MAX6692MSA(2002) Просмотр технического описания (PDF) - Maxim Integrated

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MAX6692MSA
(Rev.:2002)

Precision SMBus-Compatible Remote/Local Temperature Sensors with Overtemperature Alarms

Maxim Integrated 

Precision SMBus-Compatible Remote/Local

Temperature Sensors with Overtemperature Alarms

10MILS

10MILS

GND

10MILS

DXP

MINIMUM

DXN

10MILS

GND

Figure 4. Recommended DXP-DXN PC Traces

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 larger values 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 MAX6648/MAX6692 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

carefully 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 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 200Ω resistor in series with VCC 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 1Ω of series resistance, the error is approxi-

mately 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 ther-

mal 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 MAX6648/

MAX6692, the devices follow 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.

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