occur near full scale when the REF
pin is tied to V
(FSE) is positive, the output for the highest codes limits
at V
occur if V
Offset and linearity are defi ned and tested over the region
of the DAC transfer function where no output limiting can
occur.
Board Layout
The PC board should have separate areas for the analog and
digital sections of the circuit. A single, solid ground plane
should be used, with analog and digital signals carefully
routed over separate areas of the plane. This keeps digital
signals away from sensitive analog signals and minimizes
the interaction between digital ground currents and the
analog section of the ground plane. The resistance from
the LTC2631 GND pin to the ground plane should be as
low as possible. Resistance here will add directly to the
effective DC output impedance of the device (typically
0.1Ω). Note that the LTC2631 is no more susceptible to
this effect than any other parts of this type; on the con-
trary, it allows layout-based performance improvements
to shine rather than limiting attainable performance with
excessive internal resistance.
Another technique for minimizing errors is to use a sepa-
rate power ground return trace on another board layer.
The trace should run between the point where the power
supply is connected to the board and the DAC ground pin.
Thus the DAC ground pin becomes the common point for
analog ground, digital ground, and power ground. When
the LTC2631 is sinking large currents, this current fl ows
out the ground pin and directly to the power ground trace
without affecting the analog ground plane voltage.
It is sometimes necessary to interrupt the ground plane
to confi ne digital ground currents to the digital portion of
the plane. When doing this, make the gap in the plane only
as long as it needs to be to serve its purpose and ensure
that no traces cross over the gap.
CC
, as shown in Figure 5c. No full-scale limiting can
REF
is less than V
CC
. If V
REF
= V
CC
CC
and the DAC full-scale error
– FSE.
2631fa