Datasheet

LM87

SNAS034J –APRIL 2000–REVISED MARCH 2013

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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 sensor and the LM87 can cause temperature

conversion errors. The following guidelines should be followed:

1. Place a 0.1 μF power supply bypass capacitor as close as possible to the V

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.

2. 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.

3. 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.

4. Avoid routing diode traces in close proximity to power supply switching or filtering inductors.

5. 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.

6. 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.

7. 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.

Figure 12. Recommended Diode Trace Layout

Noise on the digital lines, overshoot greater than V

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 communication 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 placing 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

frequency noise seen at the SMBCLK input and increases the reliability of communications.

Product Folder Links: LM87