Datasheet

LTC2655
23
2655f
Board Layout
The excellent load regulation and DC crosstalk performance
of these devices is achieved in part by keeping signal and
power grounds separate.
The PC board should have separate areas for the analog
and digital sections of the circuit. This keeps digital signals
away from sensitive analog signals and facilitates the use
of separate digital and analog ground planes which have
minimal capacitive and resistive interaction with each
other.
Digital and analog ground planes should be joined at only
one point, establishing a system star ground as close to
the device’s ground pin as possible. Ideally, the analog
ground plane should be located on the component side of
the board, and should be allowed to run under the part to
shield it from noise. Analog ground should be a continuous
and uninterrupted plane, except for necessary lead pads
and vias, with signal traces on another layer.
The GND pin functions as a return path for power supply
currents in the device and should be connected to analog
ground. The REFLO pin should be connected to system
star ground. Resistance from the REFLO pin to system
star ground should be as low as possible.
Rail-to-Rail Output Considerations
In any rail-to-rail voltage output device, the output is limited
to voltages within the supply range.
Since the analog outputs of the device cannot go below
ground, they may limit for the lowest codes as shown in
Figure 3b. Similarly, limiting can occur in external refer-
ence mode near full scale when the REFIN/OUT pin is at
V
CC
/2. If V
REFIN/OUT
= V
CC
/2 and the DAC full-scale error
(FSE) is positive, the output for the highest codes limits
at V
CC
as shown in Figure 3c. No full-scale limiting can
occur if V
REFIN/OUT
≤ (V
CC
– F
SE
)/2.
Offset and linearity are defi ned and tested over the region
of the DAC transfer function where no output limiting can
occur.
OPERATION