ADT7481ARMZ datasheet(9/24 Pages) AD

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ADT7481

Rev. 0 | Page 9 of 24

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 the base-

emitter voltage (VBE) of a transistor operated at constant current.

This technique requires calibration to null the effect of the

absolute value of VBE, which varies from device to device.

The technique used in the ADT7481 measures the change in

VBE when the device is operated at two different currents.

Figure 15 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 ΔVBE, the operating current through the sensor is

switched among two related currents. The currents through the

temperature diode are switched between I, and N × I, giving

ΔVBE. The temperature can then be calculated using the ΔVBE

measurement.

The resulting ΔVBE waveforms pass through a 65 kHz 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 ΔVBE. 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.

LPF

VDD

IBIAS

×I

BIAS

DIODE

D+

D–

C11

1CAPACITOR C1 IS OPTIONAL. IT IS ONLY NECESSARY IN NOISY ENVIRONMENTS. C1 = 1000pF MAX.

REMOTE

SENSING

TRANSISTOR

fC = 65kHz

VOUT+

VOUT–

TO ADC

Figure 15. Input Signal Conditioning

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