參數(shù)資料
型號： NCT7491RQR2G
廠商： ON Semiconductor
文件頁數(shù)： 14/80頁
文件大小： 844K
描述： IC REMOTE THERMAL MONITOR 24QSOP
標準包裝： 2,500
系列： *
NCT7491
http://onsemi.com
14
Figure 10. Analog Temperature Measurement Method
Series Resistance Cancellation
Parasitic resistance to the D+ and D inputs to the
NCT7491, seen in series with the remote diode, is caused by
a variety of factors, including PCB track resistance and track
length and internal resistance in the CPU. This series
resistance appears as a temperature offset in the remote
sensors temperature measurement. This error typically
causes a 0.5 degree C offset per ohm of parasitic resistance
in series with the remote diode.
The NCT7491 automatically cancels 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 NCT7491 is designed to automatically cancel
typically up to 270 W of resistance in series with the thermal
diode. By using an advanced temperature measurement
method, this process is transparent to the user. This feature
permits resistances to be added to the sensor path to produce
a filter, allowing the part to be used in noisy environments.
Temperature Measurement Results
The results of the Local, Remote 1 and Remote 2 temperature
measurements are stored in the local (0x26), remote 1 (0x25)
and remote 2 (0x27) temperature value registers in twos
complement format or Offset 64 format, depending on bit 0
if register 0x7C (1= 2s complement, 0 = Offset 64). These
results are then compared with limits programmed into the
local, remote 1 and remote 2 high and low limit registers.
The high, low and THERM limits for the local, remote 1 and
remote 2 channels must be in the same format as the
temperature reading i.e. 2s complement or Offset 64.
All the temperature measurement data for each channel is
stored in two registers, one for the MSB and one for the LSB.
This gives the temperature measurement resolution of
0.25癈. 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.
Theoretically, the temperature sensor and ADC can
measure temperatures from 64癈 to +127.5癈 with a
resolution of +0.25癈. However, this exceeds the operating
temperature range of the device, so local temperature
measurements outside the NCT7491 operating temperature
range are not possible.
" Remote1 result registers: 0x25 (MSB), 0x77 bits <3:2>
(2 LSb)
" Local result registers: 0x26 (MSB), 0x77 bits <5:4>
(2 LSb)
" Remote1 result registers: 0x27 (MSB), 0x77 bits <7:6>
(2 LSb)
Table 6. TWOS COMPLEMENT FORMAT
Temperature
Digital Output (10Bit)
64癈
1100 0000 00
55癈
1100 1001 00
40癈
1101 1000 00
10癈
1111 0110 00
1癈
1111 1111 00
0.25癈
1111 1111 11
0癈
0000 0000 00
10.25癈
0000 1010 01
25癈
0001 1001 00
125癈
0111 1101 00
127.5癈
0111 1111 10
Diode Fault  127.75
0111 1111 11
NOTE:  Bold numbers denote the LSB bits from
extended resolution register 0x77.
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