Datasheet ADT75 (Analog Devices) - 10

ADT75 Data Sheet THEORY OF OPERATION CIRCUIT INFORMATION FUNCTIONAL DESCRIPTION
The ADT75 is a 12-bit digital temperature sensor with the 12th The conversion clock for the part is generated internally. No bit acting as the sign bit. An on-board temperature sensor generates external clock is required except when reading from and writing to a voltage precisely proportional to absolute temperature that is the serial port. In normal mode, the internal clock oscil ator runs compared to an internal voltage reference and input to a precision an automatic conversion sequence. During this automatic digital modulator. Overall accuracy for the ADT75 A Grade is conversion sequence, a conversion is initiated every 100 ms. ±2°C from −25°C to +100°C and accuracy for the ADT75 B At this time, the part powers up its analog circuitry and performs Grade is ±1°C from 0°C to +70°C. Both grades have excellent a temperature conversion. transducer linearity. The serial interface is SMBus /I2C- compatible and the open-drain output of the ADT75 is capable of sinking 3 mA. This temperature conversion typical y takes 60 ms, after which time the analog circuitry of the part automatical y shuts down. The analog The on-board temperature sensor has excellent accuracy and circuitry powers up again 40 ms later, when the 100 ms timer times linearity over the entire rated temperature range without needing out and the next conversion begins. The result of the most recent correction or calibration by the user. temperature conversion is always available in the temperature value register because the SMBus/I2C circuitry never shuts down. The sensor output is digitized by a first-order Σ-Δ modulator, also known as the charge balance type ADC. This type of converter The ADT75 can be placed in shutdown mode via the configuration uses time-domain oversampling and a high accuracy comparator to register, in which case the on-chip oscillator is shut down and deliver 12 bits of effective accuracy in an extremely compact circuit. no further conversions are initiated until the ADT75 is taken out of shutdown mode. The ADT75 can be taken out of shutdown mode
CONVERTER DETAILS
by writing 0 to Bit D0 in the configuration register. The ADT75 The ∑-∆ modulator consists of an input sampler, a summing typical y takes 1.7 ms to come out of shutdown mode. The network, an integrator, a comparator, and a 1-bit DAC. Similar conversion result from the last conversion prior to shutdown can to the voltage-to-frequency converter, this architecture creates a still be read from the ADT75 even when it is in shutdown mode. negative feedback loop and minimizes the integrator output by changing the duty cycle of the comparator output in response to In normal conversion mode, the internal clock oscillator is reset input voltage changes. The comparator samples the output of the after every read or write operation. This causes the device to start a integrator at a much higher rate than the input sampling frequency; temperature conversion, the result of which is typically available this is called oversampling. Oversampling spreads the quantization 60 ms later. Similarly, when the part is taken out of shutdown noise over a much wider band than that of the input signal, mode, the internal clock oscillator is started and a conversion is improving overall noise performance and increasing accuracy. initiated. Σ The conversion result is typically available 60 ms later. Reading
-
∆
MODULATOR INTEGRATOR
from the device before a conversion is complete causes the
COMPARATOR
ADT75 to stop converting; the part starts again when serial
VOLTAGE REF + AND VPTAT
communication is finished. This read operation provides the
–
previous conversion result.
1-BIT DAC
The measured temperature value is compared with a high temperature limit, stored in the 16-bit TOS read/write register and
1-BIT TEMPERATURE
the hysteresis temperature limit, stored in the 16-bit T
CLOCK LPF DIGITAL
HYST read/
VALUE GENERATOR FILTER REGISTER
write register. If the measured value exceeds these limits then the
12-BIT
05326-011 OS/ALERT pin is activated. This OS/ALERT pin is programmable Figure 11. First-Order Σ-Δ Modulator for mode and polarity via the configuration register. The modulated output of the comparator is encoded using a circuit technique that results in SMBus/I2C temperature data. Rev. B | Page 10 of 24 Document Outline Features Applications Product Highlights Functional Block Diagram Table of Contents Revision History General Description Specifications A Grade B Grade Timing Specifications and Diagram Absolute Maximum Ratings ESD Caution Pin Configuration and Function Descriptions Typical Performance Characteristics Theory of Operation Circuit Information Converter Details Functional Description Temperature Data Format Temperature Conversion Formulas One-Shot Mode Fault Queue Registers Address Pointer Register Temperature Value Register Configuration Register THYST Setpoint Register TOS Setpoint Register Serial Interface Serial Bus Address Writing Data Writing to the Address Pointer Register for a Subsequent Read Writing Data to a Register Reading Data OS/Alert Output OverTemperature Modes Comparator Mode Interrupt Mode SMBus Alert Applications Information Thermal Response Time Self-Heating Effects Supply Decoupling Temperature Monitoring Outline Dimensions Ordering Guide