REV. A

ADM1028

–8–

TEMPERATURE MEASUREMENT SYSTEM

Internal Temperature Measurement

The ADM1028 contains an on-chip bandgap temperature sen-

sor. The on-chip ADC performs conversions on the output of

this sensor and outputs the temperature data in 8-bit two’s

complement format. The format of the temperature data is

shown in Table I.

Table I. Temperature Data Format

Temperature

Digital Output

–128

°C

1000 0000

–125

°C

1000 0011

–100

°C

1001 1100

–75

°C

1011 0101

–50

°C

1100 1110

–25

°C

1110 0111

–1

°C

1111 1111

0

°C

0000 0000

+1

°C

0000 0001

+10

°C

0000 1010

+25

°C

0001 1001

+50

°C

0011 0010

+75

°C

0100 1011

+100

°C

0110 0100

+125

°C

0111 1101

+127

°C

0111 1111

External Temperature Measurement

The ADM1028 can measure the temperature of an external

diode sensor or diode-connected transistor, connected to Pins 9

and 10.

Pins 9 and 10 are a dedicated temperature input channel. The

default functions of Pins 11 and 12 are as

THERM outputs to

indicate over-temperature conditions.

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

is required to null this out, making the technique unsuitable

for mass production.

The technique used in the ADM1028 is to measure the change

This is given by:

where:

K is Boltzmann’s constant.

q is charge on the carrier.

T is absolute temperature in Kelvins.

N is ratio of the two currents.

Figure 3 shows the input signal conditioning used to measure

the output of an external temperature sensor. This figure shows

the external sensor as a substrate transistor, provided for tempera-

ture monitoring on some microprocessors, but it could equally

well be a discrete transistor.

If a discrete transistor is used, the collector will not be grounded,

and should be linked to the base. If a PNP transistor is used, the

base is connected to the D– input and the emitter to the D+

input. If an NPN transistor is used, the emitter is connected to

the D– input and the base to the D+ input.

To prevent ground noise interfering with the measurement, the

more negative terminal of the sensor is not referenced to ground,

but is biased above ground by an internal diode at the D– input.

If the sensor is used in a very noisy environment, a capacitor of

value up to 1000 pF may be placed between the D+ and D–

inputs to filter the noise.

To measure

currents of I and N

× I. The resulting waveform is passed

through a 65 kHz low-pass filter to remove noise, thence to a

chopper-stabilized amplifier that performs the functions of

amplification and rectification of the waveform to produce a dc

voltage proportional to

ADC to give a temperature output in 8-bit two’s complement

format. To further reduce the effects of noise, digital filtering is

performed by averaging the results of 16 measurement cycles.

An external temperature measurement nominally takes 9.6 ms.

LOW-PASS

FILTER

BIAS

DIODE

REMOTE

SENSING

TRANSISTOR

IN

D+

D–

TO

ADC

Figure 3. Signal Conditioning

LAYOUT CONSIDERATIONS

Digital boards can be electrically noisy environments, and care

must be taken to protect the analog inputs from noise, particu-

larly when measuring the very small voltages from a remote

diode sensor. The following precautions should be taken:

1. Place the ADM1028 as close as possible to the remote sens-

ing diode. Provided that the worst noise sources such as clock

generators, data/address buses and CRTs are avoided, this

distance can be 4 to 8 inches.

2. Route the D+ and D– tracks close together, in parallel with

grounded guard tracks on each side. Provide a ground plane

under the tracks if possible.

3. Use wide tracks to minimize inductance and reduce noise

pickup. Ten mil track minimum width and spacing is rec-

ommended.

4. Try to minimize the number of copper/solder joints, which

can cause thermocouple effects. Where copper/solder joints

are used, make sure that they are in both the D+ and D–

path and at the same temperature.

Thermocouple effects should not be a major problem as 1

°C

corresponds to about 200

µV, and thermocouple voltages are

about 3

two thermocouples with a big temperature differential between

them, thermocouple voltages should be much less than 200

µV.

5. Place 0.1

µF bypass and 2200 pF input filter capacitors close

to the ADM1028.

6. If the distance to the remote sensor is more than 8 inches, the

use of twisted-pair cable is recommended. This will work up

to about 6 to 12 feet.