Datasheet ADT7408 (Analog Devices) - 8

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ADT7408 Data Sheet THEORY OF OPERATION CIRCUIT INFORMATION MODES OF OPERATION
The ADT7408 is a 12-bit digital temperature sensor presented The conversion clock for the part is internally generated. in 13 bits, including the sign bit format (see the bit map in the No external clock is required except when reading from and Temperature Value Register (Read Only) section). Its output is writing to the serial port. In normal mode, the internal clock twos complement in that Bit D12 is the sign bit and Bit D0 to oscillator runs an automatic conversion sequence that initiates Bit D11 are data bits. An on-board sensor generates a voltage a conversion every 100 ms. At this time, the part powers up its precisely proportional to absolute temperature, which is analog circuitry and performs a temperature conversion. This compared to an internal voltage reference and input to a temperature conversion typically takes 60 ms, after which time precision digital modulator. Overall accuracy for the ADT7408 the analog circuitry of the part automatically shuts down. The is ±2°C from +75°C to +95°C, ±3°C from +40°C to +125°C, and analog circuitry powers up again 40 ms later, when the 100 ms ±4°C from −20°C to +125°C, with excellent transducer linearity. timer times out and the next conversion begins. Because the The serial interface is SMBus-/I2C-compatible, and the open- SMBus/I2C circuitry never shuts down, the result of the most drain output of the ADT7408 is capable of sinking 6 mA. recent temperature conversion is always available in the The on-board temperature sensor has excellent accuracy and temperature value register. linearity over the entire rated temperature range without The ADT7408 can be placed in shutdown mode via the needing correction or calibration by the user. configuration register, in which case the on-chip oscillator is A first-order, ∑-Δ modulator, also known as the charge balance shut down, and no further conversions are initiated until the type analog-to-digital converter (ADC), digitizes the sensor ADT7408 is taken out of shutdown mode by writing 0 to Bit D8 output. This type of converter uses time domain oversampling and in the configuration register. The conversion result from the last a high accuracy comparator to deliver 12 bits of effective conversion prior to shutdown can still be read from the ADT7408, accuracy in an extremely compact circuit. even when it is in shutdown mode.
CONVERTER DETAILS
In normal conversion mode, the internal clock oscillator is reset after every read or write operation. This causes the device to The Σ-Δ modulator consists of an input sampler, a summing start a temperature conversion, the result of which is typically network, an integrator, a comparator, and a 1-bit DAC, as shown available 60 ms later. Similarly, when the part is taken out of in Figure 10. This architecture creates a negative feedback loop that shutdown mode, the internal clock oscillator starts, and a minimizes the integrator output by changing the duty cycle of conversion is initiated. The conversion result is typically available the comparator output in response to input voltage changes. 60 ms later. Reading from the device before a conversion is com- There are two simultaneous but different sampling operations plete does not stop the ADT7408 from converting; the part does in the device. The comparator samples the output of the integrator not update the temperature value register immediately after the at a much higher rate than the input sampling frequency, that is, conversion but waits until communication to the part is finished. oversampling. Oversampling spreads the quantization noise This read operation provides the previous result. It is possible to over a much wider band than that of the input signal, improving miss a conversion result if the SCL frequency is very slow overall noise performance and increasing accuracy. (communication is greater than 40 ms), because the next The modulated output of the comparator is encoded using a conversion will have started. There is a 40 ms window between circuit technique that results in SMBus/I2C temperature data. the end of one conversion and the start of the next conversion for the temperature value register to be updated with a new
Σ-∆ MODULATOR INTEGRATOR
temperature value.
COMPARATOR + VOLTAGE REF
The measured temperature value is compared with the
AND VPTAT +
temperature set at the alarm temperature upper boundary trip
– –
register, the alarm temperature lower boundary trip register,
1-BIT DAC
and the critical temperature trip register. If the measured value exceeds these limits, then the EVENT# pin is activated. This
1-BIT
EVENT# output is programmable for interrupt mode, comparator
CLOCK LPF DIGITAL TEMPERATURE
mode, and the output polarity via the configuration register.
GENERATOR FILTER 12-BIT VALUE REGISTER
05 The thermal sensor continuously monitors the temperature and 0 6- 571 updates the temperature data 10 times per second. Temperature 0 Figure 10. First-Order, Σ-Δ Modulator data is latched internally by the device and can be read by software from the bus host at any time. Rev. A | Page 8 of 22 Document Outline FEATURES APPLICATIONS FUNCTIONAL BLOCK DIAGRAM GENERAL DESCRIPTION TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS TIMING CHARACTERISTICS TIMING DIAGRAM ABSOLUTE MAXIMUM RATINGS ESD CAUTION PIN CONFIGURATION AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION CIRCUIT INFORMATION CONVERTER DETAILS MODES OF OPERATION REGISTERS ADDRESS POINTER REGISTER (WRITE ONLY) CAPABILITY REGISTER (READ ONLY) CONFIGURATION REGISTER (READ/WRITE) TEMPERATURE TRIP POINT REGISTERS Alarm Temperature Upper Boundary Trip Register (Read/Write) Alarm Temperature Lower Boundary Trip Register (Read/Write) Critical Temperature Trip Register (Read/Write) Temperature Value Register (Read Only) ID REGISTERS Manufacturer ID Register (Read Only) Device ID and Revision Register (Read Only) TEMPERATURE DATA FORMAT 12-Bit Temperature Data Format 10-Bit Temperature Data Format EVENT PIN FUNCTIONALITY Event Thresholds Alarm Window Trip Critical Trip Interrupt Mode Comparator Mode SERIAL INTERFACE Serial Bus Address SMBUS/I2C COMMUNICATIONS Writing Data to a Register Reading Data From the ADT7408 Writing to the Pointer Register for a Subsequent Read Reading from Any Pointer Register APPLICATIONS INFORMATION THERMAL RESPONSE TIME SELF-HEATING EFFECTS SUPPLY DECOUPLING TEMPERATURE MONITORING OUTLINE DIMENSIONS ORDERING GUIDE