ꢁT75C
C
THEORYCOFCOPER TIONC
CIRCUIT INFORMATION
FUNCTIONAL DESCRIPTION
The ADT75 is a 12-bit digital temperature sensor with the 12th bit
acting as the sign bit. An on-board temperature sensor generates a
voltage precisely proportional to absolute temperature that is
compared to an internal voltage reference and input to a precision
digital modulator. Overall accuracy for the ADT75 A Grade is
2°C from −25°C to +100°C and accuracy for the ADT75 B Grade
is 1°C from 0°C to +70°C. Both grades have eꢀcellent transducer
linearity. The serial interface is SMBus /I2C- compatible and the
open-drain output of the ADT75 is capable of sinking 3 mA.
The conversion clock for the part is generated internally. No
eꢀternal clock is required eꢀcept 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 neꢀt conversion begins. The result of the
most recent temperature conversion is always available in the
temperature value register because the SMBus/I2C circuitry never
shuts down.
The on-board temperature sensor has eꢀcellent 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 ∑-Δ 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 eꢀtremely compact circuit.
The ADT75 can be placed in shutdown mode via the
configuration register, in which case the on-chip oscillator is
shut down and no further conversions are initiated until the
ADT75 is taken out of shutdown mode. The ADT75 can be
taken out of shutdown mode by writing 0 to Bit D0 in the
configuration register. The ADT75 typically takes 1.7 ms to
come out of shutdown mode. The conversion result from the
last conversion prior to shutdown can still be read from the
ADT75 even when it is in shutdown mode.
CONVERTER DETAILS
The ∑-Δ modulator consists of an input sampler, a summing
network, an integrator, a comparator, and a 1-bit DAC. Similar
to the voltage-to-frequency converter, 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 is 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.
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
ADT75 to stop converting; the part starts again when serial
communication is finished. This read operation provides the
previous conversion result.
Σ-Δ MODULATOR
INTEGRATOR
COMPARATOR
VOLTAGE REF
AND VPTAT
+
–
1-BIT
DAC
The measured temperature value is compared with a high
temperature limit, stored in the 16-bit TOS read/write register
and the hysteresis temperature limit, stored in the 16-bit
1-BIT
LPF DIGITAL
FILTER
TEMPERATURE
VALUE
CLOCK
GENERATOR
THYST read/write register. If the measured value eꢀceeds these
REGISTER
12-BIT
limits then the OS/ALERT pin is activated. This OS/ALERT
pin is programmable for mode and polarity via the configura-
tion register.
Figure 11. First-Order ∑-Δ Modulator
The modulated output of the comparator is encoded using a
circuit technique that results in SMBus/I2C temperature data.
Rev. 0 | Page 10 of 24
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