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SNIS143E – AUGUST 2006 – REVISED MARCH 2013
PCB LAYOUT FOR MINIMIZING NOISE
Figure 13. 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 LM95241 can cause temperature
conversion errors. Keep in mind that the signal level the LM95241 is trying to measure is in microvolts. The
following guidelines should be followed:
1. V DD should be bypassed with a 0.1μF capacitor in parallel with 100pF. The 100pF capacitor should be placed
as close as possible to the power supply pin. A bulk capacitance of approximately 10μF needs to be in the
near vicinity of the LM95241.
2. A 100pF diode bypass capacitor is recommended to filter high frequency noise but may not be necessary.
Make sure the traces to the 100pF capacitor are matched. Place the filter capacitors close to the LM95241
pins.
3. Ideally, the LM95241 should be placed within 10cm 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 simple software offset compensation.
4. 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.
5. Avoid routing diode traces in close proximity to power supply switching or filtering inductors.
6. Avoid running diode traces close to or parallel to high speed digital and bus lines. Diode traces should be
kept at least 2cm apart from the high speed digital traces.
7. 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.
8. The ideal place to connect the LM95241's GND pin is as close as possible to the Processor's GND that is
associated with the sense diode.
9. Leakage current between D+ and GND and between D+ and D ? should be kept to a minimum. Thirteen
nano-amperes of leakage can cause as much as 0.2°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 400mVp-p (typical hysteresis) and undershoot less than 500mV
below GND, may prevent successful SMBus communication with the LM95241. SMBus no acknowledge is the
most common symptom, causing unnecessary traffic on the bus. Although the SMBus maximum frequency of
communication is rather low (100kHz 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 3db
corner frequency of about 40MHz is included on the LM95241's SMBCLK input. Additional resistance can be
added in series with the SMBDAT 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 SMBDAT and SMBCLK lines.
22
Product Folder Links: LM95241
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