where temperature is measured in Kelvin and

As an example, assume you want to use the MAX6642

with a CPU that has an ideality factor of 1.002. If the

diode has no series resistance, the measured data is

related to the real temperature as follows:

For a real temperature of +85°C (358.15K), the mea-

sured temperature is +82.91°C (356.02K), an error of

-2.13°C.

Effect of Series Resistance

Series resistance in a sense diode contributes addition-

al errors. For nominal diode currents of 10µA and

100µA, the change in the measured voltage due to

series resistance is:

Since +1°C corresponds to 198.6µV, series resistance

contributes a temperature offset of:

Assume that the diode being measured has a series

resistance of 3

Ω. The series resistance contributes an

offset of:

The effects of the ideality factor and series resistance

are additive. If the diode has an ideality factor of 1.002

and series resistance of 3

Ω, the total offset can be cal-

culated by adding error due to series resistance with

error due to ideality factor:

1.36°C - 2.13°C = -0.77°C

for a diode temperature of +85°C.

In this example, the effect of the series resistance and

the ideality factor partially cancel each other.

Discrete Remote Diodes

When the remote-sensing diode is a discrete transistor,

connect its collector and base together. Table 7 lists

examples of discrete transistors that are appropriate for

use with the MAX6642.

The transistor must be a small-signal type with a rela-

tively high forward voltage; otherwise, the A/D input

voltage range can be violated. The forward voltage at

the highest expected temperature must be greater than

0.25V at 10µA, and at the lowest expected tempera-

ture, the forward voltage must be less than 0.95V at

100µA. Large power transistors must not be used. Also,

ensure that the base resistance is less than 100

Ω. Tight

specifications for forward current gain (50 < ß <150, for

example) indicate that the manufacturer has good

process controls and that the devices have consistent

Manufacturers of discrete transistors do not normally

specify or guarantee ideality factor. This is normally not

a problem since good-quality discrete transistors tend

to have ideality factors that fall within a relatively narrow

range. We have observed variations in remote tempera-

ture readings of less than

±2°C with a variety of dis-

crete transistors. Still, it is good design practice to

verify good consistency of temperature readings with

several discrete transistors from any manufacturer

under consideration.

ADC Noise Filtering

The integrating ADC used has good noise rejection for

low-frequency signals such as 60Hz/120Hz power-sup-

ply hum. In noisy environments, high-frequency noise

reduction is needed for high-accuracy remote mea-

surements. The noise can be reduced with careful PCB

layout and proper external noise filtering.

High-frequency EMI is best filtered at DXP with an

external 2200pF capacitor. Larger capacitor values can

be used for added filtering, but do not exceed 3300pF

because excessive capacitance can introduce errors

3

0 453

1 36

Ω×

°

Ω

=+

°

..

C

C

90

198 6

0 453

µ

Ω

µ

°

=

°

Ω

V

V

C

C

.

.

TT

n

n

T

T

ACTUAL

M

NOMINAL

M

M

=

⎛

⎝⎜

⎞

⎠⎟

=

⎛

⎝⎜

⎞

⎠⎟

=

1

1 008

1 002

1 00599

.

.

( .

)

±1°C, SMBus-Compatible Remote/Local

Temperature Sensor with Overtemperature Alarm

10

______________________________________________________________________________________

MANUFACTURER

MODEL NO.

Central Semiconductor (USA)

CMPT3906

Rohm Semiconductor (USA)

SST3906

Samsung (Korea)

KST3906-TF

Siemens (Germany)

SMBT3906

Zetex (England)

FMMT3906CT-ND

Table 7. Remote-Sensor Transistor

Manufacturers

Note: Discrete transistors must be diode connected (base short-

ed to collector).