ADM1024 datasheet(13/29 Pages) ONSEMI

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ADM1024

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positive and negative temperatures can be measured, the

temperature data is stored in twos complement format, as

shown in Table 4. Theoretically, the temperature sensor and

ADC can measure temperatures from −128

°C to +127°C

with a resolution of 1

°C, although temperatures 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 setting 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:

(eq. 5)

DVbe + KT q ln(N)

where:

K is Boltzmann’s constant.

q is the charge on the carrier.

T is the absolute temperature in Kelvins.

N is the ratio of the two currents.

Figure 17 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

some

microprocessors, but it could equally well be a discrete

transistor.

D+

LPF

N y I

IBIAS

VDD

BIAS

DIODE

D–

LOW−PASS

FILTER

fC = 65kHz

VOUT+

VOUT–

ADC

REMOTE

SENSING

TRANSISTOR

Figure 17. 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

Layout Considerations section for more information on C1.

To measure

DVBE, 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,

then to a chopper−stabilized amplifier that performs the

functions of amplification and rectification of the waveform

to produce a dc voltage proportional to

DVBE. 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 4.

Table 4. 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

+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 inches 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.

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