ADT7461A datasheet(10/24 Pages) AD

Part #	ADT7461A

Description	짹1째C Temperature Monitor with Series Resistance Cancellation

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ADT7461A

Rev. A | Page 10 of 24

THEORY OF OPERATION

The ADT7461A is a local and remote temperature sensor and

over/under temperature alarm, with the added ability to auto-

matically cancel the effect of 1.5 kΩ (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 kΩ 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 the base

emitter voltage (VBE) of a transistor operated at constant current.

However, 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 ADT7461A measures the change in VBE

when the device operates at three different 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 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 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 ΔVBE, the operating current through the sensor is

switched among three related currents. As shown in Figure 15,

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 ΔVBE1; and then between I and N2 × I, giving

ΔVBE2. The temperature is then calculated using the two ΔVBE

measurements. This method also cancels the effect of any series

resistance on the temperature measurement.

The resulting ΔVBE 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 ΔVBE. The ADC digitizes this voltage

producing 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 occurs.

Signal conditioning and measurement of the internal

temperature sensor are performed in the same manner.

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