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ADT75BRMZ Datasheet(PDF) 10 Page - Analog Devices |
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ADT75BRMZ Datasheet(HTML) 10 Page - Analog Devices |
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10 / 24 page  ADT75 Rev. 0 | Page 10 of 24 THEORY OF OPERATION CIRCUIT INFORMATION The ADT75 is a 12-bit digital temperature sensor with the 12th bit acting as the sign bit. An on-board temperature sensor generates a voltage precisely proportional to absolute temperature that is compared to an internal voltage reference and input to a precision digital modulator. Overall accuracy for the ADT75 A Grade is ±2°C from −25°C to +100°C and accuracy for the ADT75 B Grade is ±1°C from 0°C to +70°C. Both grades have excellent transducer linearity. The serial interface is SMBus /I2C- compatible and the open-drain output of the ADT75 is capable of sinking 3 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. The sensor output is digitized by a first-order ∑-Δ modulator, also known as the charge balance type analog-to-digital converter. This type of converter utilizes time-domain over- sampling 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. Similar to the voltage-to-frequency converter, 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 is called oversampling. Oversampling spreads the quantization noise over a much wider band than that of the input signal, improving overall noise performance and increasing accuracy. Σ-Δ MODULATOR INTEGRATOR COMPARATOR 1-BIT DAC LPF DIGITAL FILTER CLOCK GENERATOR VOLTAGE REF AND VPTAT TEMPERATURE VALUE REGISTER 1-BIT 12-BIT + – Figure 11. First-Order ∑-Δ Modulator The modulated output of the comparator is encoded using a circuit technique that results in SMBus/I2C temperature data. FUNCTIONAL DESCRIPTION The conversion clock for the part is generated internally. 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. During this automatic conversion sequence, a conversion is initiated 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. The result of the most recent temperature conversion is always available in the temperature value register because the SMBus/I2C circuitry never shuts down. The ADT75 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 ADT75 is taken out of shutdown mode. The ADT75 can be taken out of shutdown mode by writing 0 to Bit D0 in the configuration register. The ADT75 typically takes 1.7 ms to come out of shutdown mode. The conversion result from the last conversion prior to shutdown can still be read from the ADT75 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 is started and a conversion is initiated. The conversion result is typically available 60 ms later. Reading from the device before a conversion is complete causes the ADT75 to stop converting; the part starts again when serial communication is finished. This read operation provides the previous conversion result. The measured temperature value is compared with a high temperature limit, stored in the 16-bit TOS read/write register and the hysteresis temperature limit, stored in the 16-bit THYST read/write register. If the measured value exceeds these limits then the OS/ALERT pin is activated. This OS/ALERT pin is programmable for mode and polarity via the configura- tion register. |
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