Datasheet
!
last update
November 30, 2010
reference
smtutin
page
16/27
To calculate the ratio as in (1), we have to make different calculations for the 2-, 3- and 4 wire
measurement:
M
T T
T T
R
R
M
T T
T T
R
R
wire
phase phase
phase phase
x
ref
wire
phase phase
phase phase
x
ref
2 4
3 1
2 1
3
3 4
2 1
− −
−
=
−
−
=
=
−
−
=
,
(3)
The linearity is better than 13 bits provided that the amplitude of the voltages V
AB
and V
CD
is
below 0.7 V for V
DD
= 5V. For V
DD
= 3.3 V, these voltages should be less than 0.4 V. This limits
the current through the platinum resistor.
Current limitation is also required to limit the error due to self-heating. For instance, for a thermal
resistance of 200 K/W (still air) at
V
CD
= 0.7 V and 0ºC, the self-heating effect of a Pt100 causes an
error of 1 K. If this self-heating error is too large,
R
BIAS
must be increased to limit the current through the
Pt100. For
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,amounts to 2 mA which requires
R
BIAS
= 2.2 kΩ.
The relative sensitivity of a Pt100 is 3.9×10
-3
/K. When the current through the Pt100 is 2 mA, this
sensitivity corresponds to 780 µV/K. The UTI resolution in this mode is 7 µV, corresponding to 9 mK
resolution for the measurement in slow mode.
Table
13 lists the specifications of the UTI in the Pt
mode.
Parameter (V
DD
= 5 V) Typical value
K
2
56 s/V
V
0
0.36 V
R
BIAS
(Pt100, self-heating for
2.2 k (5%),
200K/W = 80 mK) I = 2 mA
6.2 k (5%)
R
BIAS
(Pt1000, self-heating for 200K/W = 80 mK)
I = 600 A
Excitation current from E and F 20 mA
Offset 10 V
Linearity 13 bits
Resolution (SF = 0) (Pt100, 2 mA) 14 bits (9 mK)
Table 13. Specifications of the Pt mode
Amplitudes of
V
CD
and
V
AB
up to 2.5 V peak-to-peak are allowed (very good resolutions can be obtained),
but self-heating effects and nonlinearity have to be taken into account.
However, linearity can
decrease to 8 bits for peak-to-peak amplitudes in the range of 0.7-2.5 V.
Platinum resistors can also be measured using mode 11.