ply hum. In noisy environments, high-frequency noise

reduction is needed for high-accuracy remote mea-

surements. The noise can be reduced with careful PC

board layout and proper external noise filtering.

High-frequency EMI is best filtered at DXP and DXN

with an external 2200pF capacitor. Larger capacitor

values can be used for added filtering, but do not

exceed 3300pF because it can introduce errors due to

the rise time of the switched current source.

PC Board Layout

Follow these guidelines to reduce the measurement

error of the temperature sensors:

1) Place the MAX6680/MAX6681 as close as is practi-

cal to the remote diode. In noisy environments, such

as a computer motherboard, this distance can be

4in to 8in (typ). This length can be increased if the

worst noise sources are avoided. Noise sources

include CRTs, clock generators, memory buses, and

ISA/PCI buses.

2) Do not route the DXP-DXN lines next to the deflec-

tion coils of a CRT. Also, do not route the traces

across fast digital signals, which can easily intro-

duce 30°C error, even with good filtering.

3) Route the DXP and DXN traces in parallel and in

close proximity to each other, away from any higher

voltage traces, such as 12VDC. Leakage currents

from PC board contamination must be dealt with care-

fully since a 20MΩ leakage path from DXP to ground

causes about 1°C error. If high-voltage traces are

unavoidable, connect guard traces to GND on either

side of the DXP-DXN traces (Figure 2).

4) Route through as few vias and crossunders as pos-

sible to minimize copper/solder thermocouple

effects.

5) When introducing a thermocouple, make sure that

both the DXP and the DXN paths have matching

thermocouples. A copper-solder thermocouple

exhibits 3µV/°C, and it takes about 200µV of voltage

error at DXP-DXN to cause a 1°C measurement

error. Adding a few thermocouples causes a negligi-

ble error.

6) Use wide traces. Narrow traces are more inductive

and tend to pick up radiated noise. The 10mil widths

and spacings that are recommended in Figure 2 are

not absolutely necessary, as they offer only a minor

improvement in leakage and noise over narrow

traces. Use wider traces when practical.

filtering (see the Typical Operating Circuit).

Twisted-Pair and Shielded Cables

Use a twisted-pair cable to connect the remote sensor

for remote-sensor distances longer than 8in or in very

noisy environments. Twisted-pair cable lengths can be

between 6ft and 12ft before noise introduces excessive

errors. For longer distances, the best solution is a

shielded twisted pair like that used for audio micro-

phones. For example, Belden 8451 works well for dis-

tances up to 100ft in a noisy environment. At the

device, connect the twisted pair to DXP and DXN and

the shield to GND. Leave the shield unconnected at the

remote sensor.

For very long cable runs, the cable’s parasitic capaci-

tance often provides noise filtering, so the 2200pF

capacitor can often be removed or reduced in value.

Cable resistance also affects remote-sensor accuracy.

For every 1Ω of series resistance, the error is approxi-

mately 1/2°C error.

Low-Power Standby Mode

Standby mode reduces the supply current to less than

10µA by disabling the ADC. Enter hardware standby by

forcing the

STBY pin low, or enter software standby by

setting the RUN/STOP bit to 1 in the Configuration Byte

register. Hardware and software standbys are very sim-

ilar: all data is retained in memory, and the SMB inter-

face is alive and listening for SMBus commands, but

the SMBus timeout is disabled. The only difference is

that in software standby mode, the One-Shot command

initiates a conversion. With hardware standby, the One-

Shot command is ignored. Activity on the SMBus caus-

es the device to draw extra supply current (see the

Typical Operating Characteristics).

Driving the

STBY pin low overrides any software con-

version command. If a hardware or software standby

command is received while a conversion is in progress,

the conversion cycle is interrupted, and the tempera-

±1°C Fail-Safe Remote/Local Temperature

Sensors with SMBus Interface

8

_______________________________________________________________________________________

MINIMUM

10mils

10mils

10mils

10mils

GND

DXN

DXP

GND

Figure 2. Recommended DXP-DXN PC Traces