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LTC2641/LTC2642

26412f

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 IDD 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 ac-

curacy. The 6nA leakage current into VOUT needed to

generate 1LSB offset error corresponds to 833M

Ωleakage

resistance from a 5V supply.

The VOUT 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 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, VREF GND and the DAC

VOUT 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 VREF input source connects to the ground

plane should be as low as possible. Excessive resistance

here will be multiplied by the code dependent IREF current

to produce an INL error similar to the error produced by

VREF source resistance.

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 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

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 ﬂow,

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 signiﬁcant 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 ﬂow the long way

around the gap, and then are typically channeled directly

into the most sensitive area of the analog GND plane.

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