ADT7408

Rev. 0 | Page 8 of 24

THEORY OF OPERATION

CIRCUIT INFORMATION

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.

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.

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.

CONVERTER 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

Σ-∆ MODULATOR

INTEGRATOR

VOLTAGE REF

AND VPTAT

CLOCK

GENERATOR

LPF DIGITAL

FILTER

TEMPERATURE

VALUE REGISTER

COMPARATOR

12-BIT

1-BIT

–

1-BIT

DAC

+

–

+

Figure 10. First-Order Σ-Δ Modulator

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

recent temperature conversion is always available in the

temperature value register.

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.

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.

The measured temperature value is compared with the

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.

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.