ADT7411ARQZ10 Просмотр технического описания (PDF) - Analog Devices

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ADT7411ARQZ10

SPI®/I2C® Compatible, 10-Bit Digital Temperature Sensor and 8-Channel ADC

Analog Devices 

ADT7411

measurement sensor could change and therefore an offset is

added to the measured 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 ADT7411 can measure the temperature of one external

diode sensor or diode-connected transistor.

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 ADT7411 is to measure the change in

VBE when the device is operated at two different currents.

This is given by:

further reduce the effects of noise, digital filtering is performed

by averaging the results of 16 measurement cycles.

Layout Considerations

Digital boards can be electrically noisy environments, and care

must be taken to protect the analog inputs from noise, particu-

larly when measuring the very small voltages from a remote

diode sensor. The following precautions should be taken:

1. Place the ADT7411 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.

3. Use wide tracks to minimize inductance and reduce noise

pickup. A 10 mil track minimum width and spacing is

recommended (see Figure 28).

∆VBE = KT q × In(N )

where:

K is Boltzmann’s constant

q is the charge on the carrier

T is the absolute temperature in Kelvin

N is the ratio of the two currents

Figure 23 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 temp-

erature monitoring on some microprocessors, 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 con-

nected to the D− input and the base to the D+ input. A 2N3906

is recommended as the external transistor.

To prevent ground noise from interfering with the measure-

ment, 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 ∆VBE, the sensor is switched between operating

currents of I, and N × I. The resulting waveform is passed

through a 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 ∆VBE. This voltage is measured by the ADC to

give a temperature output in 10-bit twos complement format. To

GND

D+

D–

GND

10 MIL

10 MIL

10 MIL

10 MIL

10 MIL

10 MIL

10 MIL

Figure 28. Arrangement of Signal Tracks

4. Try to minimize the number of copper/solder joints, which

can cause thermocouple effects. Where copper/solder

joints are used, make sure that they are in both the D+ and

D− path and at the same temperature.

Thermocouple effects should not be a major problem as

1°C corresponds to about 240 µV, and thermocouple

voltages are about 3 µV/°C of temperature difference.

Unless there are two thermocouples with a big temperature

differential between them, thermocouple voltages should

be much less than 200 mV.

5. Place 0.1 µF bypass and 2200 pF input filter capacitors

close to the ADT7411.

6. If the distance to the remote sensor is more than 8 inches,

the use of twisted-pair cable is recommended. This will

work up to about 6 feet to 12 feet.

7. For long distances (up to 100 feet) use shielded twisted-

pair cable, such as Belden #8451 microphone cable.

Connect the twisted pair to D+ and D− and the shield to

GND close to the ADT7411. Leave the remote end of the

shield unconnected to avoid ground loops.

Rev. A | Page 18 of 36

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