Datasheet

LTC2641/LTC2642
18
26412fd
For more information www.linear.com/LTC2641
will benefit from even higher bypass capacitance, as long
as the external reference remains stable with the added
capacitive loading.
Digital Inputs and Interface Logic
All of the digital inputs include Schmitt-trigger buffers to
accept slow transition interfaces. This means that opto
-
cuplers can interface directly to the LTC2641/LTC2642
without additional external logic. Digital input hysteresis
is typically 150mV.
The digital inputs are compatible with TTL/CMOS-logic
levels. However, rail-to-rail (CMOS) logic swings are
preferred, because operating the logic inputs away from
the supply rails generates additional I
DD
and GND current,
(see Typical Performance Characteristic graph Supply
Current vs Logic Input Voltage).
Digital feedthrough is only 0.2nV•s typical, but it is always
preferred to keep all logic inputs static except when loading
a new code into the DAC.
Board Layout for Precision
Even a small amount of board leakage can degrade
accuracy. The 6nA leakage current into V
OUT
needed to
generate 1LSB offset error corresponds to 833MΩ leakage
resistance from a 5V supply.
The V
OUT
node is relatively sensitive to capacitive noise
coupling, so minimum trace length, appropriate shielding
and clean board layout are imperative here.
Temperature differences at the DAC, op
amp or reference
pins can easily generate tens of microvolts of thermocou-
ple voltages. Analog signal traces should be short, close
together
and away from heat dissipating components. Air
currents across the board can also generate thermocouples.
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.
A “star ground” area should be established by attaching
the LTC2641/LTC2642 GND pin, V
REF
GND and the DAC
V
OUT
GND reference terminal to the same area on the
GND plane. Care should be taken to ensure that no large
GND return current paths flow through the “star GND”
area. In particular, the resistance from the LTC2641 GND
pin to the point where the V
REF
input source connects to
the ground plane should be as low as possible. Excessive
resistance here will be multiplied by the code dependent
I
REF
current to produce an INL error similar to the error
produced by V
REF
source resistance. For the LTC2641 in
the S8 package both GND pins, Pin 2 and Pin 7 should
be tied to the same GND plane.
Sources of ground return current in the analog area include
op amp power supply bypass capacitors and the GND
connection for single supply amps. A useful technique
for minimizing errors is to use a separate board layer for
power ground return connections, and reserve one ground
plane layer for low current “signal” GND connections.
The “signal”, or “star” GND plane must connected to the
“power” GND plane at a single point, which should be
located near the LTC2641/LTC2642 GND pin.
If separate analog and digital ground areas exist it is neces
-
sary to connect them at a single location, which should be
fairly
close to the DAC for digital signal integrity. In some
systems, large GND return currents can flow between the
digital and analog GNDs, especially if different PC boards
are involved. In such cases the digital and analog ground
connection point should not be made right at the “star”
GND area, so the highly sensitive analog signals are not
corrupted. If forced to choose, always place
analog ground
quality
ahead of digital signal ground. (A few mV of noise
APPLICATIONS INFORMATION
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