Datasheet

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SBOS371A − AUGUST 2006 − REVISED OCTOBER 2007
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7
SERIES RESISTANCE CANCELLATION
Series resistance in an application circuit that typically
results from printed circuit board (PCB) trace resistance
and remote line length (see Figure 11) is automatically
cancelled by the TMP401, preventing what would
otherwise result in a temperature offset. When using a 5V
supply voltage, a total of up to 3kΩ of series line resistance
is cancelled by the TMP401, eliminating the need for
additional characterization and temperature offset
correction. Series line resistance should be limited to
500Ω total when using a 3.3V supply voltage. See typical
characteristics curves (Figure 4 and Figure 5) for details
on the effect of series resistance and power-supply
voltage on sensed remote temperature error.
DIFFERENTIAL INPUT CAPACITANCE
The TMP401 tolerates differential input capacitance of up
to 1000pF with minimal change in temperature error. The
effect of capacitance on sensed remote temperature error
is shown in Figure 6, Remote Temperature Error vs
Differential Capacitance.
TEMPERATURE MEASUREMENT DATA
Temperature measurement data is taken over a default
range of 0°C to +127°C for both local and remote locations.
Measurements from −55°C to +150°C can be made both
locally and remotely by reconfiguring the TMP401 for the
extended temperature range. To change the TMP401
configuration from the standard to the extended
temperature range, switch bit 2 (RANGE) of the
Configuration Register from low to high.
Temperature data resulting from conversions within the
default measurement range is represented in binary form,
as shown in Table 1, Standard Binary column. Note that
any temperature below 0°C results in a data value of zero
(00h). Likewise, temperatures above +127°C result in a
value of 127 (7Fh). The device can be set to measure over
an extended temperature range by changing bit 2 of the
Configuration Register from low to high. The change in
measurement range and data format from standard binary
to extended binary occurs at the next temperature
conversion. For data captured in the extended
temperature range configuration, an offset of 64 (40h) is
added to the standard binary value, as shown in Table 1,
Extended Binary column. This configuration allows
measurement of temperatures below 0°C. Note that binary
values corresponding to temperatures as low as −64°C,
and as high as +191°C are possible; however, most
temperature sensing diodes only measure with the range
of −55°C to +150°C. Additionally, the TMP401 is rated only
for ambient temperatures ranging from −40°C to +125°C.
Parameters in the Absolute Maximum Ratings table must
be observed.
Table 1. Temperature Data Format
(Local and Remote Temperature High Bytes)
LOCAL/REMOTE TEMPERATURE REGISTER
HIGH BYTE VALUE (+15 C RESOLUTION)
TEMP
STANDARD BINARY EXTENDED BINARY
TEMP
(5C)
BINARY HEX BINARY HEX
−64 0000 0000 00 0000 0000 00
−50 0000 0000 00 0000 1110 0E
−25 0000 0000 00 0010 0111 27
0 0000 0000 00 0100 0000 40
1 0000 0001 01 0100 0001 41
5 0000 0101 05 0100 0101 45
10 0000 1010 0A 0100 1010 4A
25 0001 1001 19 0101 1001 59
50 0011 0010 32 0111 0010 72
75 0100 1011 4B 1000 1011 8B
100 0110 0100 64 1010 0100 A4
125 0111 1101 7D 1011 1101 BD
127 0111 1111 7F 1011 1111 BF
150 0111 1111 7F 1101 0110 D6
175 0111 1111 7F 1110 1111 EF
191 0111 1111 7F 1111 1111 FF
NOTE: Whenever changing between standard and
extended temperature ranges, be aware that the
temperatures stored in the temperature limit registers are
NOT automatically reformatted to correspond to the new
temperature range format. These temperature limit values
must be reprogrammed in the appropriate binary or
extended binary format.
Both local and remote temperature data use two bytes for
data storage. The high byte stores the temperature with
1°C resolution. The second or low byte stores the decimal
fraction value of the temperature and allows a higher
measurement resolution; see Table 2. The measurement
resolution for the remote channel is 0.0625°C, and is not
adjustable. The measurement resolution for the local
channel is adjustable; it can be set for 0.5°C, 0.25°C,
0.125°C, or 0.0625°C by setting the RES1 and RES0 bits
of the Resolution Register; see the Resolution Register
section.