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TMP04 Scheda tecnica(PDF) 11 Page - Analog Devices |
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TMP04 Scheda tecnica(HTML) 11 Page - Analog Devices |
11 / 16 page TMP03/TMP04 REV. A –11– DOUT TMP03 5V VLOGIC GND 4.7k V+ OPTO-COUPLER 620 a. DOUT TMP03 5V VLOGIC GND 430 V+ OPTO-COUPLER 270 4.3k 10k 2N2907 b. Figure 7. Optically Isolating the Digital Output DOUT TMP03 5V GND 4.7k V+ H11L1 680 5V Figure 8. An Opto-Isolator with Schmitt Trigger Logic Gate Improves Output Rise and Fall Times The TMP03 and TMP04 are superior to analog-output trans- ducers for measuring temperature at remote locations, because the digital output provides better noise immunity than an analog signal. When measuring temperature at a remote location, the ratio of the output pulses must be maintained. To maintain the integrity of the pulsewidth, an external buffer can be added. For example, adding a differential line driver such as the ADM485 permits precise temperature measurements at distances up to 4000 ft. (Figure 9). The ADM485 driver and receiver skew is only 5 ns maximum, so the TMP04 duty cycle is not degraded. Up to 32 ADM485s can be multiplexed onto one line by pro- viding additional decoding. As previously mentioned, the digital output of the TMP03 provides excellent noise immunity in remote measurement appli- cations. The user should be aware, however, that heat from an external cable can be conducted back to the TMP03. This heat conduction through the connecting wires can influence the temperature of the TMP03. If large temperature differences exist within the sensor environment, an opto-isolator, level shifter or other thermal barrier can be used to minimize measure- ment errors. DOUT TMP04 5V GND V+ ADM485 A B VCC DE DI NC 5V Figure 9. A Differential Line Driver for Remote Tempera- ture Measurement Microcomputer Interfaces The TMP03 output is easily decoded with a microcomputer. The microcomputer simply measures the T1 and T2 periods in software or hardware, and then calculates the temperature using the equation in the Output Encoding section of this data sheet. Since the TMP03’s output is ratiometric, precise control of the counting frequency is not required. The only timing require- ments are that the clock frequency be high enough to provide the required measurement resolution (see the Output Encoding section for details) and that the clock source be stable. The ratiometric output of the TMP03 is an advantage because the microcomputer’s crystal clock frequency is often dictated by the serial baud rate or other timing considerations. Pulsewidth timing is usually done with the microcomputer’s on-chip timer. A typical example, using the 80C51, is shown in Figure 10. This circuit requires only one input pin on the micro- computer, which highlights the efficiency of the TMP04’s pulsewidth output format. Traditional serial input protocols, with data line, clock and chip select, usually require three or more I/O pins. DOUT TMP04 5V GND V+ INPUT PORT 1.0 OSC 12 TIMER 0 (16-BITS) TIMER 1 (16-BITS) 80C51 MICROCOMPUTER TMOD REGISTER TCON REGISTER TIMER 0 TIMER 1 TIMER 0 TIMER 1 Figure 10. A TMP04 Interface to the 80C51 Microcomputer The 80C51 has two 16-bit timers. The clock source for the timers is the crystal oscillator frequency divided by 12. Thus, a crystal frequency of 12 MHz or greater will provide resolution of 1 µs or less. The 80C51 timers are controlled by two dedicated registers. The TMOD register controls the timer mode of operation, while TCON controls the start and stop times. Both the TMOD and TCON registers must be set to start the timer. |
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