ADT75 Data Sheet
Rev. B | Page 10 of 24
THEORY OF OPERATION
CIRCUIT INFORMATION
The ADT75 is a 12-bit digital temperature sensor with the 12
th
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 excellent
transducer linearity. The serial interface is SMBus /I
2
C- compatible
and the open-drain output of the ADT75 is capable of sinking 3 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 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 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. 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.
Σ-∆ MODULATOR
INTEGRATOR
COMPARATOR
1-BIT
DAC
LPF DIGITAL
FILTER
CLOCK
GENERATOR
VOLTAGE REF
AND VPTAT
TEMPERATURE
VALUE
REGISTER
1-BIT
12-BIT
+
–
05326-011
Figure 11. First-Order Σ-Δ Modulator
The modulated output of the comparator is encoded using a
circuit technique that results in SMBus/I
2
C temperature data.
FUNCTIONAL DESCRIPTION
The conversion clock for the part is generated internally. 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 because the SMBus/I
2
C circuitry never shuts down.
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.
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.
The measured temperature value is compared with a high
temperature limit, stored in the 16-bit T
OS
read/write register and
the hysteresis temperature limit, stored in the 16-bit T
HYST
read/
write register. If the measured value exceeds these limits then the
OS/ALERT pin is activated. This OS/ALERT pin is programmable
for mode and polarity via the configuration register.
