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ADT7420UCPZ-RL7 Datasheet(PDF) 10 Page - Analog Devices |
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ADT7420UCPZ-RL7 Datasheet(HTML) 10 Page - Analog Devices |
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10 / 24 page  ADT7420 Data Sheet Rev. 0 | Page 10 of 24 THEORY OF OPERATION CIRCUIT INFORMATION The ADT7420 is a high accuracy digital temperature sensor that uses a 16-bit ADC to monitor and digitize the temperature to 0.0078°C of resolution. The ADC resolution, by default, is set to 13 bits (0.0625°C). An internal temperature sensor generates a voltage proportional to absolute temperature, which is com- pared to an internal voltage reference and input into a precision digital modulator. The internal temperature sensor has high accuracy and linearity over the entire rated temperature range without needing correc- tion or calibration by the user. The sensor output is digitized by a sigma-delta (Σ-Δ) modulator, also known as the charge balance type analog-to-digital conver- ter. This type of converter utilizes time-domain oversampling and a high accuracy comparator to deliver 16 bits of resolution in an extremely compact circuit. CONVERTER DETAILS The Σ-Δ modulator consists of an input sampler, a summing network, an integrator, a comparator, and a 1-bit DAC. This architecture creates a negative feedback loop and minimizes the integrator output by changing the duty cycle of the comparator output in response to input voltage changes. The comparator samples the output of the integrator at a much higher rate than the input sampling frequency. This oversampling spreads the quantization noise over a much wider band than that of the input signal, improving overall noise performance and increasing accuracy. Σ-∆ MODULATOR INTEGRATOR COMPARATOR TEMPERATURE VALUE REGISTER CLOCK GENERATOR LPF DIGITAL FILTER 1-BIT DAC VOLTAGE REF AND VPTAT 1-BIT 13-BIT/ 16-BIT Figure 11. Σ-Δ Modulator The ADT7420 can be configured to operate in any one of the following four operating modes: normal, one-shot, 1 SPS, and shutdown. NORMAL MODE In normal mode (default power-up mode) the ADT7420 runs an automatic conversion sequence. During this automatic con- version sequence, a conversion typically takes 240 ms to complete and the ADT7420 is continuously converting. This means that as soon as one temperature conversion is completed, another temperature conversion begins. Each temperature conversion result is stored in the temperature value registers and is available through the I2C interface. In continuous conversion mode, the read operation provides the most recent converted result. On power-up, the first conversion is a fast conversion, taking typically 6 ms. If the temperature exceeds 147°C, the CT pin asserts low. If the temperature exceeds 64°C, the INT pin asserts low. Fast conversion temperature accuracy is typically within ±5°C. The conversion clock for the part is generated internally. No external clock is required except when reading from and writing to the serial port. The measured temperature value is compared with a critical temperature limit (stored in the 16-bit TCRIT setpoint read/write register), a high temperature limit (stored in the 16-bit THIGH set- point read/write register), and a low temperature limit (stored in the 16-bit TLOW setpoint read/write register). If the measured value exceeds these limits, the INT pin is activated; and if it exceeds the TCRIT limit, the CT pin is activated. The INT and CT pins are programmable for polarity via the configuration register, and the INT and CT pins are also programmable for interrupt mode via the configuration register. ONE-SHOT MODE Setting Bit 6 to 0 and Bit 5 to 1 of the configuration register (Register Address 0x03) enables the one-shot mode. When this mode is enabled, the ADT7420 immediately completes a conversion and then goes into shutdown mode. Wait for a minimum of 240 ms after writing to the operation mode bits before reading back the temperature from the tem- perature value register. This time ensures that the ADT7420 has time to power up and complete a conversion. To obtain an updated temperature conversion, reset Bit 6 to 0 and Bit 5 to 1 in the configuration register (0x03). The one-shot mode is useful when one of the circuit design priorities is to reduce power consumption. |
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