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ADT7488A Arkusz danych(PDF) 10 Page - ON Semiconductor |
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10 / 12 page ADT7488A http://onsemi.com 10 rates. Signal conditioning and measurement of the internal temperature sensor is performed in the same manner. Figure 15. Signal Conditioning for Remote Diode Temperature Sensors C1* D1+ D1– I N1 × IN2 × I VOUT+ VOUT– VDD IBIAS LOW−PASS FILTER fC = 65kHz TRANSISTOR REMOTE SENSING *CAPACITOR C1 IS OPTIONAL. BIAS DIODE TO IN NOISY ENVIRONMENTS. ADC IT SHOULD ONLY BE USED Reading Temperature Measurements The temperature data returned is two bytes in little endian format, that is, LSB before MSB. All temperatures can be read together by using Command Code 0x00 with a read length of 0x06. The command codes and returned data are described in Table 10. Table 10. Temperature Channel Command Codes Temp Channel Command Code Returned Data Internal 0x00 LSB, MSB External 1 0x01 LSB, MSB External 2 0x02 LSB, MSB All Temps 0x00 Internal LSB, internal MSB; External 1 LSB, External 1 MSB, External 2 LSB, External 2 MSB 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 11. SST Temperature Data Format Temperature (5C) Twos Complement LSB MSB −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 16 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. Figure 16. Connections for NPN and PNP Transistors D1+ D1– ADT7488A 2N3904 NPN D1+ D1– ADT7488A 2N3906 PNP 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. 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 |
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