BB XTR114

®
XTR112
XTR114
XTR
114
XTR
112
4-20mA CURRENT TRANSMITTERS
with Sensor Excitation and Linearization
FEATURES
APPLICATIONS
● LOW UNADJUSTED ERROR
● PRECISION CURRENT SOURCES
XTR112: Two 250µA
XTR114: Two 100µA
● 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
● SO-14 SOIC PACKAGE
●
●
●
●
The XTR112 and XTR114 are monolithic 4-20mA,
two-wire current transmitters. They provide complete
current excitation for high impedance platinum RTD
temperature sensors and bridges, instrumentation amplifier, and current output circuitry on a single integrated circuit. The XTR112 has two 250µA current
sources while the XTR114 has two 100µA sources for
RTD excitation.
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 transmitter is low enough to permit use without adjustment
in many applications. This includes zero output current drift, span drift and nonlinearity. The XTR112
and XTR114 operate on loop power supply voltages
down to 7.5V.
Both are available in an SO-14 surface-mount package and are specified for the –40°C to +85°C industrial temperature range.
Pt1000 NONLINEARITY CORRECTION
USING XTR112 and XTR114
5
4
Nonlinearity (%)
DESCRIPTION
INDUSTRIAL PROCESS CONTROL
FACTORY AUTOMATION
SCADA REMOTE DATA ACQUISITION
REMOTE TEMPERATURE AND PRESSURE
TRANSDUCERS
3
Uncorrected
RTD Nonlinearity
2
Corrected
Nonlinearity
1
0
–1
–200°C
+850°C
Process Temperature (°C)
IR
IR
VLIN
VREG
7.5V to 36V
+
VPS
4-20 mA
XTR112
XTR114
RG
VO
RL
RTD
–
XTR112: IR = 250µA
XTR114: IR = 100µA
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/ • Cable: BBRCORP • Telex: 066-6491 • FAX: (520) 889-1510 • Immediate Product Info: (800) 548-6132
®
©
1998 Burr-Brown Corporation
PDS-1473A
1
Printed
in U.S.A. December, 1998
XTR112,
XTR114
SPECIFICATIONS
At TA = +25°C, V+ = 24V, and TIP29C external transistor, unless otherwise noted.
XTR112U
XTR114U
PARAMETER
CONDITIONS
OUTPUT
Output Current Equation
Output Current, Specified Range
Over-Scale Limit
Under-Scale Limit: XTR112
XTR114
ZERO OUTPUT(1)
Initial Error
vs Temperature
vs Supply Voltage, V+
vs Common-Mode Voltage
vs VREG Output Current
Noise: 0.1Hz to 10Hz
SPAN
Span Equation (transconductance)
Initial Error (3)
vs Temperature(3)
Nonlinearity: Ideal Input (4)
INPUT(5)
Offset Voltage
vs Temperature
vs Supply Voltage, V+
vs Common-Mode Voltage,
RTI (CMRR)
Common-Mode Input Range(2)
Input Bias Current
vs Temperature
