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ADT7420
(Rev.:RevA)

±0.25°C Accurate, 16-Bit Digital I2C Temperature Sensor

Analog Devices 

ADT7420

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.

VOLTAGE REF

AND VPTAT

Σ-∆ MODULATOR

INTEGRATOR

COMPARATOR

1-BIT

DAC

CLOCK

GENERATOR

1-BIT

LPF DIGITAL

FILTER

13-BIT/

16-BIT

Figure 11. Σ-Δ Modulator

TEMPERATURE

VALUE

REGISTER

The ADT7420 can be configured to operate in any one of the

following four operating modes: normal, one-shot, 1 SPS, and

shutdown.

Data Sheet

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 I2C 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 TCRIT setpoint read/write

register), a high temperature limit (stored in the 16-bit THIGH set-

point read/write register), and a low temperature limit (stored in

the 16-bit TLOW setpoint read/write register). If the measured

value exceeds these limits, the INT pin is activated; and if it exceeds

the TCRIT 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.

Rev. A | Page 10 of 24

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