ADT7461A

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8

chopper−stabilized amplifier. This amplifies and rectifies

the waveform to produce a dc voltage proportional to

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

N2 y I

I

BIAS

DIODE

D+

D–

REMOTE

SENSING

TRANSISTOR

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.