Input Offset Current
vs Temperature
Impedance: Differential
Common-Mode
Noise: 0.1Hz to 10Hz
IREG = 0
MIN
4
24
0.9
0.6
VIN = 0V, RG = ∞
Full Scale (VIN) = 50mV
Full Scale (VIN) = 50mV
VCM = 2V
V+ = 7.5V to 36V
VCM = 1.25V to 3.5V(2)
MAX
MIN
TYP
IO = VIN • (40/RG) + 4mA, VIN in Volts, RG in Ω
20
✻
27
30
✻
✻
1.3
1.7
✻
✻
1
1.4
✻
✻
4
±5
±0.07
0.04
0.02
0.3
0.03
V+ = 7.5V to 36V
VCM = 1.25V to 3.5V(2)
CURRENT SOURCES
Current: XTR112
XTR114
Accuracy
vs Temperature
vs Power Supply, V+
Matching
vs Temperature
vs Power Supply, V+
Compliance Voltage, Positive
Negative(2)
Output Impedance: XTR112
XTR114
Noise: 0.1Hz to 10Hz: XTR112
XTR114
TYP
XTR112UA
XTR114UA
✻
✻
✻
✻
✻
✻
✻
±25
±0.5
0.2
MAX
UNITS
✻
✻
✻
✻
A
mA
mA
mA
mA
±50
±0.9
✻
S = 40/RG
±0.05
±3
0.003
±0.2
±25
0.01
✻
✻
✻
✻
±0.4
✻
✻
A/V
%
ppm/°C
%
±50
±0.4
±0.3
±10
±100
±1.5
±3
±50
✻
✻
✻
✻
±250
±3
✻
±100
µV
µV/°C
µV/V
µV/V
✻
50
V
nA
pA/°C
nA
pA/°C
GΩ || pF
GΩ || pF
µVp-p
1.25
5
20
±0.2
5
0.1 || 1
5 || 10
0.6
3.5
25
✻
✻
✻
✻
✻
✻
✻
✻
±3
±10
VO = 2V(6)
V+ = 7.5V to 36V
V+ = 7.5V to 36V
(V+) –3
0
VREG(2)
Accuracy
vs Temperature
vs Supply Voltage, V+
Output Current: XTR112
XTR114
Output Impedance
250
100
±0.05
±15
±10
±0.02
±3
1
(V+) –2.5
–0.2
500
1.2
0.001
0.0004
5.1
±0.02
±0.2
1
–1, +2.1
–1, +2.4
75
LINEARIZATION
RLIN (internal)
Accuracy
vs Temperature
1
±0.2
±25
POWER SUPPLY
Specified Voltage
Operating Voltage Range
TEMPERATURE RANGE
Specification, TMIN to TMAX
Operating /Storage Range
Thermal Resistance, θJA
SO-14 Surface-Mount
mA
µA
µA/°C
µA/V
µA/V
µA/mA
µAp-p
±0.2
±35
±25
±0.1
±15
10
✻
✻
✻
✻
✻
✻
✻
✻
✻
✻
✻
✻
✻
✻
✻
✻
✻
✻
✻
✻
✻
✻
✻
±0.1
✻
✻
✻
±0.5
±100
±0.4
±75
✻
±0.2
±30
✻
✻
V
V
mV/°C
mV/V
mA
mA
Ω
±1
✻
kΩ
%
ppm/°C
✻
+24
µA
µA
%
ppm/°C
ppm/V
%
ppm/°C
ppm/V
V
V
MΩ
GΩ
µAp-p
µAp-p
+7.5
+36
✻
✻
V
V
–40
–55
+85
+125
✻
✻
✻
✻
°C
°C
100
✻
°C/W
✻ Specification same as XTR112U, XTR114U.
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.
®
XTR112, XTR114
2
ABSOLUTE MAXIMUM RATINGS(1)
PIN CONFIGURATION
Top View
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
SO-14
XTR112 and XTR114
1
14 IR2
–
VIN
2
13 VIN
RG
3
12 VLIN
RG
4
11 VREG
NC
5
10 V+
IRET
6
9
B (Base)
IO
7
8
E (Emitter)
IR1
NOTE: (1) Stresses above these ratings may cause permanent damage.
Exposure to absolute maximum conditions for extended periods may degrade
device reliability.
+
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.
