RTD
XTR105
4-20 mA
V
PS
V
O
R
L
R
G
V
LIN
V
REG
+7.5V to 36V
I
R
= 0.8mA
I
R
= 0.8mA
XTR105
4-20mA CURRENT TRANSMITTER
with Sensor Excitation and Linearization
®
International Airport Industrial Park • Mailing Address: PO Box 11400, Tucson, AZ 85734 • Street Address: 6730 S. Tucson Blvd., Tucson, AZ 85706 • Tel: (520) 746-1111 • Twx: 910-952-1111
Internet: http://www.burr-brown.com/ • FAXLine: (800) 548-6133 (US/Canada Only) • Cable: BBRCORP • Telex: 066-6491 • FAX: (520) 889-1510 • Immediate Product Info: (800) 548-6132
©1997 Burr-Brown Corporation PDS-1362A Printed in U.S.A. February, 1997
FEATURES
LOW UNADJUSTED ERROR
TWO PRECISION CURRENT SOURCES
800µA EACH
RTD OR BRIDGE EXCITATION
LINEARIZATION
TWO OR THREE-WIRE RTD OPERATION
LOW OFFSET DRIFT: 0.4µV/°C
LOW OUTPUT CURRENT NOISE: 30nAp-p
HIGH PSR: 110dB min
HIGH CMR: 86dB min
WIDE SUPPLY RANGE: 7.5V TO 36V
14-PIN DIP AND SO-14 SOIC PACKAGES
APPLICATIONS
INDUSTRIAL PROCESS CONTROL
FACTORY AUTOMATION
SCADA REMOTE DATA ACQUISITION
REMOTE TEMPERATURE AND PRESSURE
TRANSDUCERS
–200°C
Pt100 NONLINEARITY CORRECTION
USING XTR105
Process Temperature (°C) +850°C
5
4
3
2
1
0
–1
Uncorrected
RTD Nonlinearity
Corrected
Nonlinearity
Nonlinearity (%)
DESCRIPTION
The XTR105 is a monolithic 4-20mA, two-wire
current transmitter with two precision current sources.
It provides complete current excitation for Platinum
RTD temperature sensors and bridges, instrumenta-
tion amplifier, and current output circuitry on a single
integrated circuit.
Versatile linearization circuitry provides a 2nd-order
correction to the RTD, typically achieving a 40:1
improvement in linearity.
Instrumentation amplifier gain can be configured for a
wide range of temperature or pressure measurements.
Total unadjusted error of the complete current trans-
mitter is low enough to permit use without adjustment
in many applications. This includes zero output cur-
rent drift, span drift and nonlinearity. The XTR105
operates on loop power supply voltages down to 7.5V.
The XTR105 is available in 14-pin plastic DIP and
SO-14 surface-mount packages and is specified for the
–40°C to +85°C industrial temperature range.
XTR105
XTR105
2
®
XTR105
IO = VIN • (40/RG) + 4mA, VIN in Volts, RG in
Specification same as XTR105P, XTR105U.
NOTES: (1) Describes accuracy of the 4mA low-scale offset current. Does not include input amplifier effects. Can be trimmed to zero. (2) Voltage measured with
respect to IRET pin. (3) Does not include initial error or TCR of gain-setting resistor, RG. (4) Increasing the full-scale input range improves nonlinearity. (5) Does not
include Zero Output initial error. (6) Current source output voltage with respect to IRET pin.
SPECIFICATIONS
At TA = +25°C, V+ = 24V, and TIP29C external transistor, unless otherwise noted.
