ADT7461ARM datasheet(9/24 Pages) AD

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ADT7461

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 kΩ (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 gen-

erate 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 will cause 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 con-

trolled 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, seen in series with the remote diode, to the

D+ and D− inputs to the ADT7461, 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 resis-

tance. The ADT7461 is designed to automatically cancel typically

up to 3 kΩ of resistance. By using an advanced temperature

measurement method, this is transparent to the user. This fea-

ture allows 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

for more details.

Noise Filtering

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 out the effect of the absolute value of VBE, which varies

from device to device.

The technique used in the ADT7461 is to measure the change

in VBE when the device is operated at three different 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 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 could equally

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

tor 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 optionally be added as a noise filter (recom-

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

noisy environments is to add a filter, as described in the section

. See the section on

for

more information on C1.

Layout Considerations

To measure ∆VBE, the operating current through the sensor is

switched among three related currents. Shown in Figure 14,

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 ∆VBE1, and then between I and N2 × I, giving

∆VBE2. The temperature may then be calculated using the two

∆VBE measurements. This method can also be shown to cancel

the effect of any series resistance on the temperature measurement.

The resulting ∆VBE 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 ∆VBE. The ADC digitizes this volt-

age 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 is performed in the same manner.

Rev. 0 | Page 9 of 24

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