ADT7463ARQZ-R7 datasheet(17/52 Pages) ONSEMI

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REV. C

ADT7463

–17–

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. Figures 15a and 15b

show how to connect the ADT7463 to an NPN or PNP transis-

tor for temperature measurement. 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.

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 and to a chopper-stabilized

amplifier that performs the functions of amplification and recti-

fication of the waveform to produce a dc voltage proportional to

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

ture output in 10-bit, twos complement format. To further reduce

the effects of noise, digital filtering is performed by averaging

the results of 16 measurement cycles. A remote temperature

measurement takes nominally 25.5 ms. The results of remote

temperature measurements are stored in 10-bit, twos complement

format, as illustrated in Table III. The extra resolution for the

temperature measurements is held in the Extended Resolution

resolution of 0.25�C.

Table III. Temperature Data Format

Temperature

Digital Output (10-Bit)*

–128�C

1000 0000 00

–125�C

1000 0011 00

–100�C

1001 1100 00

–75�C

1011 0101 00

–50�C

1100 1110 00

–25�C

1110 0111 00

–10�C

1111 0110 00

0�C

0000 0000 00

+10.25�C

0000 1010 01

+25.5�C

0001 1001 10

+50.75�C

0011 0010 11

+75�C

0100 1011 00

+100�C

0110 0100 00

+125�C

0111 1101 00

+127�C

0111 1111 00

*Bold denotes 2 LSBs of measurement in Extended

Resolution Register 2 (Reg. 0x77) with 0.25�C resolution.

ADT7463

2N3904

NPN

D+

D–

Figure 15a. Measuring Temperature Using an

NPN Transistor

2N3906

PNP

ADT7463

D+

D–

Figure 15b. Measuring Temperature Using a PNP

Transistor

Nulling Out Temperature Errors

As CPUs run faster, it is getting more difficult to avoid high

frequency clocks when routing the D+, D– traces around a

system board. Even when recommended layout guidelines are

followed, there may still be temperature errors attributed to

noise being coupled onto the D+/D– lines. High frequency noise

generally has the effect of giving temperature measurements that are

too high by a constant amount. The ADT7463 has temperature

offset registers at Addresses 0x70, 0x72 for the Remote 1 and

Remote 2 temperature channels. By doing a one-time calibration

of the system, one can determine the offset caused by system

board noise and null it out using the offset registers. The offset

registers automatically add a twos complement 8-bit reading to

every temperature measurement. The LSBs add 0.25

°C offset to

the temperature reading so the 8-bit register effectively allows

temperature offsets of up to �32�C with a resolution of 0.25�C.

This ensures that the readings in the temperature measurement

registers are as accurate as possible.

Temperature Offset Registers

Reg. 0x70 Remote 1 Temperature Offset = 0x00 (0

°C Default)

Reg. 0x71 Local Temperature Offset = 0x00 (0

°C Default)

Reg. 0x72 Remote 2 Temperature Offset = 0x00 (0

°C Default)

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