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REV. C

–16–

ADT7463

D+

D–

REMOTE

SENSING

TRANSISTOR

N � I

IBIAS

VDD

VOUT+

TO ADC

VOUT–

BIAS

DIODE

LOW-PASS

FILTER

fC = 65kHz

THERMDA

THERMDC

CPU

Figure 14. Signal Conditioning for Remote Diode Temperature Sensors

TEMPERATURE MEASUREMENT SYSTEM

Local Temperature Measurement

The ADT7463 contains an on-chip band gap temperature sensor

whose output is digitized by the on-chip 10-bit ADC. The 8-bit

MSB temperature data is stored in the local temperature register

(Address 26h). As both positive and negative temperatures can be

measured, the temperature data is stored in twos complement

format, as shown in Table III. Theoretically, the temperature sensor

and ADC can measure temperatures from –128�C to +127�C

with a resolution of 0.25�C. However, this exceeds the operating

temperature range of the device, so local temperature measure-

ments outside this range are not possible.

Remote Temperature Measurement

The ADT7463 can measure the temperature of two remote diode

sensors or diode-connected transistors connected to Pins 15 and

16, or 17 and 18.

The forward voltage of a diode or diode-connected transistor

operated at a constant current exhibits a negative temperature

coefficient of about –2 mV/�C. Unfortunately, the absolute

value of VBE varies from device to device and individual calibra-

tion is required to null this out, so the technique is unsuitable

for mass production. The technique used in the ADT7463 is to

measure the change in VBE when the device is operated at two

different currents.

This is given by

∆V

KT q

BE =× ln(

)

where:

K is Boltzmann’s constant.

q is the charge on the carrier.

T is the absolute temperature in Kelvins.

N is the ratio of the two currents.

Figure 14 shows the input signal conditioning used to measure

the output of a remote temperature sensor. This figure shows the

external sensor as a substrate transistor, provided for temperature

monitoring on some microprocessors. It could equally well be a

discrete transistor, such as a 2N3904.

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