ADT7461A
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.
BE
V
DD
I
BIAS
I
N2 y I
N1 y I
V
OUT+
D+
1
TO ADC
C1
REMOTE
SENSING
TRANSISTOR
fC = 65kHz
D–
V
BIAS
OUT–
DIODE
1
CAPACITOR C1 IS OPTIONAL. IT IS ONLY NECESSARY IN NOISY ENVIRONMENTS.
C1 = 1000pF MAX.
Figure 14. Input Signal Conditioning
Temperature Measurement Results
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.
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.
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.
Table 2. Extended Temperature Resolution
(Remote Temperature Low Byte
Extended Resolution
Remote Temperature Low Byte
0.00°C
0.25°C
0.50°C
0.75°C
0 000 0000
0 100 0000
1 000 0000
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 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.
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.
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