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ADT7488A データシート(PDF) 14 Page - Analog Devices |
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ADT7488A データシート(HTML) 14 Page - Analog Devices |
14 / 16 page ADT7488A Rev. 0 | Page 14 of 16 SST TEMPERATURE SENSOR DATA FORMAT The data for temperature is structured to allow values in the range of ±512°C to be reported. Thus, the temperature sensor format uses a twos complement, 16-bit binary value to represent values in this range. This format allows temperatures to be represented with approximately a 0.016°C resolution. Table 15. SST Temperature Data Format Twos Complement Temperature (°C) MSB LSB −125 1110 0000 1100 0000 −80 1110 1100 0000 0000 −40 1111 0110 0000 0000 −20 1111 1011 0011 1110 −5 1111 1110 1100 0000 −1 1111 1111 1100 0000 0 0000 0000 0000 0000 +1 0000 0000 0100 0000 +5 0000 0001 0100 0000 +20 0000 0100 1100 0010 +40 0000 1010 0000 0000 +80 0001 0100 0000 0000 +125 0001 1111 0100 0000 USING DISCRETE TRANSISTORS If a discrete transistor is used, the collector is not 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 is connected to the D+ input. If an NPN transistor is used, the emitter is connected to the D− input and the base is connected to the D+ input. Figure 17 shows how to connect the ADT7488A to an NPN or PNP transistor for temperature measurement. 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. D1+ D1– ADT7488A 2N3904 NPN D1+ D1– ADT7488A 2N3906 PNP Figure 17. Connections for NPN and PNP Transistors The ADT7488A shows an external temperature value of 0x8000 if the external diode is an open or short circuit. LAYOUT CONSIDERATIONS Digital boards can be electrically noisy environments. Take the following precautions to protect the analog inputs from noise, particularly when measuring the very small voltages from a remote diode sensor: • Place the ADT7488A 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 four to eight inches. • Route the D1+ and D1− 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 5 mil track minimum width and spacing is recommended. 5mil 5mil 5mil 5mil 5mil 5mil 5mil GND D1+ GND D1– Figure 18. Arrangements of Signal Tracks • 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 μV, and thermocouple voltages are about 3 μV/°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 μF bypass capacitor close to the ADT7488A. • 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 ADT7488A. 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 Ω series resistance introduces about 0.5°C error. |
同様の部品番号 - ADT7488A |
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同様の説明 - ADT7488A |
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