LM87 datasheet(18/32 Pages) NSC

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Functional Description (Continued)

η from part to part. As an example, assume a temperature

sensor has an accuracy specification of ±3˚C at room tem-

perature of 25˚C and the process used to manufacture the

diode has a non-ideality variation of ±1%. The resulting

accuracy of the temperature sensor at room temperature will

be:

ACC = ± 3˚C+(±1% of 298˚K) = ±6˚C

The additional inaccuracy in the temperature measurement

caused by

η, can be eliminated if each temperature sensor is

calibrated with the remote diode that it will be paired with.

8.5 PCB Layout Recommendations for Minimizing

Noise

In a noisy environment, such as a processor mother board,

layout considerations are very critical. Noise induced on

traces running between the remote temperature diode sen-

sor and the LM87 can cause temperature conversion errors.

The following guidelines should be followed:

Place a 0.1 µF power supply bypass capacitor as close

as possible to the V

CC pin and the recommended 2.2 nF

capacitor as close as possible to the D+ and D− pins.

Make sure the traces to the 2.2 nF capacitor are

matched.

Ideally, the LM87 should be placed within 10 cm of the

Processor diode pins with the traces being as straight,

short and identical as possible.

Diode traces should be surrounded by a GND guard ring

to either side, above and below if possible. This GND

guard should not be between the D+ and D− lines. In the

event that noise does couple to the diode lines it would

be ideal if it is coupled common mode. That is equally to

the D+ and D− lines.

Avoid routing diode traces in close proximity to power

supply switching or filtering inductors.

Avoid running diode traces close to or parallel to high

speed digital and bus lines. Diode traces should be kept

at least 2 cm. apart from the high speed digital traces.

If it is necessary to cross high speed digital traces, the

diode traces and the high speed digital traces should

cross at a 90 degree angle.

The ideal place to connect the LM87’s GND pin is as

close as possible to the Processors GND associated

with the sense diode. For the Pentium II this would be

pin A14.

Noise on the digital lines, overshoot greater than V

CC and

undershoot less than GND, may prevent successful SMBus

communication with the LM87. SMBus no acknowledge is

the most common symptom, causing unnecessary traffic on

the bus. Although, the SMBus maximum frequency of com-

munication is rather low (400 kHz max) care still needs to be

taken to ensure proper termination within a system with

multiple parts on the bus and long printed circuit board

traces. A lowpass filter, in series with the SMBCLK and

SMBData, has been added internally to the LM87 for noise

immunity. The lowpass filter has a typical cutoff frequency of

20MHz. Additional noise immunity can be achieved by plac-

ing a resistor (4.7k to 5.1k Ohms) in series with the SMBCLK

input as close to the LM87 as possible. This resistance, in

conjunction with the IC input capacitance, reduces high fre-

quency noise seen at the SMBCLK input and increases the

reliability of communications.

10099535

FIGURE 9. Recommended Diode Trace Layout

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