ADT7461A

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Differences between the ADT7461A and the ADT7461

Although the ADT7461A is pin− and register−compatible

with the ADT7461, there are some specification differences

between the two devices. A summary of these differences is

shown below in Table 1.

Table 1. Differences Between the ADT7461A

and the ADT7461

Specification

ADT7461A

ADT7461

Unit

Supply Voltage

3.0 to 3.6

3.0 to 5.5

V

Maximum Local Sensor

Accuracy

1.0

3.0

°C

Maximum Series

Resistance Cancellation

1.5

3.0

kW

Average Operating

Supply Current

16 Conversions/Sec

Standby Mode

240

5.0

170

5.5

mA

Max Conversion Time

One Shot, Averaging On

One Shot, Averaging Off

52

8.0

114.6

12.56

ms

Remote Sensor Current

Levels

High

Mid

Low

220

82

13.5

96

36

6.0

mA

Theory of Operation

The ADT7461A is a local and remote temperature sensor

and over/under temperature alarm, with the added ability to

automatically cancel the effect of 1.5 k

W (typical) of

resistance in series with the temperature monitoring diode.

When the ADT7461A 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 the

remote temperature sensor. The ADC digitizes these signals

and the results are stored in the local and remote temperature

value registers.

The local and remote measurement results are compared

with the corresponding high, low, and THERM temperature

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

comparisons generate flags that are stored in the status

register. A result that exceeds the high temperature limit or

the low temperature limit causes the ALERT output to

assert. The ALERT output also asserts if an external diode

fault is detected. Exceeding the THERM temperature limits

causes the THERM output to assert low. The ALERT output

can be reprogrammed as a second THERM output.

The limit registers are programmed and the device

controlled and configured via the serial SMBus. The

contents of any register are also read back via the SMBus.

Control and configuration functions consist of switching

the device between normal operation and standby mode,

selecting the temperature measurement range, masking or

enabling the ALERT output, switching Pin 6 between

ALERT and THERM2, and selecting the conversion rate.

Series Resistance Cancellation

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

ADT7461A, 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’s temperature measurement. This

error typically causes a 0.5

°C offset per ohm of parasitic

resistance in series with the remote diode.

The ADT7461A automatically cancels the effect of this

series resistance on the temperature reading, giving a more

accurate result, without the need for user characterization of

this resistance. The ADT7461A is designed to automatically

cancel typically up to 1.5 k

W of resistance. By using an

advanced temperature measurement method, this process is

transparent to the user. This feature permits resistances to be

added to the sensor path to produce a filter, allowing the part

to be used in noisy environments. See the section on Noise

Filtering for more details.

Temperature Measurement Method

A simple method of measuring temperature is to exploit

the negative temperature coefficient of a diode, measuring

constant current. However, this technique requires

which varies from device to device.

The technique used in the ADT7461A measures the

currents. Previous devices used only two operating currents,

but it is the use of a third current that allows automatic

cancellation of resistances in series with the external

temperature sensor.

Figure 14 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 can equally be a discrete transistor. If a discrete transistor

is used, the collector is not grounded but 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 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 on C1.

To measure

sensor is switched among three related currents. As shown

in Figure 14, N1 x I and N2 x I are different multiples of the

current, I. The currents through the temperature diode are

switched between I and N1 x I, giving

between I and N2 x I, giving

calculated using the two

also cancels the effect of any series resistance on the

temperature measurement.

The resulting

65 kHz low−pass filter to remove noise and then to a