ADT7461

http://onsemi.com

8

Figure 15. Input Signal Conditioning Error vs. Series Resistance

I

N1

TO ADC

REMOTE

SENSING

TRANSISTOR

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

D+

D–

C1*

BIAS

DIODE

LOW−PASS FILTER

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 1

shows 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, 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 2.