Datasheet
ADT7420 Data Sheet
Rev. 0 | Page 10 of 24
THEORY OF OPERATION
CIRCUIT INFORMATION
The ADT7420 is a high accuracy digital temperature sensor that
uses a 16-bit ADC to monitor and digitize the temperature to
0.0078°C of resolution. The ADC resolution, by default, is set to
13 bits (0.0625°C). An internal temperature sensor generates a
voltage proportional to absolute temperature, which is com-
pared to an internal voltage reference and input into a precision
digital modulator.
The internal temperature sensor has high accuracy and linearity
over the entire rated temperature range without needing correc-
tion 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.
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.
Σ-∆ MODULATOR
INTEGRATOR
COMPARATOR
TEMPERATURE
VALUE
REGISTER
CLOCK
GENERATOR
LPF DIGITAL
FILTER
1-BIT
DAC
VOLTAGE REF
AND VPTAT
1-BIT
13-BIT/
16-BIT
09013-012
Figure 11. Σ-Δ Modulator
The ADT7420 can be configured to operate in any one of the
following four operating modes: normal, one-shot, 1 SPS, and
shutdown.
NORMAL MODE
In normal mode (default power-up mode) the ADT7420 runs
an automatic conversion sequence. During this automatic con-
version sequence, a conversion typically takes 240 ms to complete
and the ADT7420 is continuously converting. This means that
as soon as one temperature conversion is completed, another
temperature conversion begins. Each temperature conversion
result is stored in the temperature value registers and is available
through the I
2
C 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 T
CRIT
setpoint read/write
register), a high temperature limit (stored in the 16-bit T
HIGH
set-
point read/write register), and a low temperature limit (stored
in the 16-bit T
LOW
setpoint read/write register). If the measured
value exceeds these limits, the INT pin is activated; and if it exceeds
the T
CRIT
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 6 to 0 and Bit 5 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 before reading back the temperature from the tem-
perature value register. This time ensures that the ADT7420
has time to power up and complete a conversion.
To obtain an updated temperature conversion, reset Bit 6 to 0
and Bit 5 to 1 in the configuration register (0x03).
The one-shot mode is useful when one of the circuit design
priorities is to reduce power consumption.