LM82

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SNIS113D – JANUARY 2000 – REVISED MARCH 2013

•

q is the electron charge,

•

k is the Boltzmann's constant,

•

N is the current ratio,

•

T is the absolute temperature in °K.

(1)

The temperature sensor then measures

defined universal constants, and N is a parameter controlled by the temperature sensor. The only other

parameter is

and T, the variations in

η cannot be distinguished from variations in temperature. Since the non-ideality factor is

not controlled by the temperature sensor, it will directly add to the inaccuracy of the sensor. For the Pentium II

Intel specifies a ±1% variation in

η from part to part. As an example, assume a temperature sensor has an

accuracy specification of ±3 °C at room temperature of 25 °C and the process used to manufacture the diode

has a non-ideality variation of ±1%. The resulting accuracy of the temperature sensor at room temperature will

be:

(2)

The additional inaccuracy in the temperature measurement caused by

η, can be eliminated if each temperature

sensor is calibrated with the remote diode that it will be paired with.

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 sensor and the LM82 can cause temperature

conversion errors. The following guidelines should be followed:

1. Place a 0.1

nF capacitor as close as possible to the D+ and D

− pins. Make sure the traces to the 2.2nF capacitor are

matched.

2. The recommended 2.2nF diode bypass capacitor actually has a range of 200pF to 3.3nF. The average

temperature accuracy will not degrade. Increasing the capacitance will lower the corner frequency where

differential noise error affects the temperature reading thus producing a reading that is more stable.

Conversely, lowering the capacitance will increase the corner frequency where differential noise error affects

the temperature reading thus producing a reading that is less stable.

3. Ideally, the LM82 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.

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.(See Figure 15)

5. Avoid routing diode traces in close proximity to power supply switching signals 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 LM82's GND pin is as close as possible to the Processors GND associated

with the sense diode.

9. Leakage current between D+ and GND should be kept to a minimum. One nano-ampere of leakage can

cause as much as 1°C of error in the diode temperature reading. Keeping the printed circuit board as clean

as possible will minimize leakage current.

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