ADT7461

http://onsemi.com

7

Functional Description

The ADT7461 is a local and remote temperature sensor

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

automatically cancel the effect of 3 k

W (typical) of

resistance in series with the temperature monitoring diode.

When the ADT7461 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, the low

temperature limit, or an external diode fault 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 6 between

ALERT and THERM2, and selecting the conversion rate.

Series Resistance Cancellation

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

ADT7461, 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 ADT7461 automatically cancels out 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 ADT7461 is designed to automatically

cancel typically up to 3 k

W 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.

Temperature Measurement Method

A simple method of measuring temperature is to exploit

the negative temperature coefficient of a diode by measuring

constant current. However, this technique requires

which varies from device to device.

The technique used in the ADT7461 is to measure the

currents. Previous devices have 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 15 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 collector will not be 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

may be added as a noise filter (a recommended maximum

value of 1,000 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. Figure 15

shows N1 x I and N2 x I as different multiples of the current,

I. The currents through the temperature diode are switched

between I and N1 x I, giving

N2 x I, giving

calculated using the two

can also be shown to cancel the effect of any series resistance

on the temperature measurement.

The resulting

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

ADC digitizes this voltage 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 per second, no digital averaging takes place.

Signal conditioning and measurement of the internal

temperature sensor is performed in the same manner.