ADM1024ARU datasheet(12/28 Pages) AD

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ADM1024

–12–

REV. 0

tures can be measured, the temperature data is stored in twos

complement format, as shown in Table IV. 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 –40

°C and above +125°C are outside the operating

temperature range of the device.

External Temperature Measurement

The ADM1024 can measure the temperature of two external

diode sensors or diode-connected transistors, connected to Pins

13 and 14 or 17 and 18.

Pins 13 and 14 are a dedicated temperature input channel. Pins

17 and 18 can be conﬁgured to measure a diode sensor by set-

ting Bit 2 of the Channel Mode Register to 1.

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 calibration is

required to null this out, so the technique is unsuitable for mass-

production.

The technique used in the ADM1024 is to measure the change

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

This is given by:

∆VBE = KT/q × ln(N)

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 14 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 temperature

monitoring on some microprocessors, but it could equally well

be a discrete transistor.

LOW-PASS

FILTER

fC = 65kHz

BIAS

DIODE

REMOTE

SENSING

TRANSISTOR

IIBIAS

D+

D–

VOUT+

VOUT–

ADC

VDD

Figure 14. Signal Conditioning for External Diode

Temperature Sensors

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 from 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. As the sensor is operating in a noisy environment, C1 is

provided as a noise ﬁlter. See the section on layout considerations

for more information on C1.

To measure

∆VBE, the sensor is switched between operating

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 pro-

portional to

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

a temperature output in 8-bit twos 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 takes nominally 9.6 ms.

The results of external temperature measurements are stored in

8-bit, twos-complement format, as illustrated in Table IV.

Table IV. 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

°C

0000 0000

+0.5

°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

LAYOUT CONSIDERATIONS

Digital boards can be electrically noisy environments, and care

must be taken to protect the analog inputs from noise, particularly

when measuring the very small voltages from a remote diode

sensor. The following precautions should be taken:

1. Place the ADM1024 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 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

recommended.

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