Datasheet ADT7422 (Analog Devices) - 9

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Data Sheet ADT7422 THEORY OF OPERATION CIRCUIT INFORMATION NORMAL MODE
The ADT7422 is a high accuracy, digital I2C temperature sensor In normal mode (default power-up mode) the ADT7422 runs an that uses a 16-bit ADC to monitor and digitize the temperature automatic conversion sequence. During this automatic conversion to 0.0078°C of resolution. The ADC resolution, by default, is set sequence, a conversion typically takes 240 ms to complete. The to 13 bits (0.0625°C). An internal temperature sensor generates ADT7422 continuously converts during the sequence and as soon a voltage proportional to absolute temperature, which is compared as one temperature conversion is complete, another temperature to an internal voltage reference and input into a precision digital conversion begins. Each temperature conversion result is stored modulator (see Figure 13). in the 16-bit temperature value register, which consists of the The internal temperature sensor has high accuracy and linearity 8-bit temperature value most significant byte register and the over the entire rated temperature range without needing correction 8-bit temperature value least significant byte register (see Table 7), or calibration by the user. and is available through the I2C interface. In continuous conversion mode, the read operation provides the most recent The sensor output is digitized by a Σ-Δ modulator, also known as converted result. the charge balance type ADC. This type of converter utilizes time domain oversampling and a high accuracy comparator to At power-up, the first conversion is a fast conversion and deliver 16 bits of resolution in a compact circuit. typically takes 6 ms. If the temperature exceeds 147°C, the CT pin asserts low. If the temperature exceeds 64°C, the INT pin asserts
CONVERTER DETAILS
low. Fast conversion temperature accuracy is typically within ±5°C. The Σ-Δ modulator consists of an input sampler, a summing The conversion clock for the device is generated internally. network, an integrator, a comparator, and a 1-bit digital-to-analog No external clock is required except when reading from and converter (DAC). This architecture creates a negative feedback writing to the serial port. loop and minimizes the integrator output by changing the duty cycle of the comparator output in response to input voltage The measured temperature value is compared with a critical changes. The comparator samples the output of the integrator at a temperature limit (stored in the 16-bit TCRIT setpoint read/write higher rate than the input sampling frequency. This oversampling register), a high temperature limit (stored in the 16-bit THIGH spreads the quantization noise over a wider band than that of setpoint read/write register), and a low temperature limit the input signal, which improves overall noise performance and (stored in the 16-bit TLOW setpoint read/write register). If the increases accuracy. measured value exceeds these limits, the INT pin is activated. If the measured value exceeds the TCRIT limit, the CT pin is activated. The
Σ-Δ MODULATOR
INT pin and CT pin are programmable for both polarity via the
INTEGRATOR
configuration register (Register Address 0x03) and interrupt mode
COMPARATOR VOLTAGE
via the configuration register (Register Address 0x03).
REFERENCE AND V 1 PTAT ONE SHOT MODE
To enable one shot mode, set Bit 6 to 0 and Bit 5 to 1 in the
1-BIT
configuration register (Register Address 0x03). When this mode
DAC
is enabled, the ADT7422 immediately completes a conversion and then goes into shutdown mode.
1-BIT DIGITAL CLOCK TEMPERATURE
Wait for a minimum of 240 ms after writing to the operation mode
LOW-PASS GENERATOR VALUE FILTER 13-BIT/ REGISTER
bits before reading back the temperature from the temperature
16-BIT
010 value at Register Address 0x00 and Register Address 0x01, as
1VPTAT IS A VOLTAGE PROPORTIONAL TO ABSOLUTE TEMPERATURE.
961- 20 shown in Table 7. This time ensures that the ADT7422 has time Figure 13. Σ-Δ Modulator to power up and complete a conversion. The ADT7422 can operate in four operating modes: normal, one To obtain an updated temperature conversion, reset Bit 6 to 0 and shot, 1 SPS, and shutdown. Bit 5 to 1 in the configuration register (Register Address 0x03). One shot mode is useful when a circuit design priority is to reduce power consumption. Rev. 0 | Page 9 of 23 Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION PRODUCT HIGHLIGHTS FUNCTIONAL BLOCK DIAGRAM TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS I2C TIMING SPECIFICATIONS Timing Diagram ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE ESD CAUTION PIN CONFIGURATION AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION CIRCUIT INFORMATION CONVERTER DETAILS NORMAL MODE ONE SHOT MODE CT and INT Operation in One Shot Mode 1 SPS MODE SHUTDOWN MODE FAULT QUEUE TEMPERATURE DATA FORMAT TEMPERATURE CONVERSION FORMULAS 16-Bit Temperature Data Format 13-Bit Temperature Data Format 10-Bit Temperature Data Format 9-Bit Temperature Data Format REGISTERS ADDRESS POINTER REGISTER TEMPERATURE VALUE REGISTERS STATUS REGISTER CONFIGURATION REGISTER THIGH SETPOINT REGISTERS TLOW SETPOINT REGISTERS TCRIT SETPOINT REGISTERS THYST SETPOINT REGISTER ID REGISTER SERIAL INTERFACE SERIAL BUS ADDRESS WRITING DATA READING DATA RESET GENERAL CALL INT AND CT OUTPUTS UNDERTEMPERATURE AND OVERTEMPERATURE DETECTION Comparator Mode Interrupt Mode APPLICATIONS INFORMATION THERMAL RESPONSE TIME SUPPLY DECOUPLING POWERING FROM A SWITCHING REGULATOR TEMPERATURE MEASUREMENT QUICK GUIDE TO MEASURING TEMPERATURE OUTLINE DIMENSIONS ORDERING GUIDE