ꢂT7408C
C
THEORYCOFCOPER TIONC
°IR°UIT INFORMATION
MODES OF OPERATION
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 that initiates a
conversion 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. Because the
SMBus/I2C circuitry never shuts down, the result of the most
recent temperature conversion is always available in the
temperature value register.
The ADT7408 is a 12-bit digital temperature sensor presented
in 13 bits, including the sign bit format (see the bit map in the
Temperature Value Register (Read Only) section). Its output is
twos complement in that Bit D12 is the sign bit and Bit D0 to
Bit D11 are data bits. An on-board sensor generates a voltage
precisely proportional to absolute temperature, which is
compared to an internal voltage reference and 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 ADT7408 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
ADT7408 is taken out of shutdown mode by writing 0 to Bit D8
in the configuration register. The conversion result from the last
conversion prior to shutdown can still be read from the ADT7408,
even when it is in shutdown mode.
A first-order ∑-Δ modulator, also known as the charge balance
type analog-to-digital converter (ADC), digitizes the sensor
output. This type of converter utilizes time domain oversampling
and a high accuracy comparator to deliver 12 bits of effective
accuracy in an extremely compact circuit.
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 starts, and a
conversion is initiated. The conversion result is typically available
60 ms later. Reading from the device before a conversion is com-
plete does not stop the ADT7408 from converting; the part does
not update the temperature value register immediately after the
conversion but waits until communication to the part is finished.
This read operation provides the previous result. It is possible to
miss a conversion result if the SCL frequency is very slow
(communication is greater than 40 ms), because the next
conversion will have started. There is a 40 ms window between
the end of one conversion and the start of the next conversion
for the temperature value register to be updated with a new
temperature value.
°ONVERTER DETAILS
The ∑-Δ modulator consists of an input sampler, a summing
network, an integrator, a comparator, and a 1-bit DAC, as
shown in Figure 10. 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. There are two simultaneous but different
sampling operations in the device. The comparator samples the
output of the integrator at a much higher rate than the input
sampling frequency, that is, oversampling. 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 SMBus/I2C temperature data.
Σ-∆ MODULATOR
The measured temperature value is compared with the
INTEGRATOR
COMPARATOR
temperature set at the alarm temperature upper boundary trip
register, the alarm temperature lower boundary trip register,
and the critical temperature 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 the output polarity via the configuration register.
+
VOLTAGE REF
AND VPTAT
+
–
–
1-BIT
DAC
1-BIT
The thermal sensor continuously monitors the temperature and
updates the temperature data 10 times per second. Temperature
data is latched internally by the device and can be read by
software from the bus host at any time.
CLOCK
GENERATOR
LPF DIGITAL
FILTER
TEMPERATURE
VALUE REGISTER
12-BIT
Figure 10. First-Order Σ-Δ Modulator
Rev. 0 | Page 8 of 24
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