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AD7417ARU-REEL7 Datasheet(PDF) 12 Page - Analog Devices |
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AD7417ARU-REEL7 Datasheet(HTML) 12 Page - Analog Devices |
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12 / 24 page  AD7416/AD7417/AD7418 Rev. I | Page 12 of 24 An external 2.5 V reference can be connected to the REFIN pin. This has the effect of shutting down the on-chip reference circuitry. REFIN SW1 26kΩ 2.5V 24kΩ 1.2V 1.2V EXTERNAL REFERENCE DETECT BUFFER Figure 11. On-Chip Reference TEMPERATURE MEASUREMENT 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 a constant current. Unfortu- nately, this technique requires calibration to null out the effect of the absolute value of VBE, which varies from device to device. The technique used in the AD7416/AD7417/AD7418 is to measure the current change in VBE when the device is operated at two different currents. This is given by () N q KT V BE n 1 / × = 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. SENSING TRANSISTOR TO ADC SENSING TRANSISTOR VOUT+ VDD VOUT– IN × I Figure 12. Temperature Measurement Technique Figure 12 shows the method the AD7416/AD7417/AD7418 use to measure the device temperature. To measure ΔVBE, the sensor (substrate transistor) is switched between operating currents of I and N × I. The resulting waveform is passed through a chopper-stabilized amplifier that performs the functions of amplification and rectification of the waveform to produce a dc voltage proportional to ΔVBE. This voltage is measured by the ADC to give a temperature output in 10-bit twos complement form. The temperature resolution of the ADC is 0.25°C, which corres- ponds to 1 LSB of the ADC. The ADC can theoretically measure a temperature span of 255°C; the guaranteed temperature range is −40°C to +125°C. The result of the conversion is stored in the temperature value register (0x00) as a 16-bit word. The 10 MSBs of this word store the temperature measurement (see Table 9 and Table 10). The temperature conversion formulas using the 10 MSBs of the temperature value register are Positive Temperature = ADC Code/4 (1) Negative Temperature = (ADC Code − 512)/4 (2) The MSB is removed from ADC Code in Equation 2. INTERNAL REGISTER STRUCTURE The AD7417/AD7418 have seven internal registers, as shown in Figure 13. Six of these are data registers and one is an address pointer register. The AD7416 has five internal registers (the ADC and Config2 registers are not applicable to the AD7416). TEMPERATURE VALUE REGISTER (READ-ONLY ADDRESS 0x00) CONFIGURATION REGISTER (READ/WRITE ADDRESS 0x01) THYST SETPOINT REGISTER (READ/WRITE ADDRESS 0x02) TOTI SETPOINT REGISTER (READ/WRITE ADDRESS 0x03) ADC VALUE REGISTER (READ-ONLY ADDRESS 0x04) CONFIG2 REGISTER (READ/WRITE ADDRESS 0x05) SDA DATA SCL ADDRESS POINTER REGISTER (SELCTS DATA REGISTER FOR READ/WRITE) ADDRESS SERIAL BUS INTERFACE Figure 13. AD7417/AD7418 Register Structure Address Pointer Register The address pointer register is an 8-bit register that stores an address that points to one of the six data registers. The first data byte of every serial write operation to the AD7416/AD7417/ AD7418 is the address of one of the data registers, which is stored in the address pointer register, and selects the data register to which subsequent data bytes are written. Only the three LSBs of the address pointer register are used to select a data register. Table 7. Address Pointer Register P71 P61 P51 P41 P31 P2 P1 P0 0 0 0 0 0 Register select 1 P3 to P7 must be set to 0. |
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