ADT7420 Datasheet, PDF (9/24 Pages) Analog Devices – ±0.25°C Accurate, 16-Bit Digital I2C Temperature Sensor

Preliminary Technical Data
ADT7420
THEORY OF OPERATION
CIRCUIT INFORMATION
The ADT7420 is a 13-bit digital temperature sensor that is extend-
able to 16 bits for greater resolution. An on-board temperature
sensor generates a voltage proportional to absolute temperature,
which is compared to an internal voltage reference and input to a
precision digital modulator.
The on-board temperature sensor has high accuracy and
linearity over the entire rated temperature range without
needing 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 analog-to-digital conver-
ter. This type of converter utilizes time-domain oversampling
and a high accuracy comparator to deliver 16 bits of resolution
in an extremely compact circuit.
Configuration register functions consist of
 Switching between 13-bit and 16-bit resolution
 Switching between normal operation and full power-down
 Switching between comparator and interrupt event modes
on the INT and CT pins
 Setting the active polarity of the CT and INT pins
 Setting the number of faults that activate CT and INT
 Enabling the standard one-shot mode and 1 SPS mode
CONVERTER DETAILS
The Σ-Δ modulator consists of an input sampler, a summing
network, an integrator, a comparator, and a 1-bit 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 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 I²C temperature data.
VOLTAGE REF
AND VPTAT
Σ-∆ MODULATOR
INTEGRATOR
COMPARATOR
CLOCK
GENERATOR
1-BIT
DAC
1-BIT
LPF DIGITAL
FILTER
TEMPERATURE
VALUE
13-BIT REGISTER
Figure 11. Σ-Δ Modulator
TEMPERATURE MEASUREMENT
In normal mode, the ADT7420 runs an automatic conversion
sequence. During this automatic conversion sequence, a conver-
sion takes 240 ms to complete and the ADT7420 is continuously
converting. This means that as soon as one temperature conver-
sion is completed, another temperature conversion begins. Each
temperature conversion result is stored in the temperature value
registers and is available through the I²C 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
asserts low. If the temperature exceeds 64°C, the INT pin asserts
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 TCRIT setpoint read/write
register), a high temperature limit (stored in the 16-bit THIGH set-
point read/write register), and a low temperature limit (stored
in the 16-bit TLOW setpoint read/write register). If the measured
value exceeds these limits, the INT pin is activated; and if it exceeds
the TCRIT limit, the CT pin is activated. The INT and CT pins are
programmable for polarity via the configuration register, and the
INT and CT pins are also programmable for interrupt mode via
the configuration register.
ONE-SHOT MODE
Setting Bit 5 to 0 and Bit 6 to 1 of the configuration register
(Register Address 0x03) enables the one-shot mode. When
this mode is enabled, the ADT7420 immediately completes a
conversion and then goes into shutdown mode.
Wait for a minimum of 240 ms after writing to the operation
mode bits to the one-shot bits before reading back the
temperature from the temperature value register. This time
ensures that the ADT7420 has time to power up and complete a
conversion.
The one-shot mode is useful when one of the circuit design
priorities is to reduce power consumption.
1 SPS MODE
In this mode, the part performs one measurement per second.
A conversion takes only 60 ms, and it remains in the idle state
for the remaining 940 ms period. This mode is enabled by
writing 1 to Bit 5 and 0 to Bit 6 of the configuration register
(Register Address 0x03).
Rev. PrA | Page 9 of 24