Rainbow Electronics MAX6649 User Manual

Page 13

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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
carefully 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-DXN traces (Figure 4).

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 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 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.

7) Add a 200

Ω resistor in series with V

for best noise

filtering (see Typical Operating Circuit).

8) 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 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
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 1

Ω of series resistance, the error is approxi-

mately 0.5°C.

Thermal Mass and Self-Heating

When sensing local temperature, this device is intended
to measure the temperature of the PC board to which it
is soldered. The leads provide a good thermal path
between the PC board traces and the die. Thermal con-
ductivity between the die and the ambient air is poor by
comparison, making air temperature measurements
impractical. Because the thermal mass of the PC board
is far greater than that of the MAX6649, the device
follows temperature changes on the PC board with little
or no perceivable delay.
When measuring the temperature of a CPU or other IC
with an on-chip sense junction, thermal mass has virtu-
ally 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 SOT23s, yield the best
thermal response times. Take care to account for ther-
mal gradients between the heat source and the sensor,
and ensure that 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. For the local diode, the
worst-case error occurs when autoconverting at the
fastest rate and simultaneously sinking maximum cur-
rent at the ALERT output. For example, with V

5.0V, at a 4Hz conversion rate and with ALERT sinking
1mA, the typical power dissipation is:

5.0V x 500µA + 0.4V x 1mA = 2.9mW

J-A

for the 8-pin µMAX package is about +221°C/W,

so assuming no copper PC board heat sinking, the
resulting temperature rise is:

∆T = 2.9mW x +221°C/W = +0.6409°C

Even under nearly worst-case conditions, it is difficult to
introduce a significant self-heating error.

MAX6649

+145°C Precision SMBus-Compatible Remote/

Local Sensor with Overtemperature Alarms

______________________________________________________________________________________

MINIMUM

10MILS

GND

DXN

DXP

GND

Figure 4. Recommended DXP-DXN PC Traces

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