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ADT7302ARMZ Datasheet(PDF) 10 Page - Analog Devices |
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ADT7302ARMZ Datasheet(HTML) 10 Page - Analog Devices |
10 / 17 page ADT7302 Rev. B | Page 9 of 16 THEORY OF OPERATION The ADT7302 is a 13-bit digital temperature sensor with a 14th bit that acts as a sign bit. The part houses an on-chip temperature sensor, a 13-bit ADC, a reference circuit, and serial interface logic functions in SOT-23 and MSOP packages. The ADC section consists of a conventional successive approximation converter based around a capacitor DAC. The parts can run on a 2.7 V to 5.25 V power supply. The on-chip temperature sensor allows an accurate measurement of the ambient device temperature to be made. The specified measurement range of the ADT7302 is −40°C to +125°C. The structural integrity of the device can start to deteriorate when continuously operated at absolute maximum voltage and temperature specifications. CONVERTER DETAILS The conversion clock for the part is internally generated. No external clock is required except when reading from and writing to the serial port. In normal mode, an internal clock oscillator runs an automatic conversion sequence. During this automatic conversion sequence, a conversion is initiated every 1.5 second. At this time, the part powers up its analog circuitry and performs a temperature conversion. This temperature conversion typically takes 1.2 ms, after which the analog circuitry of the part auto- matically shuts down. The analog circuitry powers up again when the 1.5 second timer times out and the next conversion begins. Since the serial interface circuitry never shuts down, the result of the most recent temperature conversion is always available in the serial output register. The ADT7302 can be placed into shutdown mode via the control register. This means that the on-chip oscillator is shut down and no further conversions are initiated until the ADT7302 is taken out of shutdown mode. The ADT7302 can be taken out of shutdown mode by writing all zeros into the control register. The conversion result from the last conversion prior to shutdown can still be read from the ADT7302 even when it is in shutdown mode. In normal conversion mode, the internal clock oscillator is reset after every read or write operation. This causes the device to start a temperature conversion, the result of which is typically available 1.2 ms later. Similarly, when the part is taken out of shutdown mode, the internal clock oscillator is started and a conversion is initiated. The conversion result is available 1.2 ms later, typically. Every result is stored in a buffer register and is only loaded into the temperature value register at the first falling SCLK edge of every serial port activity. Serial port activity does not interfere with the conversion process and every conversion completes its process even during a read operation. A conversion has to be completed before a read occurs, otherwise its result does not get loaded into the temperature value register and instead goes into the buffer register. A new conversion is triggered at the end of each serial port activity except when a conversion is already in progress. TEMPERATURE VALUE REGISTER The temperature value register is a 14-bit read-only register that stores the temperature reading from the ADC in 13-bit twos complement format plus a sign bit. The MSB (DB13) is the sign bit. The ADC can theoretically measure a 255°C temperature span. The internal temperature sensor is guaranteed to a low value limit of −40°C and a high limit of +125°C. The temperature data format is shown in Table 5, which also shows the temperature measurement range of the device (−40°C to +125°C). A typical performance curve is shown in Figure 11. Table 5. Temperature Data Format Temperature Digital Output DB13…DB0 −40°C 11, 1011 0000 0000 −30°C 11, 1100 0100 0000 −25°C 11, 1100 1110 0000 −10°C 11, 1110 1100 0000 −0.03125°C 11, 1111 1111 1111 0°C 00, 0000 0000 0000 +0.03125°C 00, 0000 0000 0001 +10°C 00, 0001 0100 0000 +25°C 00, 0011 0010 0000 +50°C 00, 0110 0100 0000 +75°C 00, 1001 0110 0000 +100°C 00, 1100 1000 0000 +125°C 00, 1111 1010 0000 Temperature Conversion Equations Positive Temperature = ADC Code(d)/32 Negative Temperature = (ADC Code(d)1 − 16384)/32 Negative Temperature = (ADC Code(d)2 − 8192)/32 1 ADC Code uses all 14 bits of the data byte, including the sign bit. 2 DB13 (the sign bit) is removed from the ADC code. |
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