Datasheet

LM95221

SNIS134B –MAY 2004–REVISED MARCH 2013

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PCB LAYOUT FOR MINIMIZING NOISE

Figure 12. 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 LM95221 can cause temperature

conversion errors. Keep in mind that the signal level the LM95221 is trying to measure is in microvolts. The

following guidelines should be followed:

1. V

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 LM95221.

2. A 2.2nF diode bypass capacitor is required to filter high frequency noise. Place the 2.2nF capacitor as close

as possible to the LM95221's D+ and D− pins. Make sure the traces to the 2.2nF capacitor are matched.

3. Ideally, the LM95221 should be placed within 10cm 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 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 LM95221'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 400mVp-p (typical hysteresis) and undershoot less than 500mV

below GND, may prevent successful SMBus communication with the LM95221. 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 LM95221'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: LM95221