XTR105P, U XTR105PA, UA
PARAMETER CONDITIONS MIN TYP MAX MIN TYP MAX UNITS
OUTPUT
Output Current Equation A
Output Current, Specified Range 4 20 ✻✻mA
Over-Scale Limit 24 27 30 ✻✻ mA
Under-Scale Limit IREG = 0V 1.8 2.2 2.6 ✻✻ mA
ZERO OUTPUT(1) VIN = 0V, RG = 4mA
Initial Error ±5±25 ±50 µA
vs Temperature ±0.15 ±0.5 ±0.9 µA/°C
vs Supply Voltage, V+ V+ = 7.5V to 36V 0.04 0.2 ✻✻ µA/V
vs Common-Mode Voltage
VCM = 1.25V to 3.5V(2) 0.02 µA/V
vs VREG Output Current 0.3 µA/mA
Noise: 0.1Hz to 10Hz 0.03 µAp-p
SPAN
Span Equation (Transconductance)
S = 40/R
G
A/V
Initial Error(3) Full Scale (VIN) = 50mV ±0.05 ±0.2 ±0.4 %
vs Temperature(3) ±3±25 ✻✻ ppm/°C
Nonlinearity: Ideal Input (4) Full Scale (VIN) = 50mV 0.003 0.01 ✻✻ %
INPUT(5)
Offset Voltage VCM = 2V ±50 ±100 ±250 µV
vs Temperature ±0.4 ±1.5 ±3µV/°C
vs Supply Voltage, V+ V+ = 7.5V to 36V ±0.3 ±3✻✻ µV/V
vs Common-Mode Voltage, VCM = 1.25V to 3.5V(2) ±10 ±50 ±100 µV/V
RTI (CMRR)
Common-Mode Input Range(2) 1.25 3.5 ✻✻V
Input Bias Current 525 50 nA
vs Temperature 20 pA/°C
Input Offset Current ±0.2 ±3±10 nA
vs Temperature 5 pA/°C
Impedance: Differential 0.1 || 1 G|| pF
Common-Mode 5 || 10 G|| pF
Noise: 0.1Hz to 10Hz 0.6 µVp-p
CURRENT SOURCES VO = 2V(6)
Current 800 µA
Accuracy ±0.05 ±0.2 ±0.4 %
vs Temperature ±15 ±35 ±75 ppm/°C
vs Power Supply, V+ V+ = 7.5V to 36V ±10 ±25 ✻✻ ppm/V
Matching ±0.02 ±0.1 ±0.2 %
vs Temperature ±3±15 ±30 ppm/°C
vs Power Supply, V+ V+ = 7.5V to 36V 1 10 ✻✻ ppm/V
Compliance Voltage, Positive (V+) –3
(V+) –2.5
✻✻ V
Negative(2) 0 –0.2 ✻✻ V
Output Impedance 150 M
Noise: 0.1Hz to 10Hz 0.003 µAp-p
VREG(2) 5.1 V
Accuracy ±0.02 ±0.1 ✻✻ V
vs Temperature ±0.2 mV/°C
vs Supply Voltage, V+ 1 mV/V
Output Current ±1mA
Output Impedance 75
LINEARIZATION
RLIN (internal) 1 k
Accuracy ±0.2 ±0.5 ±1%
vs Temperature ±25 ±100 ✻✻ ppm/°C
POWER SUPPLY
Specified +24 V
Voltage Range +7.5 +36 ✻✻V
TEMPERATURE RANGE
Specification, TMIN to TMAX –40 +85 ✻✻°C
Operating –55 +125 ✻✻°C
Storage –55 +125 ✻✻°C
Thermal Resistance,
θ
JA
14-Pin DIP 80 °C/W
SO-14 Surface-Mount 100 °C/W
3
®
XTR105
Power Supply, V+ (referenced to IO pin) ..........................................40V
Input Voltage, VIN, VIN (referenced to IO pin) ............................0V to V+
Storage Temperature Range........................................ –55°C to +125°C
Lead Temperature (soldering, 10s).............................................. +300°C
Output Current Limit ............................................................... Continuous
Junction Temperature................................................................... +165°C
NOTE: (1) Stresses above these ratings may cause permanent damage.
ABSOLUTE MAXIMUM RATINGS(1)
TOP VIEW DIP and SOIC
I
R1
V
IN
R
G
R
G
NC
I
RET
I
O
I
R2
V
IN
V
LIN
V
REG
V+
B (Base)
E (Emitter)
NC = No Internal Connection.
1
2
3
4
5
6
7
14
13
12
11
10
9
8
–+
ELECTROSTATIC
DISCHARGE SENSITIVITY
This integrated circuit can be damaged by ESD. Burr-Brown
recommends that all integrated circuits be handled with
appropriate precautions. Failure to observe proper handling
and installation procedures can cause damage.
ESD damage can range from subtle performance degradation
to complete device failure. Precision integrated circuits may
be more susceptible to damage because very small parametric
changes could cause the device not to meet its published
specifications.
PACKAGE
DRAWING TEMPERATURE
PRODUCT PACKAGE NUMBER(1) RANGE
XTR105PA 14-Pin Plastic DIP 010 –40°C to +85°C
XTR105P 14-Pin Plastic DIP 010 –40°C to +85°C
XTR105UA SO-14 Surface Mount 235 –40°C to +85°C
XTR105U SO-14 Surface Mount 235 –40°C to +85°C
NOTE: (1) For detailed drawing and dimension table, please see end of data
sheet, or Appendix C of Burr-Brown IC Data Book.
