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AD7814ART-500RL7 Folha de dados(PDF) 5 Page - Analog Devices |
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AD7814ART-500RL7 Folha de dados(HTML) 5 Page - Analog Devices |
5 / 8 page REV. E –5– AD7814 CIRCUIT INFORMATION The AD7814 is a 10-bit digital temperature sensor. The part houses an on-chip temperature sensor, a 10-bit A/D converter, a reference, and serial interface logic functions in a SOT-23 package. The A/D converter section consists of a conventional successive-approximation converter based around a capacitor DAC. The parts are capable of running on a 2.7 V to 5.5 V power supply. The on-chip temperature sensor allows an accurate measurement of the ambient device temperature to be made. The working measurement range of the AD7814 is –55 °C to +125°C. CONVERTER DETAILS The conversion clock for the part is internally generated so no external clock is required except when reading from and writing to the serial port. In normal mode, an internal clock oscillator runs the automatic conversion sequence. A conversion is initi- ated approximately every 350 µs. At this time, the part wakes up and performs a temperature conversion. This temperature con- version typically takes 25 µs, at which time the part automati- cally shuts down. The result of the most recent temperature conversion is available in the serial output register at any time. Once the conversion is finished, an internal oscillator starts counting and is designed to time out every 350 µs. The AD7814 then powers up and does a conversion. Please note that if the CS is brought low every 350 µs (±30%), then the same tempera- ture value will be output onto the DOUT line every time with- out changing. It is recommended that the CS line is not brought low every 350 µs (±30%). The ±30% covers process variation. The CS should become active (high to low) outside this range. The device is designed to autoconvert every 350 µs. If the AD7814 is accessed during the conversion process, an internal signal is generated to prevent any update of the temperature value register during the conversion. Otherwise the user could read back spurious data. The design of this feature results in this internal lockout signal being reset only at the start of the next autoconversion. Therefore, if the CS line goes active before the internal lockout signal has been reset to its inactive mode, the internal lockout signal will not be reset. To ensure that no lock- out signal is set, bring the CS low at a greater or less time than 350 µs (±30%). This will mean that the AD7814 is not inter- rupted during a conversion process. The AD7814 can be placed in a shutdown mode, via the control register, in which case the on-chip oscillator is shut down and no further conversions are initiated until the AD7814 is taken out of shutdown mode. The conversion result from the last conversion prior to shutdown can still be read from the AD7814 even when it is in shutdown mode. In the automatic conversion mode, every time a read or write operation takes place, the internal clock oscillator is restarted at the end of the read or write operation. The result of the con- version is typically available 25 µs later. Similarly, when the part is taken out of shutdown mode, the internal clock oscil- lator is restarted and the conversion result is typically available 25 µs later. Reading from the device before conversion is com- plete will provide the same set of data. TEMPERATURE VALUE REGISTER The temperature value register is a read-only register that stores the temperature reading from the ADC in 10-bit twos comple- ment format. The temperature data format is shown in Table I. This shows the full theoretical range of the ADC from –128 °C to +127 °C, but in practice the temperature measurement range is limited to the operating temperature range of the device (–55 °C to +125 °C). A typical performance curve is shown in Figure 7. Table I. Temperature Data Format Digital Output Temperature DB9 . . . DB0 –128 °C 10 0000 0000 –125 °C 10 0000 1100 –100 °C 10 0111 0000 –75 °C 10 1101 0100 –50 °C 11 0011 1000 –25 °C 11 1001 1100 –0.25 °C 11 1111 1111 0 °C 00 0000 0000 +0.25 °C 00 0000 0001 +10 °C 00 0010 1000 +25 °C 00 0110 0100 +50 °C 00 1100 1000 +75 °C 01 0010 1100 +100 °C 01 1001 0000 +125 °C 01 1111 0100 +127 °C 01 1111 1100 SERIAL INTERFACE The serial interface on the AD7814 consists of four wires CS, SCLK, DIN, and DOUT. The interface can be operated in 3-wire mode with DIN tied to Ground, in which case the inter- face has read-only capability, with data being read from the data register via the DOUT line. The DIN line is used to write the part into standby mode, if required. The CS line is used to select the device when more than one device is connected to the serial clock and data lines. The part operates in a slave mode and requires an externally applied serial clock to the SCLK input to access data from the data register. The serial interface on the AD7814 is designed to allow the part to be interfaced to systems that provide a serial clock that is synchronized to the serial data, such as the 80C51, 87C51, 68HC11, 68HC05, and PIC16Cxx microcontrollers, as well as DSP processors. A read operation from the AD7814 accesses data from the temperature value register, while a write operation to the part writes data to the control register. |
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