NC = No Connection
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/ORDERING INFORMATION
PACKAGE
PACKAGE
DRAWING
NUMBER(1)
SPECIFIED
TEMPERATURE
RANGE
ORDERING
NUMBER(2)
TRANSPORT
MEDIA
2 x 250µA
"
2 x 250µA
"
SO-14 Surface Mount
"
SO-14 Surface Mount
"
235
"
235
"
–40°C to +85°C
"
–40°C to +85°C
"
XTR112U
XTR112U/2K5
XTR112UA
XTR112UA/2K5
Rails
Tape and Reel
Rails
Tape and Reel
2 x 100µA
"
2 x 100µA
"
SO-14 Surface Mount
"
SO-14 Surface Mount
"
235
"
235
"
–40°C to +85°C
"
–40°C to +85°C
"
XTR114U
XTR114U/2K5
XTR114UA
XTR114UA/2K5
Rails
Tape and Reel
Rails
Tape and Reel
PRODUCT
CURRENT
SOURCES
XTR112U
"
XTR112UA
"
XTR114U
"
XTR114UA
"
NOTES: (1) For detailed drawing and dimension table, please see end of data sheet, or Appendix C of Burr-Brown IC Data Book. (2) Models with a slash (/) are
available only in Tape and Reel in the quantities indicated (e.g., /2K5 indicates 2500 devices per reel). Ordering 2500 pieces of “XTR112UA/2K5” will get a single
2500-piece Tape and Reel. For detailed Tape and Reel mechanical information, refer to Appendix B of Burr-Brown IC Data Book.
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.
®
3
XTR112, XTR114
FUNCTIONAL BLOCK DIAGRAM
VLIN
XTR112: IR1 = IR2 = 250µA
XTR114: IR1 = IR2 = 100µA
IR1
12
1
IR2
14
VREG
V+
IR2
IR1
11
10
+
VIN
13
5.1V
4
B
RLIN
1kΩ
Q1
9
100µA
RG
3
–
VIN
E
I = 100µA +
2
VIN
8
RG
975Ω
25Ω
7
IO = 4mA + VIN •
6
IRET
®
XTR112, XTR114
4
( R40 )
G
TYPICAL PERFORMANCE CURVES
At TA = +25°C, and V+ = 24V, unless otherwise noted.
TRANSCONDUCTANCE vs FREQUENCY
STEP RESPONSE
RG = 125Ω
RG = 500Ω
RG = 2kΩ
40
20mA
30
4mA/div
Transconductance (20 Log mA/V)
50
20
RG = 125Ω
RG = 2kΩ
4mA
10
0
100
1k
10k
25µs/div
1M
100k
Frequency (Hz)
COMMON-MODE REJECTION RATIO vs FREQUENCY
POWER-SUPPLY REJECTION RATIO vs FREQUENCY
110
Power Supply Rejection Ratio (dB)
140
Common-Mode Rejection Ratio (dB)
100
90
80
RG = 125Ω
70
60
RG = 2kΩ
50
40
30
120
RG = 125Ω
100
80
60
RG = 2kΩ
40
20
0
10
20
10
100
1k
10k
100k
100
1k
10k
100k
1M
Frequency (Hz)
1M
Frequency (Hz)
OVER-SCALE CURRENT vs TEMPERATURE
UNDER-SCALE CURRENT vs TEMPERATURE
29
1.45
1.4
Under-Scale Current (mA)
Over-Scale Current (mA)
With External Transistor
28
27
V+ = 36V
26
V+ = 7.5V
25
V+ = 24V
24
1.35
XTR112
1.3
1.25
1.2
1.15
1.1
1.05
XTR114
1
23
0.95
–75
–50
–25
0
25
50
75
100
125
–75
Temperature (°C)
–50
–25
0
25
50
75
100
125
Temperature (°C)
®
5
XTR112, XTR114
TYPICAL PERFORMANCE CURVES (CONT)
At TA = +25°C, and V+ = 24V, unless otherwise noted.
INPUT VOLTAGE AND CURRENT
NOISE DENSITY vs FREQUENCY
ZERO OUTPUT AND REFERENCE
CURRENT NOISE vs FREQUENCY
10k
10k
1k
Current Noise
100
100
Voltage Noise
10
1
10
100
1k
Zero Output Current
Noise (pA/√Hz)
1k
Input Current Noise (fA/√Hz)
Input Voltage Noise (nV/√Hz)
10k
XTR112
1
10
100
1k
10k
Frequency (Hz)
Frequency (Hz)
INPUT BIAS AND OFFSET CURRENT
vs TEMPERATURE
ZERO OUTPUT CURRENT ERROR
vs TEMPERATURE
100k
4
Zero Output Current Error (µA)
Input Bias and Offset Current (nA)
XTR114
10
25
20
+IB
15
10
–IB
5
IOS
2
0
–2
–4
–6
–8
–10
–12
0
–75
–50
–25
0
25
50
75
100
–75
125
25
50
75
ZERO OUTPUT DRIFT
PRODUCTION DISTRIBUTION
40
35
Percent of Units (%)
60
50
40
30
20
15
10
0
0.025
0.05
0.075
0.1
0.125
0.15
0.175
0.2
0.225
0.25
0.275
0.3
0.325
0.35
0.375
0.4
0.425
0.45
0.475
0.5
3.0
2.8
2.6
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0
0.8
Typical production distribution
of packaged units. XTR112
and XTR114 included.
