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ADM1032ARM

±1°C Remote and Local System Temperature Monitor

Analog Devices 

ADM1032

FUNCTIONAL DESCRIPTION

The ADM1032 is a local and remote temperature sensor and

over-temperature alarm. When the ADM1032 is operating

normally, the on-board A-to-D converter operates in a free-

running mode. The analog input multiplexer alternately selects

either the on-chip temperature sensor to measure its local tem-

perature, or the remote temperature sensor. These signals are

digitized by the ADC and the results stored in the Local and

Remote Temperature Value Registers.

The measurement results are compared with local and remote,

high, low and THERM temperature limits, stored in nine on-

chip registers. Out-of-limit comparisons generate flags that are

stored in the Status Register, and one or more out-of limit results

will cause the ALERT output to pull low. Exceeding THERM

temperature limits cause the THERM output to assert low.

The limit registers can be programmed, and the device con-

trolled and configured, via the serial System Management Bus

(SMBus). The contents of any register can also be read back via

the SMBus.

Control and configuration functions consist of:

• Switching the device between normal operation and

standby mode.

• Masking or enabling the ALERT output.

• Selecting the conversion rate.

MEASUREMENT METHOD

A simple method of measuring temperature is to exploit the

negative temperature coefficient of a diode, or the base-emitter

voltage of a transistor, operated at constant current. Unfortu-

nately, this technique requires calibration to null out the effect

of the absolute value of VBE, which varies from device to device.

The technique used in the ADM1032 is to measure the change

in VBE when the device is operated at two different currents.

This is given by:

where:

∆VBE

= (nf )

KT

q

×

In (N )

K is Boltzmann’s constant (1.38 × 10–23).

q is charge on the electron (1.6 × 10–19 Coulombs).

T is absolute temperature in Kelvins.

N is ratio of the two currents.

nf is the ideality factor of the thermal diode.

The ADM1032 is trimmed for an ideality factor of 1.008.

Figure 2 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. To prevent ground noise interfering with the measure-

ment, 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 operating in a noisy environment,

C1 may optionally be added as a noise filter. Its value is typi-

cally 2200 pF, but should be no more than 3000 pF. See the

section on Layout Considerations for more information on C1.

To measure ∆VBE, the sensor is switched between operating cur-

rents 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 pro-

portional to ∆VBE. This voltage is measured by the ADC to give

a temperature output in two’s complement format. To further

reduce the effects of noise, digital filtering is performed by aver-

aging the results of 16 measurement cycles.

Signal conditioning and measurement of the internal tempera-

ture sensor is performed in a similar manner.

TEMPERATURE DATA FORMAT

One LSB of the ADC corresponds to 0.125°C, so the ADC can

measure from 0°C to 127.875°C. The temperature data format

is shown in Tables I and II.

The results of the local and remote temperature measurements

are stored in the Local and Remote Temperature Value Registers,

and are compared with limits programmed into the Local and

Remote High and Low Limit Registers.

Table I. Temperature Data Format (Local Temperature and

Remote Temperature High Byte)

Temperature

0°C

1°C

10°C

25°C

50°C

75°C

100°C

125°C

127°C

Digital Output

0 000 0000

0 000 0001

0 000 1010

0 001 1001

0 011 0010

0 100 1011

0 110 0100

0 111 1101

0 111 1111

REV. 0

VDD

I

N؋I

IBIAS

REMOTE

SENSING

TRANSISTOR

D+

C1*

D–

BIAS

DIODE

LOW-PASS FILTER

fC = 65kHz

*CAPACITOR C1 IS OPTIONAL. IT SHOULD ONLY BE USED IN NOISY ENVIRONMENTS.

C1 = 2.2nF TYPICAL, 3nF MAX.

Figure 2. Input Signal Conditioning

–5–

VOUT+

TO ADC

VOUT–

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