ADT7461AARMZ-002 Datasheet ADT7461AARMZ, ADT7461AARMZ-002, ADT7461AARMZ-2REEL | P7-P9

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

Average Operating

Supply Current

16 Conversions/Sec

Standby Mode

240

5.0

170

5.5

Max Conversion Time

One Shot, Averaging On

One Shot, Averaging Off

8.0

114.6

12.56

Remote Sensor Current

Levels

High

Mid

Low

220

13.5

6.0

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 kW (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

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 kW 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 (V

) of a transistor operated at

constant current. However, this technique requires

calibration to null the effect of the absolute value of V

which varies from device to device.

The technique used in the ADT7461A measures the

change in V

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 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 DV

, the operating current through the

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 DV

BE1

; and then

between I and N2 x I, giving DV

BE2

. The temperature is then

calculated using the two DV

measurements. This method

also cancels the effect of any series resistance on the

temperature measurement.

The resulting DV

waveforms are passed through a

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

ADT7461A

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chopper−stabilized amplifier. This amplifies and rectifies

the waveform to produce a dc voltage proportional to DV

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.

Figure 14. Input Signal Conditioning

BIASN2 y II

BIAS

DIODE

D+

D–

CAPACITOR C1 IS OPTIONAL. IT IS ONLY NECESSARY IN NOISY ENVIRONMENTS.

REMOTE

SENSING

TRANSISTOR

= 65kHz

OUT+

OUT–

TO ADC

N1 y I

C1 = 1000pF MAX.

Temperature Measurement Results

The results of the local and remote temperature

measurements are stored in the local and remote temperature

value registers and compared with limits programmed into

the local and remote high and low limit registers.

The local temperature value is in Register 0x00 and has a

resolution of 1°C. The external temperature value is stored

in two registers, with the upper byte in Register 0x01 and the

lower byte in Register 0x10. Only the two MSBs in the

external temperature low byte are used giving the external

temperature measurement a resolution of 0.25°C. Table 2

lists the data format for the external temperature low byte.

Table 2. Extended Temperature Resolution

(Remote Temperature Low Byte

Extended Resolution Remote Temperature Low Byte

0.00°C 0 000 0000

0.25°C 0 100 0000

0.50°C 1 000 0000

0.75°C 1 100 0000

When reading the full external temperature value, read the

LSB first. This causes the MSB to be locked (that is, the

ADC does not write to it) until it is read. This feature ensures

that the results read back from the two registers come from

the same measurement.

Temperature Measurement Range

The temperature measurement range for both internal and

external measurements is, by default, 0°C to +127°C.

However, the ADT7461A can be operated using an

extended temperature range. The extended measurement

range is −64°C to +191°C. Therefore, the ADT7461A can be

used to measure the full temperature range of an external

diode, from −55°C to +150°C.

The extended temperature range is selected by setting Bit

2 of the configuration register to 1. The temperature range

is 0°C to 127°C when Bit 2 equals 0. A valid result is

available in the next measurement cycle after changing the

temperature range.

In extended temperature mode, the upper and lower

temperature that can be measured by the ADT7461A is

limited by the remote diode selection. The temperature

registers can have values from −64°C to +191°C. However,

most temperature sensing diodes have a maximum

temperature range of −55°C to +150°C. Above +150°C, they

may lose their semiconductor characteristics and

approximate conductors instead. This results in a diode

short. In this case, a read of the temperature result register

gives the last good temperature measurement. Therefore,

the temperature measurement on the external channel may

not be accurate for temperatures that are outside the

operating range of the remote sensor.

It should be noted that although both local and remote

temperature measurements can be made while the part is in

extended temperature mode, the ADT7461A itself should

not be exposed to temperatures greater than those specified

in the absolute maximum ratings section. Further, the device

is only guaranteed to operate as specified at ambient

temperatures from −40°C to +120°C.

Temperature Data Format

The ADT7461A has two temperature data formats. When

the temperature measurement range is from 0°C to 127°C

(default), the temperature data format for both internal and

external temperature results is binary. When the measurement

range is in extended mode, an offset binary data format is used

for both internal and external results. Temperature values are

offset by 64°C in the offset binary data format. Examples of

temperatures in both data formats are shown in Table 3.