FUNCTIONAL BLOCK DIAGRAM
PACKAGE/ORDERING INFORMATION
PIN CONFIGURATION
+
975
6
I = 100µA +
100µA
800µA 800µA
25
V+
Q
1
9
B
10
11
3
13
2
4
8
E
V
IN
R
G
I
O
= 4mA + V
IN
40
R
G
( )
5.1V
R
G
R
LIN
1k
V
IN
+
V
IN
I
RET
7
V
REG
14
1
12 I
R2
I
R1
V
LIN
4
®
XTR105
–75 –50 –25 0 25 50 75 100
Temperature (°C)
UNDER-SCALE CURRENT vs TEMPERATURE
125
2.40
2.35
2.30
2.25
2.20
2.15
Under-Scale Current (mA)
V+ = 7.5V to 36V
TYPICAL PERFORMANCE CURVES
At TA = +25°C, V+ = 24V, unless otherwise noted.
20mA
STEP RESPONSE
25µs/div
4mA/div
RG = 125
RG = 2k
4mA
10 100 1k 10k 100k
Frequency (Hz)
POWER-SUPPLY REJECTION vs FREQUENCY
1M
140
120
100
80
60
40
20
0
Power Supply Rejection (dB)
R
G
= 2k
R
G
= 125
100 1k 10k 100k
Frequency (Hz)
TRANSCONDUCTANCE vs FREQUENCY
1M
50
40
30
20
10
0
Transconductance (20 Log mA/V)
R
G
= 125
R
G
= 500
R
G
= 2k
10 100 1k 10k 100k
Frequency (Hz) 1M
110
100
90
80
70
60
50
40
30
20
Common-Mode Rejection (dB)
COMMON-MODE REJECTION vs FREQUENCY
R
G
= 2k
R
G
= 125
Full-Scale Input = 50mV
–75 –50 –25 0 25 50 75 100
Temperature (°C)
OVER-SCALE CURRENT vs TEMPERATURE
125
29
28
27
26
25
24
23
Over-Scale Current (mA)
V+ = 7.5V
V+ = 36V
V+ = 24V
With External Transistor
5
®
XTR105
TYPICAL PERFORMANCE CURVES (CONT)
At TA = +25°C, V+ = 24V, unless otherwise noted.
–75 –50 –25 0 25 50 75 100
Temperature (°C)
INPUT BIAS AND OFFSET CURRENT
vs TEMPERATURE
125
25
20
15
10
5
0
Input Bias and Offset Current (nA)
+IB
IOS
–IB
1 10 100 1k 10k
Frequency (Hz)
INPUT VOLTAGE AND CURRENT
NOISE DENSITY vs FREQUENCY
100k
10k
1k
100
10
Input Voltage Noise (nV/Hz)
10k
1k
100
10
Input Current Noise (fA/Hz)
Current Noise
Voltage Noise
1 10 100 1k 10k
Frequency (Hz)
ZERO OUTPUT AND REFERENCE
CURRENT NOISE vs FREQUENCY
100k
10k
1k
100
10
Noise (pA/Hz)
Zero Output Current
Reference Current
–75 –50 –25 0 25 50 75 100
Temperature (°C)
ZERO OUTPUT CURRENT ERROR
vs TEMPERATURE
125
4
2
0
–2
–4
–6
–8
–10
–12
Zero Output Current Error (µA)
Input Offset Voltage Drift (µV/°C)
INPUT OFFSET VOLTAGE DRIFT
PRODUCTION DISTRIBUTION
50
45
40
35
30
25
20
15
10
5
0
Percent of Units (%)
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
2.6
2.8
3.0
0.1% 0.02%
Typical Production Distribution
of Packaged Units.
Zero Output Drift (µA/°C)
ZERO OUTPUT DRIFT
PRODUCTION DISTRIBUTION
40
35
30
25
20
15
10
5
0
Percent of Units (%)
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0.55
0.60
0.65
0.70
0.75
0.80
0.85
0.90
0.95
1.00
0.3% 0.05% 0.02%
Typical Production Distribution
of Packaged Units.
6
®
XTR105
TYPICAL PERFORMANCE CURVES (CONT)
At TA = +25°C, V+ = 24V, unless otherwise noted.
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’s 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.
