Data Sheet ADT7320
Rev. 0 | Page 9 of 24
THEORY OF OPERATION
CIRCUIT INFORMATION
The ADT7320 is a high accuracy digital temperature sensor that
uses a 16-bit ADC to monitor and digitize the temperature to a
resolution of 0.0078°C. By default, the ADC resolution is set to
13 bits (0.0625°C). An internal temperature sensor generates a
voltage proportional to absolute temperature; this voltage is
compared 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 the need for
correction or calibration by the user.
The sensor output is digitized by a sigma-delta (Σ-Δ) modulator,
also known as the charge balance type ADC. This type of converter
uses 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 digital-to-analog
converter (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 the 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.
The modulated output of the comparator is encoded using a
circuit technique that results in SPI temperature data.
Σ-∆ MODULATOR
INTEGRATOR
COMPARATOR
TEMPERATURE
VALUE
REGISTER
CLOCK
GENERATOR
LPF DIGITAL
FILTER
1-BIT
DAC
VOLTAGE
REFERENCE
1-BIT
13-/16-BIT
09012-012
Figure 11. Σ-∆ Modulator
The ADT7320 can be configured to operate in any one of the
following four operating modes: normal mode, one-shot mode,
1 SPS mode, and shutdown mode.
CONTINUOUS CONVERSION MODE
In continuous conversion mode (default power-up mode), the
ADT7320 runs an automatic conversion sequence. During this
automatic conversion sequence, a conversion takes 240 ms to
complete and the ADT7320 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 register and is available
through the SPI 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 is
asserted low. If the temperature exceeds 64°C, the INT pin is
asserted 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 T
CRIT
setpoint register), a
high temperature limit (stored in the 16-bit T
HIGH
setpoint
register), and a low temperature limit (stored in the 16-bit T
LOW
setpoint register). If the measured value exceeds the high or low
temperature limits, the INT pin is activated; if it exceeds the T
CRIT
limit, the CT pin is activated. The polarity of the INT and CT
pins is programmable for via the configuration register, and the
INT and CT pins are also programmable for interrupt mode via
the configuration register.
