廬
FOUR-WIRE RTD CURRENT-LOOP TRANSMITTER:
Four-Wire Connections to an RTD Allow the RTD to be Remotely
Located from Active Circuitry, Yet Maintain Accuracy
by David Jones, and Mark Stitt
Platinum Resistance Temperature Detectors (RTD)s are
favored for demanding temperature measurement applica-
tions, such as process control, because of their excellent
accuracy and sensor interchangeability. Combining an RTD
with a Burr-Brown XTR105 forms a complete precision
temperature to 4-to-20mA current-loop transmitter solution.
Current-mode transmission gives superior noise rejection
and is immune to errors resulting from series line resistance.
XTR105 is a two-wire transmitter that uses the same wire-
pair for both signal and power supply, which reduces
expensive wiring cost and simplifies installation.
XTR105 is an application-specific IC containing onboard
transducer excitation, signal conditioning instrumentation
amplifier, linearization circuitry, and two-wire current-loop
output on a single chip. Excitation consists of two on-chip
current sources. One current source excites the RTD and a
second current source drives a reference resistor to set
system 鈥渮ero.鈥?/div>
When the RTD is located near the XTR105, a basic 鈥渢wo-
wire鈥?solution can be implemented, as shown in Figure 1.
Errors due to wiring resistance are minimal and the XTR105
will provide good accuracy. However, when the RTD is
R
LIN
I
REF
(2 ea.) 0.8mA
4-to-20mA
+IN
14
13
3
R
G
100鈩?/div>
RTD
4
鈥揑N
R
Z
100鈩?/div>
2
6
1
12
10
9
XTR105
8
7
B
E
0.01碌F
R
CM
2k鈩?/div>
0.01碌F
FIGURE 1. Basic RTD to 4-to-20mA Two-Wire Tansmitter Using XTR105. Internal 0.8mA current sources excite the RTD
and zero-setting resistor. R
CM
provides additional voltage drop to set the XTR105 inputs within their valid
common-mode input range. Optional resistor, R
LIN
can be used for linearization of the RTD. See the XTR105 data
sheet for additional details.
The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN assumes
no responsibility for the use of this information, and all use of such information shall be entirely at the user鈥檚 own risk. Prices and specifications are subject to change
without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not authorize or warrant
any BURR-BROWN product for use in life support devices and/or systems.
漏
1999 Burr-Brown Corporation
AB-151
Printed in U.S.A. October, 1999
1
2
3