–1 0.5 0 0.5 1 1.5
V
REG
Output Current (mA)
V
REG
OUTPUT VOLTAGE vs V
REG
OUTPUT CURRENT
2
5.35
5.30
5.25
5.20
5.15
5.10
5.05
5
V
REG
Output Voltage (V)
125°C
NOTE: Above 1mA,
Zero Output Degrades
–55°C
25°C
Temperature (°C)
REFERENCE CURRENT ERROR
vs TEMPERATURE
+0.05
0
–0.05
–0.10
–0.15
–0.20
Reference Current Error (%)
–75 –50 –25 0 25 50 75 100 125
Current Source Drift (ppm/°C)
CURRENT SOURCE DRIFT
PRODUCTION DISTRIBUTION
40
35
30
25
20
15
10
5
0
Percent of Units (%)
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
0.04% 0.01%
Typical Production Distribution
of Packaged Units.
I
R1
AND I
R2
Included.
Current Source Matching Drift (ppm/°C)
CURRENT SOURCE MATCHING
DRIFT PRODUCTION DISTRIBUTION
80
70
60
50
40
30
20
10
0
Percent of Units (%)
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
0.07% 0.02%
Typical Production Distribution
of Packaged Units.
7
®
XTR105
APPLICATION INFORMATION
Figure 1 shows the basic connection diagram for the XTR105.
The loop power supply, VPS, provides power for all cir-
cuitry. Output loop current is measured as a voltage across
the series load resistor, RL.
Two matched 0.8mA current sources drive the RTD and
zero-setting resistor, RZ. The instrumentation amplifier in-
put of the XTR105 measures the voltage difference between
the RTD and RZ. The value of RZ is chosen to be equal to
the resistance of the RTD at the low-scale (minimum)
measurement temperature. RZ can be adjusted to achieve
4mA output at the minimum measurement temperature to
correct for input offset voltage and reference current mis-
match of the XTR105.
RCM provides an additional voltage drop to bias the inputs of
the XTR105 within their common-mode input range. RCM
should be bypassed with a 0.01µF capacitor to minimize
common-mode noise. Resistor RG sets the gain of the instru-
mentation amplifier according to the desired temperature
range. RLIN1 provides second-order linearization correction
to the RTD, typically achieving a 40:1 improvement 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:
IO = 4mA + VIN • (40/RG)
(VIN in volts, RG in ohms)
where VIN is the differential input voltage. As evident from
the transfer function, if no RG is used the gain is zero and the
output is simply the XTR105’s zero current. The value of RG
varies slightly for two-wire RTD and three-wire RTD con-
nections with linearization. RG can be calculated from the
equations given in Figure 1 (two-wire RTD connection) and
Table I (three-wire RTD connection).
The IRET pin is the return path for all current from the current
sources and VREG. The IRET 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 VREG 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. VREG is capable of sourcing
approximately 1mA of current. Exceeding 1mA may affect
the 4mA zero output.
FIGURE 1. Basic Two-Wire RTD Temperature Measurement Circuit with Linearization.
14 11
12
13
3
4
2
R
G
XTR105
R
CM
= 1k
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
= 2R
1
(R
2
+R
Z
) – 4(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
= 1k (Internal)
R
LIN
(R
2
– R
1
)
2(2R
1
– R
2
– R
Z
)
(3)
V
PS
8
4-20 mA
I
O
0.01µF
I
R
= 0.8mA
I
R
= 0.8mA
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).
8
®
XTR105
to 36V. The loop supply voltage, VPS, will differ from the
voltage applied to the XTR105 according to the voltage drop
on the current sensing resistor, RL (plus any other voltage
drop in the line).
If a low loop supply voltage is used, RL (including the loop
wiring resistance) must be made a relatively low value to
assure that V+ remains 7.5V or greater for the maximum
loop current of 20mA:
It is recommended to design for V+ equal or greater than
7.5V with loop currents up to 30mA to allow for out-of-
range input conditions.
The low operating voltage (7.5V) of the XTR105 allows
operation directly from personal computer power supplies
(12V ±5%). When used with the RCV420 Current Loop
Receiver (Figure 7), load resistor voltage drop is limited to
3V.
ADJUSTING INITIAL ERRORS
Many applications require adjustment of initial errors. Input
offset and reference current mismatch errors can be cor-
rected by adjustment of the zero resistor, RZ. Adjusting the
gain-setting resistor, RG, corrects any errors associated with
gain.
TWO-WIRE AND
THREE-WIRE RTD CONNECTIONS
In Figure 1, the RTD can be located remotely simply by
extending the two connections to the RTD. With this remote
two-wire connection to the RTD, line resistance will intro-
duce error. This error can be partially corrected by adjusting
the values of RZ, RG, and RLIN1.
