SA56004HD,118【PDF 24/43 页】IC TEMP SENSOR DIGITAL 8SOIC

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SA56004X

All information provided in this document is subject to legal disclaimers.

Product data sheet

Rev. 7 25 February 2013

24 of 43

NXP Semiconductors

SA56004X

Digital temperature sensor with overtemperature alarms

8.1.3 Layout considerations

Digital boards can be electrically noisy environments, and the SA56004X is measuring

very small voltages from the remote sensor, so care must be taken to minimize noise

induced at the sensor inputs. The following precautions should be taken.

1. Place the SA56004X as close as possible to the remote sensing diode. Provided that

the worst noise sources, that is, 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. 10 mil track

minimum width and spacing is recommended (see Figure 10

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 since 1 癈 corresponds to about

200 mV and thermocouple voltages are about 3 mV/癈 of temperature difference.

Unless there are two thermocouples with a large temperature differential between

them, thermocouple voltages should be much less than 200 mV.

5. Place a 0.1 mF bypass capacitor close to the V

pin. In very noisy environments,

place a 1000 pF input filter capacitor across D+ and D- close to the SA56004X.

6. 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.

7. For really long distances (up to 100 feet), use shielded twisted pair, such as

Belden #8451 microphone cable. Connect the twisted pair to D+ and D- and the

shield to GND close to the SA56004X. 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. 1 W resistance introduces about 1 癈 error.

Fig 10. Typical arrangement of signal tracks

002aag953

GND

D+

10 mil

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