ADT7408
Preliminary Technical Data
Rev. PrC | Page 6 of 22
THEORY OF OPERATION
CIRCUIT INFORMATION
The ADT7408 is a 12-bit digital temperature sensor. Its output
is Two’s complement that the 12th bit is the sign bit. An on-
board sensor generates a voltage precisely proportional to
absolute temperature, which is compared to an internal voltage
reference and is input to a precision digital modulator. Overall
accuracy for the ADT7408 is ±2°C from 75°C to 95°C, ±3°C
from 40°C to +125°C, and ±4°C from -20°C to +125°C with
excellent transducer linearity. The serial interface is SMBus-
/I2C- compatible and the open-drain output of the ADT7408 is
capable of sinking 6 mA.
The on-board temperature sensor has excellent accuracy and
linearity over the entire rated temperature range without
needing correction or calibration by the user.
The sensor output is digitized by a first-order ∑- modulator,
also known as the charge balance type analog-to-digital
converter. This type of converter utilizes time-domain over-
sampling and a high accuracy comparator to deliver 12 bits of
effective accuracy in an extremely compact circuit.
CONVERTER DETAILS
The ∑- 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 that 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, called oversampling.
Oversampling spreads the quantization noise over a much
wider band than that of the input signal, improving overall
noise performance and increasing accuracy.
Figure 4. First-Order
∑- Modulator
The modulated output of the comparator is encoded using a
circuit technique that results in SMBus/I2C temperature data.
FUNCTIONAL DESCRIPTION
The conversion clock for the part is internally generated. No
external clock is required except when reading from and
writing to the serial port. In normal mode, the internal clock
oscillator runs an automatic conversion sequence. During this
automatic conversion sequence, a conversion is initiated every
100 ms. At this time, the part powers up its analog circuitry and
performs a temperature conversion. This temperature
conversion typically takes 60 ms, after which time the analog
circuitry of the part automatically shuts down. The analog
circuitry powers up again 40 ms later, when the 100 ms timer
times out and the next conversion begins. The result of the most
recent temperature conversion is always available in the
temperature value register as the SMBus/I2C circuitry never
shuts down.
The ADT7408 can be placed in a shutdown mode via the
configuration register, in which case the on-chip oscillator is
shut down and no further conversions are initiated until the
ADT7408 is taken out of shutdown mode. The ADT7408 can be
taken out of shutdown mode by writing zero to Bit D8 in the
configuration register. The conversion result from the last
conversion prior to shut-down can still be read from the
ADT7408 even when it is in shutdown mode.
In normal conversion mode, the internal clock oscillator is reset
after every read or write operation. This causes the device to
start a temperature conversion, the result of which is typically
available 60 ms later. Similarly, when the part is taken out of
shutdown mode, the internal clock oscillator is started and a
conversion is initiated. The conversion result is typically
available 60 ms later. Reading from the device before a
conversion is complete causes the ADT7408 to stop converting;
the part starts again when serial communication is finished.
This read operation provides the previous result.
The measured temperature value is compared with the
temperature set at the Alarm Temp Upper Boundary Trip
Register, the Alarm Temp Lower Boundary Trip Register, and
the Critical Temp Trip Register. If the measured value exceeds
these limits then the EVENT# pin is activated. This EVENT#
output is programmable for interrupt mode, comparator mode,
and also the output polarity via the configuration register.