LTC2642-16 Linear Technology Corporation, LTC2642-16 Datasheet - Page 17

• Manufacturer Part Number: LTC2642-16
• Description: Ltc2642-16 - 16-bit Vout Dac In 3mm X 3mm Dfn
• Manufacturer: Linear Technology Corporation

Digital Inputs and Interface Logic
All of the digital inputs include Schmitt-trigger buffers
to accept slow transition interfaces. This means that op-
tocuplers 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
(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 ac-
curacy. The 6nA leakage current into V
generate 1LSB offset error corresponds to 833MΩ leakage
resistance from a 5V supply.
The V
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 thermo-
couple voltages. Analog signal traces should be short,
close together and away from heat dissipating compo-
nents. 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
V
plane. Care should be taken to ensure that no large GND
APPLICATIONS INFORMATION
OUT
GND reference terminal to the same area on the GND
OUT
node is relatively sensitive to capacitive noise
REF
DD
GND and the DAC
and GND current,
OUT
needed to
return current paths fl ow through the “star GND” area. In
particular, the resistance from the LTC2641 GND pin to the
point where the V
plane should be as low as possible. Excessive resistance
here will be multiplied by the code dependent I
to produce an INL error similar to the error produced by
V
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 connec-
tions. 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 fl ow 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
on the digital inputs is imperceptible, thanks to the digital
input hysteresis)
Just by maintaining separate areas on the GND plane
where analog and digital return currents naturally fl ow,
good results are generally achieved. Only after this has
been done, it is sometimes useful to interrupt the ground
plane with strategically placed “slots”, to prevent the digital
ground currents from fringing into the analog portion of
the plane. When doing this, the gap in the plane should be
only as long as it needs to be to serve its purpose.
Caution: if a GND plane gap is improperly placed, so that
it interrupts a signifi cant GND return path, or if a signal
traces crosses over the gap, then adding the gap may
greatly degrade performance! In this case, the GND and
signal return currents are forced to fl ow the long way
around the gap, and then are typically channeled directly
into the most sensitive area of the analog GND plane.
REF
source resistance.
REF
LTC2641/LTC2642
input source connects to the ground
REF
17
current
26412f