20
0
0.6
125
25
5
Input Offset Voltage Drift (µV/°C)
Zero Output Drift (µA/°C)
®
XTR112, XTR114
100
30
10
0.4
0
INPUT OFFSET VOLTAGE DRIFT
PRODUCTION DISTRIBUTION
70
0
–25
Temperature (°C)
Typical production distribution
of packaged units. XTR112 and
XTR114 included.
0.2
–50
Temperature (°C)
80
Percent of Units (%)
100
Reference Current
10
100k
10k
1k
6
TYPICAL PERFORMANCE CURVES (CONT)
At TA = +25°C, and V+ = 24V, unless otherwise noted.
CURRENT SOURCE DRIFT
PRODUCTION DISTRIBUTION
CURRENT SOURCE MATCHING
DRIFT PRODUCTION DISTRIBUTION
40
90
Typical production distribution
of packaged units.
XTR112 and XTR114 included.
Typical production distribution
of packaged units. XTR112 and
XTR114 included.
80
Percent of Units (%)
30
25
20
15
10
70
60
50
40
30
Current Source Matching Drift (ppm/°C)
XTR114 VREG OUTPUT VOLTAGE
vs VREG OUTPUT CURRENT
XTR112 VREG OUTPUT VOLTAGE
vs VREG OUTPUT CURRENT
30
28
26
24
22
20
18
16
14
12
8
10
6
Current Source Drift (ppm/°C)
5.35
5.35
125°C
125°C
5.30
VREG Output Voltage (V)
5.30
5.25
25°C
5.20
5.15
–55°C
5.10
NOTE: Above 2.4mA,
zero output degrades
5.05
25°C
5.25
5.20
5.15
–55°C
NOTE: Above 2.1mA,
zero output degrades
5.10
5.05
5.00
5.00
–1
–0.5
0
0.5
1
1.5
2
2.5
3
–1
–0.5
VREG Output Current (mA)
0
0.5
1
1.5
2
2.5
3
VREG Output Current (mA)
REFERENCE CURRENT ERROR
vs TEMPERATURE
+0.05
Reference Current Error (%)
VREG Output Voltage (V)
4
0
75
70
65
60
55
50
45
40
35
30
25
20
15
5
0
10
10
0
2
20
5
0
Percent of Units (%)
35
0
–0.05
–0.10
–0.15
–0.20
–75
–50
–25
0
25
50
75
100
125
Temperature (°C)
®
7
XTR112, XTR114
APPLICATION INFORMATION
The transfer function through the complete instrumentation
amplifier and voltage-to-current converter is:
Figure 1 shows the basic connection diagram for the XTR112
and XTR114. The loop power supply, VPS, provides power
for all circuitry. Output loop current is measured as a voltage
across the series load resistor, RL.
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 RG is not used the gain is zero and
the output is simply the XTR’s zero current. The value of RG
varies slightly for two-wire RTD and three-wire RTD connections with linearization. RG can be calculated from the
equations given in Figure 1 (two-wire RTD connection) and
Table I (three-wire RTD connection).
Two matched current sources drive the RTD and zerosetting resistor, RZ. 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 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 mismatch of the
XTR112 and XTR114.
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 XTR112 and XTR114
and to be included in the output current without causing an
error.