A better method for remotely located RTDs is the three-wire
RTD connection shown in Figure 3. This circuit offers
improved accuracy. RZ’s current is routed through a third
wire to the RTD. Assuming line resistance is equal in RTD
lines 1 and 2, this produces a small common-mode voltage
which is rejected by the XTR105. A second resistor, RLIN2,
is required for linearization.
Note that although the two-wire and three-wire RTD con-
nection circuits are very similar, the gain-setting resistor,
RG, has slightly different equations:
Two-wire:
Three-wire:
where RZ = RTD resistance at TMIN
R1 = RTD resistance at (TMIN + TMAX)/2
R2 = RTD resistance at TMAX
A negative input voltage, VIN, will cause the output current
to be less than 4mA. Increasingly negative VIN will cause the
output current to limit at approximately 2.2mA. Refer to the
typical curve “Under-Scale Current vs Temperature.”
Increasingly positive input voltage (greater than the full-
scale input) will produce increasing output current according
to the transfer function, up to the output current limit of
approximately 27mA. Refer to the typical curve “Over-
Scale Current vs Temperature.”
EXTERNAL TRANSISTOR
Transistor Q1 conducts the majority of the signal-dependent
4-20mA loop current. Using an external transistor isolates
the majority of the power dissipation from the precision
input and reference circuitry of the XTR105, maintaining
excellent accuracy.
Since the external transistor is inside a feedback loop its
characteristics are not critical. Requirements are: VCEO =
45V min,
β
= 40 min and PD = 800mW. Power dissipation
requirements may be lower if the loop power supply voltage
is less than 36V. Some possible choices for Q1 are listed in
Figure 1.
The XTR105 can be operated without this external transis-
tor, however, accuracy will be somewhat degraded due to
the internal power dissipation. Operation without Q1 is not
recommended for extended temperature ranges. A resistor
(R = 3.3k) connected between the IRET pin and the E
(emitter) pin may be needed for operation below 0°C with-
out Q1 to guarantee the full 20mA full-scale output, espe-
cially with V+ near 7.5V.
LOOP POWER SUPPLY
The voltage applied to the XTR105, V+, is measured with
respect to the IO connection, pin 7. V+ can range from 7.5V
FIGURE 2. Operation Without External Transistor.
R
L
max =(V+)–7.5V
20mA
–R
WIRING
R
G
=2R
1
(R
2
+R
Z
)–4(R
2
R
Z
)
R
2
–R
1
R
G
=2(R
2
–R
Z
)(R
1
–R
Z
)
R
2
–R
1
8
XTR105 0.01µF
E
I
O
I
RET
V+
10
7
6
R
Q
= 3.3k
For operation without external
transistor, connect a 3.3k
resistor between pin 6 and 
pin 8. See text for discussion 
of performance.
9
®
XTR105
To maintain good accuracy, at least 1% (or better) resistors
should be used for RG. Table I provides standard 1% RG
resistor values for a three-wire Pt100 RTD connection with
linearization.
LINEARIZATION
RTD temperature sensors are inherently (but predictably)
nonlinear. With the addition of one or two external resistors,
RLIN1 and RLIN2, it is possible to compensate for most of this
nonlinearity resulting in 40:1 improvement in linearity over
the uncompensated output.
A typical two-wire RTD application with linearization is
shown in Figure 1. Resistor RLIN1 provides positive feed-
back and controls linearity correction. RLIN1 is chosen ac-
cording to the desired temperature range. An equation is
given in Figure 1.
In three-wire RTD connections, an additional resistor, RLIN2,
is required. As with the two-wire RTD application, RLIN1
provides positive feedback for linearization. RLIN2 provides
an offset canceling current to compensate for wiring resis-
tance encountered in remotely located RTDs. RLIN1 and
RLIN2 are chosen such that their currents are equal. This
makes the voltage drop in the wiring resistance to the RTD
a common-mode signal which is rejected by the XTR105.
The nearest standard 1% resistor values for RLIN1 and RLIN2
should be adequate for most applications. Table I provides
the 1% resistor values for a three-wire Pt100 RTD connec-
tion.
If no linearity correction is desired, the VLIN pin should be
left open. With no linearization, RG = 2500 • VFS, where
VFS = 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
common-mode biasing of the inputs. As mentioned earlier,
RCM can be adjusted to provide an additional voltage drop to
bias the inputs of the XTR105 within their common-mode
input range.
OPEN-CIRCUIT PROTECTION
The optional transistor Q2 in Figure 3 provides predictable
behavior with open-circuit RTD connections. It assures that
FIGURE 3. Three-Wire Connection for Remotely Located RTDs.
