ADT7483A

http://onsemi.com

7

Theory of Operation

The ADT7483A is a local and 2 remote temperature

sensor and over/under temperature alarm. When the

ADT7483A is operating normally, the on-board ADC

operates in a freerunning 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:

Standby Mode

Temperature Measurement Method

A simple method of measuring temperature is to exploit

the negative temperature coefficient of a diode, measuring

constant current. Unfortunately, this technique requires

which varies from device to device. The technique used in

device is operated at two different currents.

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 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

is switched among two related currents, I and N  I. The

currents through the temperature diode are switched

between I and N  I, giving

calculated 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 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.

Figure 14. Input Signal Conditioning

LOW-PASS FILTER

REMOTE

SENSING

TRANSISTOR

BIAS

DIODE

D+

D−

IN  I

To ADC

C1*

*CAPACITOR C1 IS OPTIONAL. IT IS ONLY NECESSARY IN NOISY ENVIRONMENTS. C1 = 1,000 pF 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 eight MSBs

stored in one register and two LSBs stored in another

register. Table 6 lists the temperature measurement registers.