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Table 3. Temperature Data Format

(Temperature High Byte)

Temperature Binary Offset Binary (Note 1)

–55°C 0 000 0000

(Note 2)

0 000 1001

0°C 0 000 0000 0 100 0000

+1°C 0 000 0001 0 100 0001

+10°C 0 000 1010 0 100 1010

+25°C 0 001 1001 0 101 1001

+50°C 0 011 0010 0 111 0010

+75°C 0 100 1011 1 000 1011

+100°C 0 110 0100 1 010 0100

+125°C 0 111 1101 1 011 1101

+127°C 0 111 1111 1 011 1111

+150°C 0 111 1111

(Note 3)

1 101 0110

1. Offset binary scale temperature values are offset by 64°C.

2. Binary scale temperature measurement returns 0°C for all

temperatures < 0°C.

3. Binary scale temperature measurement returns 127°C for all

temperatures > 127°C.

The user can switch between measurement ranges at any

time. Switching the range likewise switches the data format.

The next temperature result following the switching is

reported back to the register in the new format. However, the

contents of the limit registers do not change. It is up to the

user to ensure that when the data format changes, the limit

registers are reprogrammed as necessary. More information

on this is found in the Limit Registers section.

ADT7461A Registers

The ADT7461A contains 22, 8−bit registers in total.

These registers store the results of remote and local

temperature measurements, high and low temperature

limits, and configure and control the device. See the Address

Pointer Register section through the Consecutive ALERT

the ADT7461A registers. Additional details are shown in

Table 4 through Table 8. The entire register map is available

in Table 9.

Address Pointer Register

The address pointer register itself does not have, nor does

it require, an address because the first byte of every write

operation is automatically written to this register. The data

in this first byte always contains the address of another

pointer register. It is to this register address that the second

byte of a write operation is written, or to which a subsequent

read operation is performed.

The power−on default value of the address pointer register

is 0x00. Therefore, if a read operation is performed

immediately after power−on, without first writing to the

address pointer, the value of the local temperature is returned

because its register address is 0x00.

Temperature Value Registers

The ADT7461A has three registers to store the results of

local and remote temperature measurements. These

registers can only be written to by the ADC and can be read

by the user over the SMBus. The local temperature value

The external temperature value high byte register is at

Address 0x01, with the low byte register at Address 0x10.

The power−on default for all three registers is 0x00.

Configuration Register

The configuration register is Address 0x03 at read and

Address 0x09 at write. Its power−on default is 0x00. Only

four bits of the configuration register are used. Bit 0, Bit 1,

Bit 3, and Bit 4 are reserved; the user does not write to them.

Bit 7 of the configuration register masks the ALERT

output. If Bit 7 is 0, the ALERT output is enabled. This is the

power−on default. If Bit 7 is set to 1, the ALERT

output is

disabled. This applies only if Pin 6 is configured as ALERT

If Pin 6 is configured as THERM2

, then the value of Bit 7

has no effect.

If Bit 6 is set to 0, which is power−on default, the device

is in operating mode with ADC converting. If Bit 6 is set to

1, the device is in standby mode and the ADC does not

convert. The SMBus does, however, remain active in

standby mode; therefore, values can be read from or written

to the ADT7461A via the SMBus. The ALERT

and THERM

outputs are also active in standby mode. Changes made to

the registers in standby mode that affect the THERM

ALERT

outputs cause these signals to be updated.

Bit 5 determines the configuration of Pin 6 on the

ADT7461A. If Bit 5 is 0 (default), then Pin 6 is configured

as an ALERT

output. If Bit 5 is 1, then Pin 6 is configured

as a THERM2

output. Bit 7, the ALERT mask bit, is only

active when Pin 6 is configured as an ALERT

output. If Pin

6 is set up as a THERM2

output, then Bit 7 has no effect.

Bit 2 sets the temperature measurement range. If Bit 2 is

0 (default value), the temperature measurement range is set

between 0°C to +127°C. Setting Bit 2 to 1 sets the

measurement range to the extended temperature range

(−64°C to +191°C).

Table 4. Configuration Register Bit Assignments

Bit Name Function Power−On

Default

7 MASK1 0 = ALERT Enabled

1 = ALERT

Masked

6 RUN/STOP 0 = Run

1 = Standby

5 ALERT/

THERM2

0 = ALERT

1 = THERM2

4, 3 Reserved 0

2 Temperature

Range Select

0 = 0°C to 127°C

1 = Extended range

1, 0 Reserved 0

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P19

ADT7461AARMZ-002

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Manufacturer:

ON Semiconductor

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SENSOR DIGITAL -40C-120C MICRO8

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ADT7461AARMZ-002

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