REV. A

ADM1025/ADM1025A

–10–

TEMPERATURE MEASUREMENT SYSTEM

Internal Temperature Measurement

The ADM1025/ADM1025A contains an on-chip bandgap tem-

perature sensor, whose output is digitized by the on-chip ADC.

The temperature data is stored in the Local Temperature Value

Register (address 27h). As both positive and negative tempera-

tures can be measured, the temperature data is stored in two’s

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 1°C, although tempera-

tures below 0

°C and above 100°C are outside the operating

temperature range of the device.

External Temperature Measurement

The ADM1025/ADM1025A can measure temperature using an

external diode sensor or diode-connected transistor, connected to

Pins 9 and 10.

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

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

for mass production.

The technique used in the ADM1025/ADM1025A is to measure

ent 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 ratio of the two currents

Figure 11 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 tem-

perature 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.

Bit 6 of Status Register 2 (42h) is set if a remote diode fault is

detected. The ADM1025/ADM1025A detects shorts from D+

to GND or supply, as well as shorts/opens between D+/D–.

LOW-PASS

FILTER

BIAS

DIODE

REMOTE

SENSING

TRANSISTOR

IN

D+

D–

TO

ADC

Figure 11. Signal Conditioning for External Diode

Temperature Sensors

Table III. 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

0

°C

0000 0000

+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

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 1 nF may be placed between the D+ and D– inputs

to filter the noise.

To measure

∆V

currents of I and N

× I. The resulting waveform is passed through

a 65 kHz low-pass filter to remove noise, then to a chopper-

stabilized amplifier that performs the functions of amplification

and rectification of the waveform to produce a dc voltage pro-

portional to

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 sixteen measurement cycles. An

external temperature measurement takes nominally 34.8 ms.

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 ADM1025/ADM1025A as close as possible to the

remote sensing diode. Provided that the worst noise sources

such as clock generators, data/address buses and CRTs are

avoided, this distance can be four to eight 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. 10 mil track minimum width and spacing is

recommended.

10MIL

10MIL

10MIL

10MIL

10MIL

10MIL

10MIL

GND

D+

D–

GND

Figure 12. Arrangement of Signal Tracks