ADT75BRMZ-REEL7 View Datasheet(PDF) - Analog Devices

Part Name

Description

MFG CO.

ADT75BRMZ-REEL7

±1°C Accurate, 12-Bit Digital Temperature Sensor

Analog Devices 

ADT75

THEORY OF OPERATION

CIRCUIT INFORMATION

The ADT75 is a 12-bit digital temperature sensor with the 12th 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 /I2C- 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 ∑-Δ modulator,

also known as the charge balance type analog-to-digital

converter. This type of converter utilizes time-domain over-

sampling and a high accuracy comparator to deliver 12 bits of

effective accuracy in an extremely compact circuit.

CONVERTER DETAILS

The ∑-Δ 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.

VOLTAGE REF

AND VPTAT

Σ-Δ MODULATOR

INTEGRATOR

COMPARATOR

+

–

1-BIT

DAC

CLOCK

GENERATOR

1-BIT

TEMPERATURE

LPF DIGITAL

VALUE

FILTER 12-BIT REGISTER

Figure 11. First-Order ∑-Δ Modulator

The modulated output of the comparator is encoded using a

circuit technique that results in SMBus/I2C 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/I2C 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 TOS read/write register

and the hysteresis temperature limit, stored in the 16-bit

THYST 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 configura-

tion register.

Rev. 0 | Page 10 of 24

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