Datasheet

10
SBOS061B
www.ti.com
XTR105
In 3-wire RTD connections, an additional resistor, R
LIN2
, is
required. As with the 2-wire RTD application, R
LIN1
provides
positive feedback for linearization. R
LIN2
provides an offset
canceling current to compensate for wiring resistance en-
countered in remotely located RTDs. R
LIN1
and R
LIN2
are
chosen such that their currents are equal. This makes the
voltage drop in the wiring resistance to the RTD a common-
mode signal that is rejected by the XTR105. The nearest
standard 1% resistor values for R
LIN1
and R
LIN2
should be
adequate for most applications. Table I provides the 1%
resistor values for a 3-wire Pt100 RTD connection.
If no linearity correction is desired, the V
LIN
pin should be left
open. With no linearization, R
G
= 2500 • V
FS
, where
V
FS
= full-scale input range.
RTDs
The text and figures thus far have assumed a Pt100 RTD. With
higher resistance RTDs, the temperature range and input
voltage variation should be evaluated to ensure proper com-
mon-mode biasing of the inputs. As mentioned earlier, R
CM
can
be adjusted to provide an additional voltage drop to bias the
inputs of the XTR105 within their common-mode input range.
ERROR ANALYSIS
See Table II for how to calculate the effect various error
sources have on circuit accuracy. A sample error calculation
for a typical RTD measurement circuit (Pt100 RTD, 200°C
measurement span) is provided. The results reveal the
XTR105’s excellent accuracy, in this case 1.1% unadjusted.
Adjusting resistors R
G
and R
Z
for gain and offset errors
improves circuit accuracy to 0.32%. Note that these are
worst-case errors; ensured maximum values were used in
the calculations and all errors were assumed to be positive
(additive). The XTR105 achieves performance that is difficult
to obtain with discrete circuitry and requires less space.
OPEN-CIRCUIT PROTECTION
The optional transistor Q
2
in Figure 3 provides predictable
behavior with open-circuit RTD connections. It assures that
if any one of the three RTD connections is broken, the
XTR105’s output current will go to either its high current limit
(≈ 27mA) or low current limit (≈ 2.2mA). This is easily
detected as an out-of-range condition.
FIGURE 3. Remotely Located RTDs with 3-Wire Connection.
Resistance in this line causes
a small common-mode voltage
which is rejected by the XTR105.
OPEN RTD
TERMINAL
I
O
1
2
3
≈ 2.2mA
≈27mA
≈2.2mA
RTD
(R
LINE2
)(R
LINE1
)
R
Z
(1)
R
LIN2
(1)
R
LIN1
(1)
(R
LINE3
)
2
1
3
0.01µF
R
CM
= 1000Ω
0.01µF
Q
2
(2)
2N2222
NOTES: (1) See Table I for resistor equations and
1% values. (2) Q
2
optional. Provides predictable
output current if any one RTD connection is
broken:
13
4
3
2
R
G
XTR105
7
6
(1)
R
G
R
G
V
IN
–
V
IN
+
V
LIN
I
R1
I
R2
V
REG
V+
I
RET
I
O
E
B
8
9
Q
1
I
O
I
O
14
11
12
1
10
EQUAL line resistances here
creates a small common-mode
voltage which is rejected by
the XTR105.