Datasheet

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last update
November 30, 2010
reference
smtutin
page
15/27
7 RESISTIVE MODES (MODE 5 – 15)
7.1 Mode 5. Pt: 1 platinum resistor Pt100/ Pt1000, 4-wire
In this mode, one platinum resistor and one reference resistor can be measured. The connection of the
resistors to the UTI is depicted in Figure 9 . Because of the use of force/sense wires, both resistors R
x
and R
ref
are measured in a 4-wire setup, thereby completely eliminating the effect of lead resistances.
The driving voltage V
EF
is a square wave with amplitude V
DD
at 1/4 of the internal oscillator frequency.
The resistor R
BIAS
is used to set the current through the chain. For a measurement of a Pt100, the
measurement accuracy amounts to ±40 mΩ. This 40 mΩ can be considered as a systematic error
caused by the internal design of the chip.
Excitation of the measurement chain is done with an AC voltage. The cable capacitance has
influence on the measurement accuracy. For applications with any significant length of
(shielded) cable a special interface is developed. Please refer to the relevant application
notes in the support shop of our website and you will see how cable lengths up to 200 meter
can be implemented without loss of precision. Smartec recommends this solution for all
resistive applications, where the sensing element is not close to the UTI.
One measurement cycle consists of 4 phases. These phases contain the information for a 2-, 3- or 4-
wire measurement.
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A) B) C)
Figure 9. Connection of platinum resistors to the UTI in a 4-wire (a), 3-wire (b) and a 2-wire (c)
connection
Phase Measured voltages Output periods
1 V
offset
Toffset=N*K
2
*Voffset
2 V
ab
+V
offset
Tab= N*K
2
*(Vab+Voffset)
3 Vcd+V
offset
Tcd=N*K
2
*(Vcd+Voffset)
4 V
bc
+V
offset
Tbc= N*K
2
*(Vbc+Voffset)
Table 12. Measured voltages during the measurement of platinum resistors
E
A
B
C
D
F
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R
bIAS
R
REF
R
X
E
A
B
C
D
F
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R
bIAS
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R
REF
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R
X