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ADT7317ARQ
(Rev.:RevPrN)

SPI/I2C® Compatible, 10-Bit Digital Temperature Sensor and Quad Voltage Output 12/10/8-Bit DAC

Analog Devices 

PRELIMINARY TECHNICAL DATA

ADT7316/7317/7318

VDD

I

OPTIONAL CAPACITOR, UP TO

3nF MAX. CAN BE ADDED TO

IMPROVE HIGH FREQUENCY

NOISE REJECTION IN NOISY

ENV IRO NME NTS

D+

REM OT E

S ENSI NG

TR ANSISTOR

( 2N3906)

C1

D-

LOWPASS FILTER

fc = 65kHz

N x I IBIAS

BIAS

DIODE

V OUT +

TO ADC

VO UT-

Figure 10. Signal Conditioning for External Diode temperature Sensors

MEASUREMENT METHOD

INTERNAL TEMPERATURE MEASUREMENT

The ADT7316/7317/7318 contains an on-chip bandgap

temperature sensor, whose output is digitized by the on-

chip ADC. The temperature data is stored in the Internal

Temperature Value Register. As both positive and nega-

tive temperatures can be measured, the temperature data is

stored in two's complement format, as shown in Table 3.

The thermal characteristics of the measurement sensor

could change and therefore an offset is added to the mea-

sured value to enable the transfer function to match the

thermal characteristics. This offset is added before the

temperature data is stored. The offset value used is stored

in the Internal Temperature Offset Register.

EXTERNAL TEMPERATURE MEASUREMENT

The ADT7316/7317/7318 can measure the temperature of

one external diode sensor or diode-connected transistor.

The forward voltage of a diode or diode-connected tran-

sistor, operated at a constant current, exhibits a negative

temperature coefficient of about -2mV/oC. 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 time taken to measure the external temperature can

be reduced by setting C0 of Control Config. 3 register

(1Ah). This increases the ADC clock speed from 1.4KHz

to 22KHz but the analog filters on the D+ and D- input

pins are switched off to accommodate the higher clock

speeds. Running at the slower ADC speed, the time taken

to measure the external temperature is TBD while on the

fast ADC this time is reduced to TBD.

The technique used in the ADT7316/7317/7318 is to

measure the change in Vbe when the device is operated at

two different currents.

This is given by:

∆Vbe = KT/q x 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 10 shows the input signal conditioning used to

measure the output of an external temperature sensor.

This figure shows the external sensor as a substrate tran-

sistor, provided for temperature monitoring on some mi-

croprocessors, 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.

We recommend that a 2N3906 be used as the external

transistor.

To prevent ground noise interfering with the measure-

ment, the more negative terminal of the sensor is not ref-

erenced 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 informa-

tion on C1.

To measure ∆Vbe, the sensor is switched between operating

currents of I and N x I. The resulting waveform is passed

through a lowpass filter to remove noise, thence to a chop-

per-stabilized amplifier that performs the functions of

amplification and rectification of the waveform to produce

a DC voltage proportional to ∆Vbe. This voltage is mea-

sured by the ADC to give a temperature output in 8-bit

two’s complement format. To further reduce the effects of

noise, digital filtering is performed by averaging the re-

sults of 16 measurement cycles.

–16–

REV. PrN

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