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ADT7490

Rev. 0 | Page 21 of 76

Remote Temperature Measurement

THERMAL DIODE TEMPERATURE MEASUREMENT

METHOD

The ADT7490 can measure the temperature of two remote

diode sensors or diode-connected transistors connected to

Pin 10 and Pin 11, or Pin 12 and Pin 13.

A simple method of measuring temperature is to exploit the

negative temperature coefficient of a diode, measuring the base-

emitter voltage (VBE) of a transistor operated at constant

current. Unfortunately, this technique requires calibration to

null out the effect of the absolute value of VBE, which varies

from device to device.

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. Therefore, the technique

is unsuitable for mass production. The technique used in the

ADT7490 is to measure the change in VBE when the device is

operated at three different currents. This is given by

The technique used in the ADT7490 is to measure the change

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

Previous devices have used only two operating currents, but the

use of a third current allows automatic cancellation of resis-

tances in series with the external temperature sensor.

)

ln(N

Figure 29 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, but it could equally

be a discrete transistor, such as a 2N3904/2N3906.

where:

K is the Boltzmann constant.

q is the charge on the carrier.

T is the absolute temperature in Kelvin.

N is the ratio of the two currents.

If a discrete transistor is used, the collector is not 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. Figure 26 and Figure

show how to connect the ADT7490 to an NPN or PNP

transis-

tor for temperature measurement.

To measure ΔVBE, the operating current through the sensor is

switched among three related currents. N1 × I and N2 × I are

different multiples of the current I, as shown in Figure 28. The

currents through the temperature diode are switched between

I and N1 × I, giving ΔVBE1, and then between I and N2 × I,

giving ΔVBE2. The temperature can then be calculated using the

two ΔVBE measurements. This method can also cancel the effect

of any series resistance on the temperature measurement.

2N3904

NPN

ADT7490

D+

D–

The resulting ΔVBE waveforms are passed through a 65 kHz

low-pass filter to remove noise and then to a chopper-stabilized

amplifier. This amplifies and rectifies the waveform to produce

a dc voltage proportional to ΔVBE. The ADC digitizes this

voltage, and a temperature measurement is produced. To reduce

the effects of noise, digital filtering is performed by averaging

the results of 16 measurement cycles.

Figure 26. Measuring Temperature Using an NPN Transistor

2N3906

PNP

ADT7490

D+

D–

The results of remote temperature measurements are stored in

10-bit, twos complement format, as listed in Table 15. The extra

resolution for the temperature measurements is held in the

Extended Resolution Register 2 (0x77). This gives temperature

readings with a resolution of 0.25°C.

Figure 27. Measuring Temperature Using a PNP Transistor

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. C1 can optionally be added as a noise

filter (recommended maximum value of 1000 pF). However, a

better option in noisy environments is to add a filter, as

described in the Series Resistance Cancellation section.

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