APPLICATION INFORMATION
Figure 1 shows the basic connection diagram for the XTR105.
The loop power supply, V
PS
, provides power for all circuitry.
Output loop current is measured as a voltage across the
series load resistor, R
L
.
Two matched 0.8mA current sources drive the RTD and
zero-setting resistor, R
Z
. The instrumentation amplifier input
of the XTR105 measures the voltage difference between the
RTD and R
Z
. The value of R
Z
is chosen to be equal to the
resistance of the RTD at the low-scale (minimum) measure-
ment temperature. R
Z
can be adjusted to achieve 4mA output
at the minimum measurement temperature to correct for
input offset voltage and reference current mismatch of the
XTR105.
R
CM
provides an additional voltage drop to bias the inputs of
the XTR105 within their common-mode input range. R
CM
should be bypassed with a 0.01µF capacitor to minimize
common-mode noise. Resistor R
G
sets the gain of the instru-
mentation amplifier according to the desired temperature
range. R
LIN1
provides 2nd-order linearization correction to the
RTD, typically achieving a 40:1 improvement in linearity. An
additional resistor is required for 3-wire RTD connections
(see Figure 3).
The transfer function through the complete instrumentation
amplifier and voltage-to-current converter is:
I
O
= 4mA + V
IN
• (40/R
G
)
(V
IN
in volts, R
G
in ohms)
where V
IN
is the differential input voltage.
As evident from the transfer function, if no R
G
is used the
gain is zero and the output is simply the XTR105’s zero
current. The value of R
G
varies slightly for 2-wire RTD and 3-
wire RTD connections with linearization. R
G
can be calcu-
lated from the equations given in Figure 1 (2-wire RTD
connection) and Table I (3-wire RTD connection).
The I
RET
pin is the return path for all current from the current
sources and V
REG
. The I
RET
pin allows any current used in
external circuitry to be sensed by the XTR105 and to be
included in the output current without causing an error.
The V
REG
pin provides an on-chip voltage source of approxi-
mately 5.1V and is suitable for powering external input
circuitry (refer to Figure 6). It is a moderately accurate
voltage reference—it is not the same reference used to set
the 800µA current references. V
REG
is capable of sourcing
approximately 1mA of current. Exceeding 1mA may affect
the 4mA zero output.
I
R
= 0.8mA
I
R
= 0.8mA
Possible choices for Q
1
(see text).
TYPE
2N4922
TIP29C
TIP31C
14
I
R2
7.5V to 36V
11
10
V+
I
O
4-20 mA
XTR105
B
9
Q
1
0.01µF
V
O
E
I
O
8
R
L
+
V
PS
–
PACKAGE
TO-225
TO-220
TO-220
12
13
V
LIN
+
V
IN
1
I
R1
V
REG
4
(2)
R
G
R
G
R
LIN1(3)
3
R
G
–
V
IN
2
RTD
R
Z
(1)
7
I
RET
6
I
O
= 4mA + V
IN
• ( 40 )
R
G
NOTES: (1) R
Z
= RTD resistance at minimum measured temperature.
R
CM
= 1kΩ
(2) R
G
=
(3) R
LIN1
=
2R
1
(R
2
+R
Z
) – 4(R
2
R
Z
)
R
2
– R
1
R
LIN
(R
2
– R
1
)
2(2R
1
– R
2
– R
Z
)
0.01µF
where R
1
= RTD Resistance at (T
MIN
+ T
MAX
)/2
R
2
= RTD Resistance at T
MAX
R
LIN
= 1kΩ (Internal)
FIGURE 1. Basic 2-Wire RTD Temperature Measurement Circuit with Linearization.
XTR105
SBOS061B
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TI [ TEXAS INSTRUMENTS ]
