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REV. A

–14–

ADM1027

TEMPERATURE MEASUREMENT SYSTEM

Local Temperature Measurement

The ADM1027 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 temp

tures 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

oC to +127oC with a resolution of 0.25oC. However, this

exceeds the operating temperature range of the device (0

oC to

105

oC), so local temperature measurements outside this range

are not possible. Temperature measurement from –127

oC to

+127

oC is possible using a remote sensor.

Remote Temperature Measurement

The ADM1027 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/

oC. 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 ADM1027 is to

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

different currents. This is given by

where:

K is Boltzmann’s constant.

q is charge on the carrier.

T is absolute temperature in kelvins.

N is the ratio of the two currents.

Figure 13 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 2N3904/06.

D+

D–

REMOTE

SENSING

TRANSISTOR

IBIAS

VDD

VOUT+

TO ADC

VOUT–

BIAS

DIODE

LOW-PASS

FILTER

fC = 65kHz

THERMDA

THERMDC

CPU

Figure 13. Signal Conditioning for Remote Diode Temperature Sensors

DVKT q

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