LM40 datasheet(26/27 Pages) NSC

26 / 27 page

3.0 Application Hints (Continued)

Ideally, the LM40 should be placed within 10cm of the

Processor diode pins with the traces being as straight,

short and identical as possible. Trace resistance of 0.7

Ω

can cause as much as 1˚C of error. This error can be

compensated for by adding or subtracting an offset to

the remote temperature reading(s).

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.

Avoid routing diode traces in close proximity to power

supply switching or filtering inductors.

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.

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.

The ideal place to connect the LM40’s GND pin is as

close as possible to the Processors GND associated

with the sense diode.

Leakage current between D+ and GND should be kept

to a minimum. Seventeen nano-amperes of leakage can

cause as much as 0.2˚C of error in the diode tempera-

ture reading (see curve in Section Typical Performance

Characteristics). Keeping the printed circuit board as

clean as possible will minimize leakage current.

The SensorPath Bus is less sensitive to noise than its pre-

decessor the SMBus due to the inherent filtering present in

the pulse-width encoding of the data. Care still needs to be

taken such that induced noise is analyzed and minimized.

SensorPath Bus corrupt data is the most common symptom

for noise coupled in SWD. A no-ACK is the symptom for

noise coupled into the Device Number Select pin (ADD). An

RC lowpass filter as well as a debouncing circuit are in-

cluded in the LM40 that filter noise spikes less than 2.5 µsec

in duration on the SWD signal.

LM40 Arkusz danych(PDF) 26 Page - National Semiconductor (TI)