RCM provides an additional voltage drop to bias the inputs of
the XTR112 and XTR114 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 instrumentation 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 threewire RTD connections, see Figure 3.
The VREG pin provides an on-chip voltage source of approximately 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. VREG 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.
IR2
Possible choices for Q1 (see text).
IR1
12
13
4
VLIN
+
VIN
1
IR1
TO-225
TO-220
TO-220
7.5V to 36V
14
11
IR2
10
VREG
V+
IO
4-20 mA
(2)
RLIN1(3)
PACKAGE
RG
RG
XTR112
XTR114
3
TYPE
2N4922
TIP29C
TIP31C
RG
B
9
Q1
0.01µF
VO
E
8
RL
IO
2
–
VIN
+
VPS
–
7
IRET
(1)
RTD
RZ
6
IO = 4mA + VIN • ( 40 )
RG
NOTES: (1) RZ = RTD resistance at minimum measured temperature.
RCM
(2) RG =
2.5 • IREF [R1(R2 + RZ) – 2(R2RZ)]
(3) RLIN1 =
0.01µF
R2 – R1
0.4 • RLIN(R2 – R1)
IREF (2R1 – R2 – RZ)
where R1 = RTD Resistance at (TMIN + TMAX)/2
R2 = RTD Resistance at TMAX
RLIN = 1kΩ (Internal)
IREF = 0.25 for XTR112
IREF = 0.1 for XTR114
XTR112: IR1 = IR2 = 250µA, RCM = 3.3kΩ
XTR114: IR1 = IR2 = 100µA, RCM = 8.2kΩ
FIGURE 1. Basic Two-Wire RTD Temperature Measurement Circuit with Linearization.
®
XTR112, XTR114
8
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 1.3mA for the
XTR112 and 1mA for the XTR114. Refer to the typical
curve “Under-Scale Current vs Temperature.”
range from 7.5V to 36V. The loop supply voltage, VPS, will
differ from the applied voltage 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:
Increasingly positive input voltage (greater than the fullscale 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 “OverScale Current vs Temperature.”
R L max = 

(V+) – 7.5V 
– R WIRING
20mA 
EXTERNAL TRANSISTOR
It is recommended to design for V+ equal or greater than
7.5V with loop currents up to 30mA to allow for out-ofrange input conditions.
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 XTR112 and XTR114,
maintaining excellent accuracy.
The low operating voltage (7.5V) of the XTR112 and
XTR114 allow 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.
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.
ADJUSTING INITIAL ERRORS
Many applications require adjustment of initial errors. Input
offset and reference current mismatch errors can be corrected by adjustment of the zero resistor, RZ. Adjusting the
gain-setting resistor, RG, corrects any errors associated with
gain.
The XTR112 and XTR114 can be operated without this
external transistor, 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 = 3.3kΩ) connected between the IRET
pin and the E (emitter) pin may be needed for operation
below 0°C without Q1 to guarantee the full 20mA full-scale
output, especially with V+ near 7.5V.
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 introduce error. This error can be partially corrected by adjusting
the values of RZ, RG, and RLIN1.
LOOP POWER SUPPLY
The voltage applied to the XTR112 and XTR114, V+, is
measured with respect to the IO connection, pin 7. V+ can
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 XTR112 and XTR114. A second
resistor, RLIN2, is required for linearization.
10
V+
E
XTR112
XTR114
Note that although the two-wire and three-wire RTD connection circuits are very similar, the gain-setting resistor,
RG, has slightly different equations:
8
0.01µF
Two-wire: R G =
2.5 • I REF [ R1 ( R 2 + R Z ) – 2( R 2 R Z )]
R 2 – R1
IO
7
Three-wire: R G =
IRET
6
RQ = 3.3kΩ
For operation without external
transistor, connect a 3.3kΩ
resistor between pin 6 and
pin 8. See text for discussion
of performance.
2.5 • I REF ( R 2 – R Z )( R1 – R Z )
R 2 – R1
where RZ = RTD resistance at TMIN
R1 = RTD resistance at (TMIN + TMAX)/2
R2 = RTD resistance at TMAX
IREF = 0.25 for XTR112
IREF = 0.1 for XTR114
FIGURE 2. Operation Without External Transistor.
