ADT7421

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7

Figure 9. Input Signal Conditioning

NOTE:

N2 y I

I

BIAS

DIODE

D+

D–

C1

REMOTE

SENSING

TRANSISTOR

TO ADC

N1 y I

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

lists the data format for the external temperature low byte.

Table 1. 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 +125°C.

However, the ADT7421 can be operated using an extended

temperature range. The extended measurement range is

−40

°C to +125°C.

The extended temperature range is selected by setting

Bit 2 of the configuration register to 1. The temperature

range is 0

°C to 125°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 ADT7421 is limited

by the remote transistor selection. The temperature registers

can have values from −40

°C to +125°C. Most temperature

sensing transistors have a maximum temperature range of

−55

°C to +150°C. Above +150°C, they may lose their

semiconductor characteristics and approximate conductors

instead.

It should be noted that although both local and remote

temperature measurements can be made while the part is in

extended temperature mode, the ADT7421 itself should not

be exposed to temperatures greater than those specified in

the Absolute Maximum Ratings Table. Further, the device

is only guaranteed to operate as specified at ambient

temperatures from −40

°C to +125°C.

Temperature Data Format

The ADT7421 has two temperature data formats. When

the temperature measurement range is from 0

°C to 125°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 the following table.

Table 2. Temperature Data Format

(Temperature High Byte)

Temperature

Binary

Offset Binary

−40°C

0 000 0000

0 001 1000

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

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 125°C for all

temperatures > 125°C.