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NE1617ADS Datasheet PDF : 30 Pages
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NXP Semiconductors
NE1617A
Temperature monitor for microprocessor systems
9.1.3 Layout considerations
Digital boards can be electrically noisy environments, and the NE1617A 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 NE1617A 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 four to eight 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 4).
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 C corresponds to about
200 V and thermocouple voltages are about 3 V/C of temperature difference.
Unless there are two thermocouples with a big temperature differential between them,
thermocouple voltages should be much less than 200 V.
5. Place a 0.1 F bypass capacitor close to the VDD pin. In very noisy environments,
place a 1000 pF input filter capacitor across D+ and D close to the NE1617A.
6. If the distance to the remote sensor is more than eight inches, the use of twisted pair
cable is recommended. This works up to about six 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 NE1617A. 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 resistance introduces about 1 C error.
GND
D+
D−
GND
Fig 4. Typical arrangement of signal tracks
002aag953
10 mil
10 mil
10 mil
10 mil
10 mil
10 mil
10 mil
NE1617A
Product data sheet
All information provided in this document is subject to legal disclaimers.
Rev. 5 — 20 March 2012
© NXP B.V. 2012. All rights reserved.
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