ADT7461ARMZ-REEL7 Datasheet ADT7461ARZ, ADT7461ARMZ-002, ADT7461ARMZ-R7 | P7-P9

ADT7461

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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 kW (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 kW 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

the base-emitter voltage (V

) of a transistor operated at

constant current. However, this technique requires

calibration to null out the effect of the absolute value of V

which varies from device to device.

The technique used in the ADT7461 is to measure the

change in V

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 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 DV

, the operating current through the

sensor is switched among three related currents. Figure 15

shows N1 × I and N2 × I as different multiples of the

current, I. The currents through the temperature diode are

switched between I and N1 × I, giving DV

BE1

, and then

between I and N2 × I, giving DV

BE2

. The temperature may

then be calculated using the two DV

measurements. This

method can also be shown to cancel 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

chopper-stabilized amplifier. This amplifies and rectifies the

waveform to produce a dc voltage proportional to DV

. The

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.

ADT7461

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Figure 15. Input Signal Conditioning

LOW-PASS FILTER

= 65 kHz

REMOTE

SENSING

TRANSISTOR

BIAS

DIODE

D+

D−

BIAS

I N1 × I

OUT+

OUT−

To ADC

N2 × I

C1*

*CAPACITOR C1 IS OPTIONAL. IT SHOULD ONLY BE USED IN NOISY ENVIRONMENTS.

Temperature Measurement Results

The results of the local and remote temperature

measurements are stored in the local and remote temperature

value registers and are 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. This gives the external

temperature measurement a resolution of 0.25°C. Table 6

shows the data format for the external temperature low byte.

Table 6. 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, both the

high and low byte, the two registers should be read in

succession. Reading one register does not lock the other, so

both should be read before the next conversion finishes. In

practice, there is more than enough time to read both

registers, as transactions over the SMBus are significantly

faster than a conversion time.

Temperature Measurement Range

The temperature measurement range for both internal and

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

However, the ADT7461 can be operated using an extended

temperature range. It can measure the full temperature range

of an external diode, from −55°C to +150°C. The user can

switch between these two temperature ranges by setting or

clearing Bit 2 in the configuration register. 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 ADT7461 is

limited by the remote diode selection. The temperature

registers themselves 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.

The user should be aware that the temperature measurement

on the external channel may not be accurate for temperatures

that are outside the operating range of the remote sensor.

While both local and remote temperature measurements

can be made while the part is in extended temperature mode,

the ADT7461 itself should not be exposed to temperatures

greater than those specified in the Absolute Maximum

Ratings section. Also, the device is guaranteed to operate only

as specified at ambient temperatures from −40°C to +120°C.

Temperature Data Format

The ADT7461 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 in

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

temperatures in both data formats are shown in Table 7.

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Table 7. TEMPERATURE DATA FORMAT

(LOCAL AND REMOTE 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 also 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 are not changed. The user must

ensure that the limit registers are reprogrammed, as

necessary, when the data format changes. See the Limit

Registers section for more

information.

ADT7461 Registers

The ADT7461 contains a total of 22 8-bit registers. These

registers are used to store the results of remote and local

temperature measurements and high and low temperature

limits and to configure and control the device. A description

of these registers follows. Additional details are provided in

Table 8 to Table 12.

Address Pointer Register

The address pointer register does not have or require an

address, as 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 register on the

ADT7461, which is stored in the address pointer register.

This register address is written to by the second byte of a

write operation or is used for a subsequent read operation.

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, since its register address is 0x00.

Temperature Value Registers

The ADT7461 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. Bits 0, 1, 3,

and 4 are reserved and should not be written to by the user.

Bit 7 of the configuration register is used to mask 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 only applies if Pin 6 is configured as

ALERT

. If Pin 6 is configured as THERM2, the value of

Bit 7 has no effect.

If Bit 6 is set to 0 (the power-on default), the device is in

operating mode with the 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, so values can be read from or written to the

ADT7461 via the SMBus in this mode. The ALERT

and

THERM

outputs are also active in standby mode. Changes

made to the registers in standby mode that affect the

THERM

or ALERT outputs cause these signals to be

updated.

Bit 5 determines the configuration of Pin 6 on the

ADT7461. 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), the temperature measurement range is set

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

measurement range is set to the extended temperature range.

Table 8. 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-P20

ADT7461ARMZ-REEL7

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Manufacturer:

ON Semiconductor

Description:

SENSOR DIGITAL -40C-120C MICRO8

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ADT7461ARMZ-REEL7

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