ADT7481

http://onsemi.com

7

Theory of Operation

The ADT7481 is a local and dual remote temperature

sensor and over/under temperature alarm. When the

ADT7481 is operating normally, the on−board ADC

operates in a free−running mode. The analog input

multiplexer alternately selects either the on−chip

temperature sensor to measure its local temperature, or

either of the remote temperature sensors. The ADC digitizes

these signals and the results are stored in the local, Remote 1,

and Remote 2 temperature value registers.

The local and remote measurement results are compared

with the corresponding high, low, and THERM temperature

limits, stored in on−chip registers. Out−of−limit comparisons

generate flags that are stored in the status register. A result that

exceeds the high temperature limit, the low temperature limit,

or remote diode open circuit will cause the ALERT output to

assert low. Exceeding THERM temperature limits causes the

THERM output to assert low. The ALERT output can be

reprogrammed as a second THERM output.

The limit registers can be programmed, and the device

controlled and configured via the serial SMBus. The

contents of any register can also be read back via the SMBus.

Control and configuration functions consist of switching

the device between normal operation and standby mode,

selecting the temperature measurement scale, masking or

enabling the ALERT output, switching Pin 8 between

ALERT and THERM2, and selecting the conversion rate.

Temperature Measurement Method

A simple method of measuring temperature is to exploit

the negative temperature coefficient of a diode, measuring

constant current.

This technique requires calibration to null the effect of the

The technique used in the ADT7481 measures the change

Figure 14 shows the input signal conditioning used to

measure the output of a remote temperature sensor. This

figure shows the remote sensor as a substrate transistor, but

it could equally be a discrete transistor. If a discrete

transistor is used, the collector is not grounded and is linked

to the base. To prevent ground noise interfering with the

measurement, the more negative terminal of the sensor is not

referenced to ground, but is biased above ground by an

internal diode at the D− input. C1 may optionally be added

as a noise filter with a recommended maximum value of

1,000 pF.

To measure

sensor is switched among two related currents. The currents

through the temperature diode are switched between I, and

N x I, giving

using the

The resulting

low−pass filter to remove noise and then to a

chopper−stabilized amplifier. This amplifies and rectifies

the waveform to produce a dc voltage proportional to

The ADC digitizes this voltage producing a temperature

measurement. To reduce the effects of noise, digital filtering

is performed by averaging the results of 16 measurement

cycles for low conversion rates. At rates of 16, 32, and 64

conversions/second, no digital averaging takes place.

Signal conditioning and measurement of the local

temperature sensor is performed in the same manner.

Figure 14. Input Signal Conditioning

N ×I

I

BIAS

DIODE

D+

D–

C1

NOTE:

REMOTE

SENSING

TRANSISTOR

TO ADC

CAPACITOR C1 IS OPTIONAL. IT IS ONLY NECESSARY IN NOISY ENVIRONMENTS. C1 = 1000pF MAX.

Temperature Measurement Results

The results of the local and remote temperature

measurements are stored in the local and remote temperature

value registers and are compared with limits programmed

into the local and remote high and low limit registers.

The local temperature measurement is an 8−bit

measurement with 1

°C resolution. The remote temperature

measurements are 10−bit measurements, with the 8 MSBs

stored in one register and the 2 LSBs stored in another

register. Table 1 is a list of the temperature measurement

registers.