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Data Sheet

ADT7312

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THEORY OF OPERATION

CIRCUIT DESCRIPTION

The ADT7312 is a ±1°C accurate digital temperature sensor

that uses a 16-bit ADC to monitor and digitize the temperature

to a resolution of 0.0078°C. By default, the ADC resolution is

set to 13 bits (0.0625°C).

An internal temperature sensor generates a voltage proportional

to absolute temperature; this voltage is compared 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 the need for

correction or calibration by the user.

The sensor output is digitized by a sigma-delta (Σ-∆) modu-

lator, also known as a charge balance type ADC. This type of

converter uses time domain oversampling and a high accuracy

comparator to deliver 16 bits of resolution in an extremely

compact circuit.

CONVERTER ARCHITECTURE

The Σ-∆ modulator consists of an input sampler, a summing

network, an integrator, a comparator, and a 1-bit digital-to-

analog converter (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 fre-

quency. This 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 SPI temperature data.

Σ-Δ MODULATOR

INTEGRATOR

COMPARATOR

TEMPERATURE

VALUE

CLOCK

GENERATOR

VOLTAGE

REFERENCE

LPF DIGITAL

FILTER

1-BIT

DAC

1 BIT

13/16 BITS

Figure 10. Σ-∆ Modulator

The ADT7312 can be configured to operate in any one of the

following operating modes:

•

Normal mode (continuous conversion mode)

•

One-shot mode

•

1 SPS mode

•

Shutdown mode

NORMAL MODE (CONTINUOUS CONVERSION

MODE)

In normal mode (default power-on mode), the ADT7312 runs

an automatic conversion sequence. During this automatic con-

version sequence, a conversion typically takes 240 ms to complete.

The ADT7312 is continuously converting: as soon as one tempera-

ture conversion is completed, another temperature conversion

begins. Each temperature conversion result is stored in the

temperature value register (Register Address 0x02) and is avail-

able through the SPI 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 is

asserted low. If the temperature exceeds 64°C, the INT pin is

asserted 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 register),

a high temperature limit (stored in the 16-bit THIGH setpoint

setpoint register). If the measured value exceeds the THIGH or

TLOW limit, the INT pin is activated; if the measured value

exceeds the TCRIT limit, the CT pin is activated. The polarity of

the INT and CT pins is programmable using the configuration

programmable for interrupt mode via the configuration register.

Rev. A

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