ADT7516 datasheet(27/44 Pages) AD

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ADT7516/ADT7517/ADT7519

Rev. B | Page 27 of 44

input. If an NPN transistor is used, the emitter is connected 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

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 ΔVBE, 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 ΔVBE. This voltage is measured by the ADC to

give a temperature 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.

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:

•

Place the ADT7516/ADT7517/ADT7519 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.

•

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.

•

Use wide tracks to minimize inductance and reduce noise

pickup. A 10 mil track minimum width and spacing is

recommended.

GND

D+

D–

GND

10MIL

Figure 54. Arrangement of Signal Tracks

•

Try to minimize the number of copper/solder joints

because they can cause thermocouple effects. Where

copper/solder joints are used, make sure that they are in

both the D+ and D− path and are at the same temperature.

Thermocouple effects should not be a major problem

because 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.

•

Place 0.1 μF bypass and 2200 pF input filter capacitors

close to the ADT7516/ADT7517/ADT7519.

•

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

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

up to about 6 feet to 12 feet.

•

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 ADT7516/ADT7517/ADT7519. Leave the remote

end of the shield unconnected to avoid ground loops.

Because the measurement technique uses switched current

sources, excessive cable and/or filter capacitance can affect the

measurement. When using long cables, the filter capacitor can

be reduced or removed.

Cable resistance can also introduce errors. Series resistance of

1 Ω introduces about 0.5°C error.

Temperature Value Format

One LSB of the ADC corresponds to 0.25°C. The ADC can

theoretically measure a temperature span of 255°C. The internal

temperature sensor is guaranteed to a low value limit of −40°C.

It is possible to measure the full temperature span using the

external temperature sensor. The temperature data format is

shown in Table 9.

The result of the internal or external temperature measurements

is stored in the temperature value registers, and is compared

with limits programmed into the internal or external high and

low registers.

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