range. We have observed variations in remote tempera-

ture readings of less than

±2°C with a variety of dis-

crete transistors. Still, it is good design practice to

verify good consistency of temperature readings with

several discrete transistors from any manufacturer

under consideration.

ADC Noise Filtering

The integrating ADC used has good noise rejection for

low-frequency signals such as 60Hz/120Hz power-sup-

ply hum. In noisy environments, high-frequency noise

reduction is needed for high-accuracy remote mea-

surements. The noise can be reduced with careful PCB

layout and proper external noise filtering.

High-frequency EMI is best filtered at DXP with an

external 2200pF capacitor. Larger capacitor values can

be used for added filtering, but do not exceed 3300pF

because excessive capacitance can introduce errors

due to the rise time of the switched current source.

Nearly all noise sources tested cause the temperature

conversion results to be higher than the actual temper-

ature, typically by +1°C to +10°C, depending on the

frequency and amplitude (see the

Typical Operating

Characteristics).

PCB Layout

Follow these guidelines to reduce the measurement

error of the temperature sensors:

1) Connect the thermal-sense diode to the MAX6642

using two traces—one between DXP and the

anode, the other between the MAX6642’s GND and

the cathode. Do not connect the cathode to GND at

the sense diode.

2) Place the MAX6642 as close as is practical to the

remote thermal 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.

3) Do not route the thermal diode lines next to the

deflection coils of a CRT. Also, do not route the

traces across fast digital signals, which can easily

introduce a 30°C error, even with good filtering.

4) Route the thermal diode traces in parallel and in

close proximity to each other, away from any higher

voltage traces, such as +12VDC. Leakage currents

from PCB contamination must be dealt with careful-

ly since a 20M

Ω 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 trace (Figure 4).

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

sible to minimize copper/solder thermocouple

effects.

6) When introducing a thermocouple, make sure that

both the thermal diode paths have matching ther-

mocouples. A copper-solder thermocouple exhibits

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

DXP to cause a +1°C measurement error. Adding a

few thermocouples causes a negligible error.

7) Use wide traces. Narrow traces are more inductive

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

widths and spacing recommended in Figure 4 are

not absolutely necessary, as they offer only a minor

improvement in leakage and noise over narrow

traces. Use wider traces when practical.

8) Add a 47

filtering (see the

Typical Operating Circuit).

9) Copper cannot be used as an EMI shield; only fer-

rous materials such as steel work well. Placing a

copper ground plane between the DXP-DXN traces

and traces carrying high-frequency noise signals

does not help reduce EMI.

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 GND and

the shield to GND. Leave the shield unconnected at the

remote diode.

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.

±1°C, SMBus-Compatible Remote/Local

Temperature Sensor with Overtemperature Alarm

______________________________________________________________________________________

11

MINIMUM

10 mils

10 mils

10 mils

10 mils

THERMAL DIODE CATHODE/GND

THERMAL DIODE ANODE/DXP

GND

GND

Figure 4. Recommended DXP PC Traces