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AD7415ARTZ-1REEL7 Arkusz danych(PDF) 7 Page - Analog Devices |
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AD7415ARTZ-1REEL7 Arkusz danych(HTML) 7 Page - Analog Devices |
7 / 20 page AD7414/AD7415 Rev. F | Page 7 of 20 THEORY OF OPERATION CIRCUIT INFORMATION The AD7414/AD7415 are standalone digital temperature sensors. The on-chip temperature sensor allows an accurate measurement of the ambient device temperature to be made. The 10-bit analog-to-digital converter converts the temperature measured into a twos complement format for storage in the temperature register. The ADC is made up of a conventional successive-approximation converter based around a capacitor digital-to-analog (DAC). The serial interface is I2C-and SMBus- compatible. The AD7414/AD7415 require a 2.7 V to 5.5 V power supply. The temperature sensor has a working measurement range of −40°C to +125°C. FUNCTIONAL DESCRIPTION Temperature measurement is initiated by two methods. The first uses an internal clock countdown of 800 ms, and a conversion is performed. The internal oscillator is the only circuit that is powered up between conversions, and once it times out, every 800 ms, a wake-up signal is sent to power up the rest of the circuitry. A monostable is activated at the beginning of the wake-up signal to ensure that sufficient time is given to the power-up process. The monostable typically takes 4 μs to time out. It then takes typically 25 μs for each conversion to be completed. The new temperature value is loaded into the temperature value register and ready for reading by the I2C interface. A temperature measurement is also initiated every time the one-shot method is used. This method requires the user to write to the one-shot bit in the configuration register when a temperature measurement is needed. Setting the one-shot bit to 1 starts a temperature conversion directly after the write operation. The track-and-hold goes into hold approximately 4 μs (monostable time out) after the STOP condition, and a conversion is then initiated. Typically 25 μs later, the conversion is complete and the temperature value register is loaded with a new temperature value. The measurement modes are compared with a high tempera- ture limit, stored in an 8-bit read/write register. This is applica- ble only to the AD7414, because the AD7415 does not have an ALERT pin and subsequently does not have an overtemperature monitoring function. If the measurement is greater than the high limit, the ALERT pin is activated (if it has already been enabled in the configuration register). There are two ways to deactivate the ALERT pin again: when the alert reset bit in the configuration register is set to 1 by a write operation, and when the temperature measured is less than the value in the TLOW register. This ALERT pin is compatible with the SMBus SMBALERT option. Configuration functions consist of • Switching between normal operation and full power- down • Enabling or disabling the SCL and SDA filters • Enabling or disabling the ALERT function • Setting the ALERT pin polarity SUPPLY 2.7V TO 5.5V 10μF 1k Ω 0.1 μF SDA SCL ALERT AS GND AD7414 VDD 10k Ω 10kΩ 10kΩ VDD VDD VDD μC/μP Figure 6. Typical Connection Diagram MEASUREMENT TECHNIQUE A common method of measuring temperature is to exploit the negative temperature coefficient of a diode, or the base-emitter voltage of a transistor, operated at constant current. Unfortunately, this technique requires calibration to null the effect of the absolute value of VBE, which varies from device to device. The technique used in the AD7414/AD7415 is to measure the change in VBE when the device is operated at two different currents. This is given by ( ) N ln q KT V BE × = Δ where: K is Boltzmann’s constant. q is the charge on the electron (1.6 × 10–19 Coulombs). T is the absolute temperature in Kelvins. N is the ratio of the two currents. |
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