®
9
XTR112, XTR114
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.
Table I summarizes the resistor equations for two-wire and
three-wire RTD connections. An example calculation is also
provided. To maintain good accuracy, at least 1% (or better)
resistors should be used for RG. Table II provides standard
1% RG values for a three-wire Pt1000 RTD connection with
linearization for the XTR112. Table III gives RG values for
the XTR114.
TWO-WIRE
THREE-WIRE
RG
General Equations
=
XTR112 (IREF = 0.25)
(see Table II)
=
XTR114 (IREF = 0.1)
(see Table III)
=
IREF • 2.5 [R1 (R2 + RZ) – 2 (R2RZ)]
(R2 – R1)
0.625 • [R1 (R2 + RZ) – 2 (R2RZ)]
(R2 – R1)
0.25 • [R1 (R2 + RZ) – 2 (R2RZ)]
(R2 – R1)
RLIN1
=
=
=
0.4 • RLIN (R2 – R1)
IREF • (2R1 – R2 – RZ)
1.6 • RLIN (R2 – R1)
(2R1 – R2 – RZ)
4 • RLIN (R2 – R1)
(2R1 – R2 – RZ)
RG
=
=
=
RLIN1
IREF • 2.5 (R2 – RZ) (R1 – RZ)]
=
(R2 – R1)
0.625 • (R2 – RZ) (R1 – RZ)]
=
(R2 – R1)
0.25 • (R2 – RZ) (R1 – RZ)]
(R2 – R1)
=
0.4 • RLIN (R2 – R1)
RLIN2
=
IREF • (2R1 – R2 – RZ)
1.6 • RLIN (R2 – R1)
=
(2R1 – R2 – RZ)
4 • RLIN (R2 – R1)
(2R1 – R2 – RZ)
=
0.4 • (RLIN + RG)(R2 – R1)
IREF • (2R1 – R2 – RZ)
1.6 • (RLIN + RG)(R2 – R1)
(2R1 – R2 – RZ)
4 • (RLIN + RG)(R2 – R1)
(2R1 – R2 – RZ)
where RZ = RTD resistance at the minimum measured temperature, TMIN
R1 = RTD resistance at the midpoint measured temperature, TMID = (TMIN + TMAX)/2
R2 = RTD resistance at maximum measured temperature, TMAX
RLIN = 1kΩ (internal)
XTR112 RESISTOR EXAMPLE:
The measurement range is –100°C to +200°C for a 3-wire Pt100 RTD connection. Determine the values for RS, RG, RLIN1, and RLIN2. Look up the values
from the chart or calculate the values according to the equations provided.
METHOD 1: TABLE LOOK UP
TMIN = –100°C and ∆T = 300°C (TMAX = +200°C),
Using Table II the 1% values are:
RZ = 604Ω
RLIN1 = 33.2kΩ
RG = 750Ω
RLIN2 = 59kΩ
Calculation of Pt1000 Resistance Values
(according to DIN IEC 751)
Equation (1) Temperature range from –200°C to 0°C:
R(T) = 1000 [1 + 3.90802 • 10–3 • T – 0.5802 • 10–6 • T2
– 4.27350 • 10–12 • (T – 100) • T3]
METHOD 2: CALCULATION
Step 1: Determine RZ, R1, and R2.
RZ is the RTD resistance at the minimum measured temperature, TMIN = –100°C.
Using Equation (1) at right gives RZ = 602.5Ω (1% value is 604Ω).
R2 is the RTD resistance at the maximum measured temperature, TMAX = 200°C.
Using Equation (2) at right gives R2 = 1758.4Ω.
R1 is the RTD resistance at the midpoint measured temperature,
TMID = (TMIN + TMAX) /2 = (–100 + 200)/2 = 50°C. R1 is NOT the average of RZ and R2.
Using Equation (2) at right gives R1 = 1194Ω.