Resistance in this line causes 
a small common-mode voltage
which is rejected by 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
)
21
3
0.01µF
R
CM
= 10000.01µF
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
3
4
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
9Q
1
I
O
I
O
14 11
12 1
10
EQUAL line resistances here
creates a small common-mode
voltage which is rejected by
XTR105.
10
®
XTR105
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.
REVERSE-VOLTAGE PROTECTION
The XTR105’s low compliance rating (7.5V) permits the
use of various voltage protection methods without compro-
mising operating range. Figure 4 shows a diode bridge
circuit which allows normal operation even when the volt-
age connection lines are reversed. The bridge causes a two
diode drop (approximately 1.4V) loss in loop supply volt-
age. This results in a compliance voltage of approximately
9V—satisfactory for most applications. If 1.4V drop in loop
supply is too much, a diode can be inserted in series with the
loop supply voltage and the V+ pin. This protects against
reverse output connection lines with only a 0.7V loss in loop
supply voltage.
SURGE PROTECTION
Remote connections to current transmitters can sometimes be
subjected to voltage surges. It is prudent to limit the maximum
surge voltage applied to the XTR105 to as low as practical.
Various zener diode and surge clamping diodes are specially
designed for this purpose. Select a clamp diode with as low a
voltage rating as possible for best protection. For example, a
36V protection diode will assure proper transmitter operation
at normal loop voltages, yet will provide an appropriate level
of protection against voltage surges. Characterization tests on
three production lots showed no damage to the XTR105
within loop supply voltages up to 65V.
Most surge protection zener diodes have a diode character-
istic in the forward direction that will conduct excessive
current, possibly damaging receiving-side circuitry if the
loop connections are reversed. If a surge protection diode is
used, a series diode or diode bridge should be used for
protection against reversed connections.
TABLE I. RZ, RG, RLIN1, and RLIN2 Standard 1% Resistor Values for Three-Wire Pt100 RTD Connection with Linearization.
TMIN 100°C 200°C 300°C 400°C 500°C 600°C 700°C 800°C 900°C 1000°C
–200°C 18.7/86.6 18.7/169 18.7/255 18.7/340 18.7/422 18.7/511 18.7/590 18.7/66.5 18.7/750 18.7/845
15000 9760 8060 6650 5620 4750 4020 3480 3090 2740
16500 11500 10000 8870 7870 7150 6420 5900 5360 4990
–100°C 60.4/80.6 60.4/162 60.4/243 60.4/324 60.4/402 60.4/487 60.4/562 60.4/649 60.4/732
27400 15400 10500 7870 6040 4990 4220 3570 3090
29400 17800 13000 10200 8660 7500 6490 5900 5360
0°C 100/78.7 100/158 100/237 100/316 100/392 100/475 100/549 100/634
33200 16200 10500 7680 6040 4870 4020 3480
35700 18700 13000 10000 8250 7150 6340 5620
100°C 137/75 137/150 137/226 137/301 137/383 137/453 137/536
31600 15400 10200 7500 5760 4750 3920
34000 17800 12400 9760 8060 6810 6040
200°C 174/73.2 174/147 174/221 174/294 174/365 174/442
30900 15000 9760 7150 5620 4530
3320 17400 12100 9310 7680 6490
300°C 210/71.5 210/143 210/215 210/287 210/357
30100 14700 9530 6980 5360
32400 16500 11500 8870 7320
400°C 249/68.1 249/137 249/205 249/274
28700 14000 9090 6650
30900 16200 11000 8450
500°C 280/66.5 280/133 280/200
28000 13700 8870
30100 15400 10500
600°C 316/64.9 313/130
26700 13000
28700 14700
700°C 348/61.9
26100
27400
800°C 374/60.4
24900
26700
MEASUREMENT TEMPERATURE SPAN T (°C)
RZ/RG
RLIN1
RLIN2
NOTE: The values listed in the table are 1%
resistors, exact values may be calculated
from the following equations:
RZ = RTD resistance at minimum measured
temperature.
