LM99CIMMX NSC [National Semiconductor], LM99CIMMX Datasheet - Page 19

• Manufacturer Part Number: LM99CIMMX
• Manufacturer: NSC [National Semiconductor]

3.0 Application Hints
3.2 PCB LAYOUT 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 sen-
sor and the LM99 can cause temperature conversion errors.
Keep in mind that the signal level the LM99 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
2. Ideally, the LM99 should be placed within 10 cm of the
3. Diode traces should be surrounded by a GND guard ring
as possible to the V
capacitor as close as possible to the LM99’s D+ and D−
pins. Make sure the traces to the 2.2 nF capacitor are
matched.
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.
to either side, above and below if possible. This GND
FIGURE 11. Ideal Diode Trace Layout
DD
pin and the recommended 2.2 nF
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4. Avoid routing diode traces in close proximity to power
5. Avoid running diode traces close to or parallel to high
6. If it is necessary to cross high speed digital traces, the
7. The ideal place to connect the LM99’s GND pin is as
8. Leakage current between D+ and GND should be kept
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 LM99. 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 LM99’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 cou-
pling 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.
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.
supply switching or filtering inductors.
speed digital and bus lines. Diode traces should be kept
at least 2 cm apart from the high speed digital traces.
diode traces and the high speed digital traces should
cross at a 90 degree angle.
close as possible to the Processors GND associated
with the sense diode.
to a minimum. One nano-ampere of leakage can cause
as much as 1˚C of error in the diode temperature read-
ing. Keeping the printed circuit board as clean as pos-
sible will minimize leakage current.
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