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ADT7408 データシート(PDF) 8 Page - Analog Devices |
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ADT7408 データシート(HTML) 8 Page - Analog Devices |
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8 / 24 page  ADT7408 Rev. 0 | Page 8 of 24 THEORY OF OPERATION CIRCUIT INFORMATION The ADT7408 is a 12-bit digital temperature sensor presented in 13 bits, including the sign bit format (see the bit map in the Temperature Value Register (Read Only) section). Its output is twos complement in that Bit D12 is the sign bit and Bit D0 to Bit D11 are data bits. An on-board sensor generates a voltage precisely proportional to absolute temperature, which is compared to an internal voltage reference and input to a precision digital modulator. Overall accuracy for the ADT7408 is ±2°C from 75°C to 95°C, ±3°C from 40°C to +125°C, and ±4°C from −20°C to +125°C, with excellent transducer linearity. The serial interface is SMBus-/I2C-compatible, and the open- drain output of the ADT7408 is capable of sinking 6 mA. The on-board temperature sensor has excellent accuracy and linearity over the entire rated temperature range without needing correction or calibration by the user. A first-order ∑-Δ modulator, also known as the charge balance type analog-to-digital converter (ADC), digitizes the sensor output. This type of converter utilizes time domain oversampling and a high accuracy comparator to deliver 12 bits of effective accuracy in an extremely compact circuit. CONVERTER DETAILS The ∑-Δ modulator consists of an input sampler, a summing network, an integrator, a comparator, and a 1-bit DAC, as shown in Figure 10. This architecture creates a negative feedback loop that minimizes the integrator output by changing the duty cycle of the comparator output in response to input voltage changes. There are two simultaneous but different sampling operations in the device. The comparator samples the output of the integrator at a much higher rate than the input sampling frequency, that is, oversampling. Oversampling spreads the quantization noise over a much wider band than that of the input signal, improving overall noise performance and increasing accuracy. The modulated output of the comparator is encoded using a circuit technique that results in SMBus/I2C temperature data. Σ-∆ MODULATOR INTEGRATOR VOLTAGE REF AND VPTAT CLOCK GENERATOR LPF DIGITAL FILTER TEMPERATURE VALUE REGISTER COMPARATOR 12-BIT 1-BIT – 1-BIT DAC + – + Figure 10. First-Order Σ-Δ Modulator MODES OF OPERATION 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, the internal clock oscillator runs an automatic conversion sequence that initiates a conversion every 100 ms. At this time, the part powers up its analog circuitry and performs a temperature conversion. This temperature conversion typically takes 60 ms, after which time the analog circuitry of the part automatically shuts down. The analog circuitry powers up again 40 ms later, when the 100 ms timer times out and the next conversion begins. Because the SMBus/I2C circuitry never shuts down, the result of the most recent temperature conversion is always available in the temperature value register. The ADT7408 can be placed in shutdown mode via the configuration register, in which case the on-chip oscillator is shut down, and no further conversions are initiated until the ADT7408 is taken out of shutdown mode by writing 0 to Bit D8 in the configuration register. The conversion result from the last conversion prior to shutdown can still be read from the ADT7408, 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 60 ms later. Similarly, when the part is taken out of shutdown mode, the internal clock oscillator starts, and a conversion is initiated. The conversion result is typically available 60 ms later. Reading from the device before a conversion is com- plete does not stop the ADT7408 from converting; the part does not update the temperature value register immediately after the conversion but waits until communication to the part is finished. This read operation provides the previous result. It is possible to miss a conversion result if the SCL frequency is very slow (communication is greater than 40 ms), because the next conversion will have started. There is a 40 ms window between the end of one conversion and the start of the next conversion for the temperature value register to be updated with a new temperature value. The measured temperature value is compared with the temperature set at the alarm temperature upper boundary trip register, the alarm temperature lower boundary trip register, and the critical temperature trip register. If the measured value exceeds these limits, then the EVENT# pin is activated. This EVENT# output is programmable for interrupt mode, comparator mode, and the output polarity via the configuration register. The thermal sensor continuously monitors the temperature and updates the temperature data 10 times per second. Temperature data is latched internally by the device and can be read by software from the bus host at any time. |
同様の部品番号 - ADT7408 |
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同様の説明 - ADT7408 |
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