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ADT7485A Datasheet(PDF) 11 Page - ON Semiconductor |
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ADT7485A Datasheet(HTML) 11 Page - ON Semiconductor |
11 / 12 page ADT7485A http://onsemi.com 11 • Use wide tracks to minimize inductance and reduce noise pickup. A 5 mil track minimum width and spacing is recommended. Figure 17. Arrangements of Signal Tracks 5mil 5mil 5mil 5mil 5mil 5mil 5mil GND D1+ GND D1– • 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 D1+ and D1− paths and are at the same temperature. • Thermocouple effects should not be a major problem because 1 °C corresponds to about 240 mV, and thermocouple voltages are about 3 mV/°C of the temperature difference. Unless there are two thermocouples with a big temperature differential between them, thermocouple voltages should be much less than 200 mV. • Place a 0.1 mF bypass capacitor close to the ADT7485A. • If the distance to the remote sensor is more than eight inches, the use of a twisted pair cable is recommended. This works for distances of about 6 feet to 12 feet. • For very long distances (up to 100 feet), use shielded twisted pair cables, such as Belden #8451 microphone cables. Connect the twisted pair cable to D+ and D− and the shield to GND, close to the ADT7485A. 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. A 1 W series resistance introduces about 0.5 °C error. Temperature Offset As CPUs run faster, it is more difficult to avoid high frequency clocks when routing the D+ and D− tracks around a system board. Even when the recommended layout guidelines are followed, there may still be temperature errors, attributed to noise being coupled onto the D+ and D− lines. High frequency noise generally has the effect of producing temperature measurements that are consistently too high by a specific amount. The ADT7485A has temperature offset command code of 0xe0 through which a desired offset can be set. By doing a one−time calibration of the system, the offset caused by system board noise can be calculated and nulled by specifying it in the ADT7485A. The offset is automatically added to every temperature measurement. The maximum offset is ±128°C with 0.25°C resolution. The offset format is the same as the temperature data format; 16−bit, twos complement notation, as shown in Table 11. The offset should be programmed in little endian format, that is, LSB before MSB. The offset value is also returned in little endian format when read. ORDERING INFORMATION Device Order Number* Package Type Package Option Shipping† ADT7485AARMZ−R 10−Lead MSOP RM−10 3000 Tape & Reel ADT7485AARMZ−R7 1000 Tape & Reel †For information on tape and reel specifications, including part orientation and tape sizes, please refer to our Tape and Reel Packaging Specifications Brochure, BRD8011/D. *These are Pb−Free packages. |
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