LTC2625 LINER [Linear Technology], LTC2625 Datasheet - Page 13

• Manufacturer Part Number: LTC2625
• Description: Octal 16-/14-/12-Bit Rail-to-Rail DACs in 16-Lead SSOP
• Manufacturer: LINER [Linear Technology]

OPERATIO
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 0.020Ω 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
30Ω typical channel resistance of the output devices;
e.g., when sinking 1mA, the minimum output voltage =
30Ω • 1mA = 30mV. See the graph Headroom at Rails vs
Output Current in the Typical Performance Characteristics
section.
The amplifiers are stable driving capacitive loads of up
to 1000pF.
Board Layout
The excellent load regulation and DC crosstalk perfor-
mance of these devices is achieved in part by keeping
“signal” and “power” grounds separated internally and by
reducing shared internal resistance to just 0.005Ω.
The GND pin functions both as the node to which the
reference and output voltages are referred and as a return
path for power currents in the device. Because of this,
careful thought should be given to the grounding scheme
and board layout in order to ensure rated performance.
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.
U
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 continu-
ous and uninterrupted plane, except for necessary lead
pads and vias, with signal traces on another layer.
The GND pin of the part should be connected to analog
ground. Resistance from the GND pin to system star
ground should be as low as possible. Resistance here will
add directly to the effective DC output impedance of the
device (typically 0.020Ω), and will degrade DC crosstalk.
Note that the LTC2605/LTC2615/LTC2625 are no more
susceptible to these effects than other parts of their type;
on the contrary, they allow layout-based performance
improvements to shine rather than limiting attainable
performance with excessive internal resistance.
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 4b. Similarly, limiting can occur near full scale
when the REF pin is tied to V
full-scale error (FSE) is positive, the output for the highest
codes limits at V
limiting can occur if V
Offset and linearity are defined and tested over the region
of the DAC transfer function where no output limiting
can occur.
LTC2605/LTC2615/LTC2625
CC
as shown in Figure 4c. No full-scale
REF
is less than V
CC
. If V
REF
= V
CC
CC
– FSE.
and the DAC
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