9 Layout

9.1 PCB Layout for Minimizing Noise

Figure 9-1. Ideal Diode Trace Layout

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 LM64 can cause temperature

conversion errors. Keep in mind that the signal level the LM64 is trying to measure is in microvolts. The following

guidelines should be followed:

1. Place a 0.1 µF power supply bypass capacitor as close as possible to the VDD pin and the recommended

2.2 nF capacitor as close as possible to the LM64's D+ and D− pins. Make sure the traces to the 2.2 nF

capacitor are matched.

2. Ideally, the LM64 should be placed within 10 cm of the Processor diode pins with the traces being as

straight, short and identical as possible. Trace resistance of 1 Ω can cause as much as 1°C of error. This

error can be compensated by using the Remote Temperature Offset Registers, since the value placed in

these registers will automatically be subtracted from or added to the remote temperature reading.

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 LM64's GND pin is as close as possible to the Processor's GND associated

with the sense diode.

8. Leakage current between D+ and GND should be kept to a minimum. One nano-ampere of leakage can

cause as much as 1°C of error in the diode temperature reading. Keeping the printed circuit board as clean

as possible will minimize leakage current.

Noise coupling into the digital lines greater than 400 mVp-p (typical hysteresis) and undershoot less than 500

mV below GND, may prevent successful SMBus communication with the LM64. SMBus no acknowledge is the

most common symptom, causing unnecessary traffic on the bus. Although the SMBus maximum frequency of

communication is rather low (100 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. An RC lowpass filter with a 3 dB

corner frequency of about 40 MHz is included on the LM64's SMBCLK input. Additional resistance can be added

in series with the SMBData and SMBCLK lines to further help filter noise and ringing. Minimize noise coupling by

keeping digital traces out of switching power supply areas as well as ensuring that digital lines containing high

speed data communications cross at right angles to the SMBData and SMBCLK lines.

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LM64

SNAS207B – MAY 2004 – REVISED JANUARY 2024

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