MAX6642
±1°C, SMBus-Compatible Remote/
Local Temperature Sensor with
Overtemperature Alarm
Maxim Integrated | 11www.maximintegrated.com
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 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 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 47Ω resistor in series with V
CC
for best noise
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.
