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ADT7483A

Dual Channel Temperature Sensor and Over Temperature Alarm

ON Semiconductor 

ADT7483A

THEORY OF OPERATION

The ADT7483A is a local and 2x remote temperature sensor

and over/under temperature alarm. When the ADT7483A 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 or one of the remote

temperature sensors to measure its local temperature. 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. Likewise, 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 13 between ALERT and THERM2.

• 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. Unfortunately, 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 ADT7483A is to

measure 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 should be 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 can be optionally added as a noise filter (recommended

maximum value 1,000 pF).

To measure ΔVBE, the operating current through the sensor is

switched among two related currents, I and N × I. The currents

through the temperature diode are switched between I and N × I,

giving ΔVBE. The temperature is then 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 and produces 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.

VDD

I

N×I

IBIAS

REMOTE

SENSING

TRANSISTOR

D+

C11

D–

BIAS

DIODE

LPF

fC = 65kHz

VOUT+

TO ADC

VOUT–

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

Figure 15. Input Signal Conditioning

Rev. 1 | Page 9 of 24 | www.onsemi.com

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