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ADT7482

Dual Channel Temperature Sensor and Overtemperature Alarm

Analog Devices 

THEORY OF OPERATION

The ADT7482 is a local and 2× remote temperature sensor and

overtemperature/undertemperature alarm. When the ADT7482

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 a

remote diode open circuit causes 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.

SERIES RESISTANCE CANCELLATION

Parasitic resistance to the D+ and D− inputs to the ADT7482,

seen in series with the remote diode, is caused by a variety of

factors, including PCB track resistance and track length. This

series resistance appears as a temperature offset in the remote

sensor temperature measurement. This error typically causes a

0.5°C offset per ohm of parasitic resistance in series with the

remote diode.

The ADT7482 automatically cancels out the effect of this series

resistance on the temperature reading, providing a more

accurate result, without the need for user characterization of

this resistance. The ADT7482 is designed to automatically cancel

typically up to 1.5 kΩ of resistance. By using an advanced

temperature measurement method, this is transparent to the

user. This feature allows resistances to be added to the sensor

path to produce a filter, allowing the part to be used in noisy

environments. See the Noise Filtering section for more details.

ADT7482

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. However, this technique requires calibration to null out

the effect of the absolute value of VBE, which varies from device

to device.

The technique used in the ADT7482 is to measure the change

in VBE when the device is operated at three different currents.

Previous devices have used only two operating currents. The

use of a third current allows automatic cancellation of

resistances in series with the external temperature sensor.

Figure 16 shows the input signal conditioning used to measure

the output of an external temperature sensor. This figure shows

the external sensor as a substrate transistor, but it could equally

be a discrete transistor. If a discrete transistor is used, the collec-

tor is not grounded and should be linked to the base. To prevent

ground noise from 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.

Capacitor C1 can be added as a noise filter (a recommended

maximum value of 1000 pF). However, a better option in noisy

environments is to add a filter, as described in the Noise

Filtering section. See the Layout Considerations section for

more information.

To measure ΔVBE, the operating current through the sensor is

switched among three related currents. Shown in Figure 16,

N1 × I and N2 × I are different multiples of the current, I. The

currents through the temperature diode are switched between I

and N1 × I, giving ΔVBE1, and then between I and N2 × I, giving

ΔVBE2. The temperature can then be calculated using the two

ΔVBE measurements. This method can also be shown to cancel

the effect of any series resistance on the temperature measurement.

The resulting ΔVBE waveforms are passed 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 vol-

tage and a temperature measurement is produced. 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 internal tempera-

ture sensor are performed in the same manner.

Rev. 0 | Page 9 of 24

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