Datasheet

LM95234

SNIS136D –AUGUST 2006–REVISED MARCH 2013

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PCB LAYOUT FOR MINIMIZING NOISE

Figure 24. 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 LM95234 can cause temperature

conversion errors. Keep in mind that the signal level the LM95234 is trying to measure is in microvolts. The

following guidelines should be followed:

1. V

should be bypassed with a 0.1 µF capacitor in parallel with 100 pF. The 100 pF capacitor should be

placed as close as possible to the power supply pin. A bulk capacitance of approximately 10 µF needs to be

in the near vicinity of the LM95234.

2. A 100 pF diode bypass capacitor is recommended to filter high frequency noise but may not be necessary.

Make sure the traces to the 100 pF capacitor are matched. Place the filter capacitors close to the LM95234

pins.

3. Ideally, the LM95234 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 1Ω can cause as much as 0.62°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 2 cm 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 LM95234's GND pin is as close as possible to the Processors GND

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 400 mVp-p (typical hysteresis) and undershoot less than 500 mV

below GND, may prevent successful SMBus communication with the LM95234. 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 LM95234'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.

Product Folder Links: LM95234