R
G
=2(R
2
–R
Z
)(R
1
–R
Z
)
(R
2
–R
1
)
R2 = RTD resistance at TMAX
where R1 = RTD resistance at (TMIN + TMAX)/2
RLIN = 1k (Internal)
R
LIN1
=R
LIN
(R
2
–R
1
)
2(2R
1
–R
2
–R
Z
)
R
LIN2
=(R
LIN
+R
G
)(R
2
–R
1
)
2(2R
1
–R
2
–R
Z
)
11
®
XTR105
SAMPLE ERROR CALCULATION
RTD value at 4mA Output (RRTD MIN) 100
RTD Measurement Range 200°C (0.38/°C)
Ambient Temperature Range (TA)20°C
Supply Voltage Change (V+) 5V
Common-Mode Voltage Change (CM) 0.1V
SAMPLE
ERROR SOURCE ERROR EQUATION ERROR CALCULATION(1) UNADJ. ADJUST.
INPUT
Input Offset Voltage VOS/(VIN MAX) • 106100µV/(800µA • 0.38/°C • 200 °C) • 1061645 0
vs Common-Mode CMRR • CM/(VIN MAX) • 10650µV/V • 0.1V/(800µA * 0.38/°C • 200°C) • 10682 82
Input Bias Current IB/IREF • 1060.025µA/800µA • 10631 0
Input Offset Current IOS • RRTD MIN/(VIN MAX) • 1063nA • 100/(800µA • 0.38/°C • 200°C) • 10650
Total Input Error: 1763 82
EXCITATION
Current Reference Accuracy IREF Accuracy (%)/100% • 1060.2%/100% • 1062000 0
vs Supply (IREF vs V+) • V+ 25ppm/V • 5V 125 125
Current Reference Matching IREF Matching (%)/100% • 800µA • 0.1%/100% • 800µA • 100/(800µA • 0.38/°C • 200°C) • 1061316 0
RRTD MIN/(VIN MAX) • 106
vs Supply (IREF matching vs V+) • V+ • 10ppm/V • 5V • 800µA • 100/(800µA • 0.38/°C • 200°C) 66 66
RRTD MIN/(VIN MAX)Total Excitation Error: 3507 191
GAIN
Span Span Error (%)/100% • 1060.2%/100% • 1062000 0
Nonlinearity Nonlinearity (%)/100% • 1060.01%/100% • 106100 100
Total Gain Error: 2100 100
OUTPUT
Zero Output (IZERO - 4mA) /16000µA • 10625µA/16000µA • 1061563 0
vs Supply (IZERO vs V+) • V+/16000µA • 1060.2µA/V • 5V/16000 µA • 10663 63
Total Output Error: 1626 63
DRIFT (TA = 20°C)
Input Offset Voltage Drift • TA/(VIN MAX) • 1061.5µV/°C • 20°C/(800µA • 0.38/°C • 200°C) • 106493 493
Input Bias Current (typical) Drift • TA/800µA • 10620pA/°C • 20°C/800µA • 1060.5 0.5
Input Offset Current (typical) Drift • TA • RRTD MIN/(VIN MAX) • 1065pA/°C • 20°C • 100/(800µA • 0.38/°C • 200 °C) • 1060.2 0.2
Current Reference Accuracy Drift • TA35ppm/°C • 20°C 700 700
Current Reference Matching Drift • TA • 800µA • RRTD MIN/(VIN MAX) 15ppm/°C • 20°C • 800µA • 100/(800µA • 0.38/°C • 200°C) 395 395
Span Drift • TA25ppm/°C • 20°C 500 500
Zero Output Drift • TA/16000µA • 1060.5µA/°C • 20°C/16000µA • 106626 626
Total Drift Error: 2715 2715
NOISE (0.1 to 10Hz, typ)
Input Offset Voltage vn/(VIN MAX) • 1060.6µV/(800µA • 0.38/°C • 200°C) • 10610 10
Current Reference IREF Noise • RRTD MIN/(VIN MAX) • 1063nA • 100/(800µA • 0.38 /°C • 200°C) • 10 655
Zero Output IZERO Noise/16000µA • 1060.03µA/16000µA • 10 622
Total Noise Error: 17 17
TOTAL ERROR: 11728 3168
(1.17%) (0.32%)
NOTE (1): All errors are min/max and referred to input unless otherwise stated.
TABLE II. Error Calculation.
ERROR
(ppm of Full Scale)
RADIO FREQUENCY INTERFERENCE
The long wire lengths of current loops invite radio frequency
interference. RF can be rectified by the sensitive input
circuitry of the XTR105 causing errors. This generally
appears as an unstable output current that varies with the
position of loop supply or input wiring.
If the RTD sensor is remotely located, the interference may
enter at the input terminals. For integrated transmitter as-
semblies with short connection to the sensor, the interfer-
ence more likely comes from the current loop connections.
