Twisted-Pair and Shielded Cables
Use a twisted-pair cable to connect the remote sensor
for remote-sensor distance 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 shield-
ed twisted pair like that used for audio microphones. For
example, Belden 8451 works well for distances 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 capac-
itor can often be removed or reduced in value. Cable
resistance also affects remote-sensor accuracy. For every
1Ω of series resistance, the error is approximately 0.5°C.
Thermal Mass and Self-Heating
When sensing local temperature, these devices are
intended to measure the temperature of the PC board
to which they are soldered. The leads provide a good
thermal path between the PC board traces and the die.
Thermal conductivity between the die and the ambient
air is poor by comparison, making air temperature mea-
surements impractical. Because the thermal mass of
the PC board is far greater than that of the MAX6641,
the devices follow temperature changes on the PC
board with little or no perceivable delay. When measur-
ing the temperature of a CPU or other IC with an on-
chip sense junction, thermal mass has virtually no
effect. The measured temperature of the junction tracks
the actual temperature within a conversion cycle.
When measuring temperature with discrete remote sen-
sors, smaller packages, such as µMAXes, yield the
best thermal response times. Take care to account for
thermal gradients between the heat source and the
sensor, and ensure stray air currents across the sensor
package do not interfere with measurement accuracy.
Self-heating does not significantly affect measurement
accuracy. Remote-sensor self-heating due to the diode
current source is negligible.
MAX6641
SMBus-Compatible Temperature Monitor with
Automatic PWM Fan-Speed Controller
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