Datasheet

8
®
XTR112, XTR114
APPLICATION INFORMATION
Figure 1 shows the basic connection diagram for the XTR112
and XTR114. 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 current sources drive the RTD and zero-
setting resistor, R
Z
. These current sources are 250µA for the
XTR112 and 100µA for the XTR114. Their instrumentation
amplifier input 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
XTR112 and XTR114.
R
CM
provides an additional voltage drop to bias the inputs of
the XTR112 and XTR114 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 instrumentation amplifier according to the desired tem-
perature range. R
LIN1
provides second-order linearization
correction to the RTD, typically achieving a 40:1 improve-
ment in linearity. An additional resistor is required for three-
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 R
G
is not used the gain is zero and
the output is simply the XTR’s zero current. The value of R
G
varies slightly for two-wire RTD and three-wire RTD con-
nections with linearization. R
G
can be calculated from the
equations given in Figure 1 (two-wire RTD connection) and
Table I (three-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 XTR112 and XTR114
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 precision current references. V
REG
is capable of sourcing
approximately 2.1mA of current for the XTR112 and 2.4mA
for the XTR114. Exceeding these values may affect the 4mA
zero output. Both products can sink approximately 1mA.
FIGURE 1. Basic Two-Wire RTD Temperature Measurement Circuit with Linearization.
14
11
12
13
4
3
2
R
G
XTR112
XTR114
R
CM
7
1
0.01µF
I = 4mA + V
IN
• ( )
O
40
R
G
R
Z
RTD
6
(2)
NOTES: (1) R
Z
= RTD resistance at minimum measured temperature.
R
G
R
G
V
IN
–
V
IN
+
V
LIN
I
R1
I
R2
V
REG
V+
I
RET
I
O
E
B
(1)
R
G
=
2.5 • I
REF
[R
1
(R
2
+ R
Z
) – 2(R
2
R
Z
)]
R
2
– R
1
(2)
R
LIN1
=
where R
1
= RTD Resistance at (T
MIN
+ T
MAX
)/2
R
2
= RTD Resistance at T
MAX
R
LIN
= 1kΩ (Internal)
I
REF
= 0.25 for XTR112
I
REF
= 0.1 for XTR114
0.4 • R
LIN
(R
2
– R
1
)
I
REF
(2R
1
– R
2
– R
Z
)
(3)
V
PS
8
4-20 mA
I
O
0.01µF
I
R1
I
R2
7.5V to 36V
–
+
9
10
R
LIN1
(3)
R
L
V
O
Q
1
TYPE
2N4922
TIP29C
TIP31C
PACKAGE
TO-225
TO-220
TO-220
Possible choices for Q
1
(see text).
XTR112: I
R1
= I
R2
= 250µA, R
CM
= 3.3kΩ
XTR114: I
R1
= I
R2
= 100µA, R
CM
= 8.2kΩ