Bypass capacitors on the input reduce or eliminate this input
interference. Connect these bypass capacitors to the IRET
terminal as shown in Figure 5. Although the dc voltage at the
IRET terminal is not equal to 0V (at the loop supply, VPS) this
circuit point can be considered the transmitter’s “ground.”
The 0.01µF capacitor connected between V+ and IO may
help minimize output interference.
ERROR ANALYSIS
Table II shows 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 RG and RZ for gain and offset errors
improves circuit accuracy to 0.32%. Note that these are worst
case errors; guaranteed maximum values were used in the
calculations and all errors were assumed to be positive (addi-
tive). The XTR105 achieves performance which is difficult to
obtain with discrete circuitry and requires less space.
12
®
XTR105
FIGURE 4. Reverse Voltage Operation and Over-Voltage Surge Protection.
FIGURE 5. Input Bypassing Technique with Linearization.
XTR105
7
V+
I
O
E
B
V
PS
10
0.01µF
R
L
D
1(1)
9
8
NOTE: (1) Zener Diode 36V: 1N4753A or General
Semiconductor Transorb
TM
1N6286A. Use lower
voltage zener diodes with loop power supply
voltages less than 30V for increased protection.
See “Over-Voltage Surge Protection.”
Maximum V
PS
must be
less than minimum
voltage rating of zener
diode.
The diode bridge causes
a 1.4V loss in loop supply
voltage.
1N4148
Diodes
6
I
RET
14 11
12
13
3
4
2
R
G
XTR105
R
CM
7
1
0.01µF
R
Z
(1)
RTD
6
NOTE: (1) Alternative connection for bypass capacitors.
R
G
R
G
V
IN
V
IN
+
V
LIN
I
R1
I
R2
V
REG
V+
I
RET
I
O
E
B
8
0.01µF
9
10
1k
R
LIN1
R
LIN2
1k
13
®
XTR105
FIGURE 6. Thermocouple Low Offset, Low Drift Loop Measurement with Diode Cold Junction Compensation.
FIGURE 7. ±12V Powered Transmitter/Receiver Loop.
0.01µFQ
1
1N4148
–12V
1µF
5
4
2
3
15
13 14
11
10 12
1µF
V
O
= 0 to 5V
RCV420
16
+12V
8
7
9
E
B
14 11
12
13
3
4
2
XTR105
R
CM
= 1k
1
0.01µF
R
Z
137
R
LIN1
5760R
G
402
RTD
Pt100
100°C to
600°C
6
R
G
R
G
V
IN
V
IN
+
V
LIN
I
R1
I
R2
V
REG
V+
I
RET
I
O
10
I
O
= 4mA – 20mA
NOTE: A two-wire RTD connection is shown. For remotely
located RTDs, a three-wire RTD conection is recommended.
R
G
becomes 383, R
LIN2
is 8060. See Figure 3 and
Table I.
14
1
12
5V
11
13
I
REG
< 1mA
3
4
2
6
R
G
1250XTR105
1/2
LTC1047 7
R
G
R
G
V
IN
V
IN
+
V
REG
I
R2
V+
I
RET
I
O
E
B
8
I
O
= 4mA + (V
IN
–V
IN
)
+–
40
R
G
9
10
R
F
10k
R
412
1/2
LTC1047
V+
V–
Type J
25
(G = 1 + = 50)
2R
F
R
50
1k
R
F
10k
I
R1
V
LIN
R
CM
= 1250
14
®
XTR105
FIGURE 8. Isolated Transmitter/Receiver Loop.
FIGURE 9. Bridge Input, Current Excitation.
3
4
2
R
G
XTR105
7
6
R
G
R
G
V+10
13
B
E
9
8
V
IN
V
IN
+
I
RET
R
CM
= 1k
(1)
1.6mA
NOTE: (1) Use R
CM
to adjust the
common-mode voltage to within
1.25V to 3.5V.
14
1
12
11
V
REG
I
R2
I
R1
V
LIN
5
4
2
3
15
13 14
11
10 12
RCV420
16
16 2
15
10 87
9
V–
VO
V+
0 – 5V
ISO122
1
+15V
0
–15V
1µF
1µF
Isolated Power
from PWS740
0.01µFQ1
1N4148
8
7
9
E
B
14 11
12
13
3
4
2
XTR105
RCM = 1k
1
0.01µF
RLIN1
RG
RLIN2
RTD
6
RG
RG
VLIN IR1 IR2 V
REG
V+
IRET
IO
10
IO = 4mA – 20mA
VIN
VIN
+
RZ
NOTE: A three-wire RTD connection is shown.
For a two-wire RTD connection eliminate RLIN2.