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MAX1617A

Remote/Local Temperature Sensor with SMBus Serial Interface

Maxim Integrated 

MAX1617A

Remote/Local Temperature Sensor

with SMBus Serial Interface

PC Board Layout

1) Place the MAX1617A as close as practical to the

remote diode. In a noisy environment, such as a

computer motherboard, this distance can be 4 in. to 8

in. (typical) or more as long as the worst noise sourc-

es (such as CRTs, clock generators, memory buses,

and ISA/PCI buses) are avoided.

2) Do not route the DXP–DXN lines next to the deflection

coils of a CRT. Also, do not route the traces across a

fast memory bus, which can easily introduce +30°C

error, even with good filtering. Otherwise, most noise

sources are fairly benign.

3) Route the DXP and DXN traces in parallel and in

close proximity to each other, away from any high-

voltage traces such as +12VDC. Leakage currents

from PC board contamination must be dealt with care-

fully, since a 20MΩ leakage path from DXP to ground

causes about +1°C error.

4) Connect guard traces to GND on either side of the

DXP–DXN traces (Figure 2). With guard traces in

place, routing near high-voltage traces is no longer

an issue.

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 DXP and the DXN paths have matching ther-

mocouples. In general, PC board-induced thermo-

couples are not a serious problem. A copper-solder

thermocouple exhibits 3μV/°C, and it takes about

200μV of voltage error at DXP–DXN to cause a +1°C

measurement error. So, most parasitic thermocouple

errors are swamped out.

7) Use wide traces. Narrow ones are more inductive

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

and spacings recommended in Figure 2 aren’t ab-

solutely necessary (as they offer only a minor im-

provement in leakage and noise), but try to use them

where practical.

8) Keep in mind that copper can’t be used as an EMI

shield, and only ferrous 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.

PC Board Layout Checklist

●● Place the MAX1617A close to a remote diode.

●● Keep traces away from high voltages (+12V bus).

●● Keep traces away from fast data buses and CRTs.

●● Use recommended trace widths and spacings.

●● Place a ground plane under the traces.

10 MILS

10 MILS

GND

10 MILS

DXP

MINIMUM

DXN

10 MILS

GND

Figure 2. Recommended DXP/DXN PC Traces

●● Use guard traces flanking DXP and DXN and connect-

ing to GND.

●● Place the noise filter and the 0.1μF VCC bypass

capacitors close to the MAX1617A.

Twisted Pair and Shielded Cables

For remote-sensor distances longer than 8 in., or in partic-

ularly noisy environments, a twisted pair is recommended.

Its practical length is 6 feet to 12 feet (typical) before

noise becomes a problem, as tested in a noisy electron-

ics laboratory. For longer distances, the best solution is a

shielded twisted pair like that used for audio microphones.

For example, the Belden 8451 works well for distances up

to 100 feet in a noisy environment. Connect the twisted

pair to DXP and DXN and the shield to GND, and leave

the shield’s remote end unterminated.

Excess capacitance at DX_ limits practical remote sensor

distances (see Typical Operating Characteristics). For

very long cable runs, the cable’s parasitic capacitance

often provides noise filtering, so the 2200pF capacitor can

often be removed or reduced in value.

Cable resistance also affects remote-sensor accuracy;

1Ω series resistance introduces about +1/2°C error.

Low-Power Standby Mode

Standby mode disables the ADC and reduces the sup-

ply- current drain to less than 10μA. Enter standby mode

by forcing the STBY pin low or via the RUN/STOP bit in

the configuration byte register. Hardware and software

standby modes behave almost identically: all data is

retained in memory, and the SMB interface is alive and

listening for reads and writes. The only difference is that

in hardware standby mode, the one-shot command does

not initiate a conversion.

Standby mode is not a shutdown mode. With activity on

the SMBus, extra supply current is drawn (see Typical

Operating Characteristics). In software standby mode,

the MAX1617A can be forced to perform A/D conversions

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