Step 2: Calculate RG, RLIN1, and RLIN2 using equations above.
Equation (2) Temperature range from 0°C to +850°C:
R(T) = 1000 (1 + 3.90802 • 10–3 • T – 0.5802 • 10–6 • T2)
where: R(T) is the resistance in Ω at temperature T.
T is the temperature in °C.
NOTE: Most RTD manufacturers provide reference tables for
resistance values at various temperatures.
Resistor values for other RTD types (such as Pt2000) can be
calculated using the XTR resistor selection program in the
Applications Section on Burr-Brown’s web site (www.burrbrown.com)
RG = 757Ω (1% value is 750Ω)
RLIN1 = 33.322kΩ (1% value is 33.2kΩ)
RLIN2 = 58.548kΩ (1% value is 59kΩ)
TABLE I. Summary of Resistor Equations for Two-Wire and Three-Wire Pt1000 RTD Connections.
®
XTR112, XTR114
10
XTR112 1% RESISTOR VALUES FOR A THREE-WIRE RTD CONNECTION
MEASUREMENT TEMPERATURE SPAN ∆T (°C)
TMIN
100°C
200°C
300°C
400°C
500°C
600°C
700°C
800°C
900°C
1000°C
–200°C
187/267
48700
61900
604/255
86600
110000
187/536
31600
48700
604/499
49900
75000
187/806
25500
46400
604/4750
33200
59000
187/1050
21500
44200
604/1000
24900
49900
187/1330
17800
41200
604/1270
19600
44200
187/1580
15000
39200
604/1500
15800
40200
187/1820
13000
36500
604/1780
13300
37400
187/2100
11300
34800
604/2050
11500
34800
187/2370
9760
33200
604/2260
10000
32400
187/2670
8660
31600
1000/243
105000
130000
1370/237
102000
127000
1000/487
51100
76800
1370/475
49900
73200
1000/732
33200
57600
1370/715
32400
56200
1000/976 1000/1210 1000/1470 1000/1740 1000/1960
24300
19100
15400
13000
11000
48700
42200
38300
35700
33200
1370/953 1370/1180 1370/1430 1370/1690
23700
18700
15000
12400
46400
40200
36500
33200
1740/232
100000
121000
2100/221
95300
118000
1740/464
48700
69800
2100/442
46400
68100
1740/698
31600
53600
2100/665
30100
51100
2490/215
93100
113000
2800/210
887000
107000
2490/432
45300
64900
2800/412
43200
61900
2490/649
29400
48700
2800/619
28000
45300
1740/931 1740/1150 1740/1400
23200
17800
14300
44200
38300
34800
2100/887 2100/1130
22100
17400
NOTE: The values listed in the table are 1% resistors (in Ω).
42200
36500
Exact values may be calculated from the following equations:
2490/866
RZ = RTD resistance at minimum measured temperature, TMIN.
21500
40200
0.625 • (R 2 – R Z ) (R1 – R Z )
RG =
(R 2 – R1)
3160/200
86600
102000
3480/191
82500
100000
3160/402
42200
59000
–100°C
0°C
100°C
200°C
300°C
400°C
500°C
600°C
700°C
800°C
RZ /RG
RLIN1
RLIN2
RLIN1 =
1.6 • RLIN (R 2 – R1)
(2R1 – R 2 – R Z )
RLIN2 =
1.6 • (RLIN + RG ) (R 2 – R1)
(2R1 – R 2 – R Z )
where R1 = RTD resistance at the midpoint measured temperature, (TMIN + TMAX)/2
R2 = RTD resistance at TMAX
3740/187
80600
95300
RLIN = 1kΩ (Internal)
TABLE II. XTR112 RZ, RG, RLIN1, and RLIN2 Standard 1% Resistor Values for Three-Wire Pt1000 RTD Connection with Linearization.
XTR114 1% RESISTOR VALUES FOR A THREE-WIRE RTD CONNECTION
MEASUREMENT TEMPERATURE SPAN ∆T (°C)
TMIN
100°C
200°C
300°C
400°C
500°C
600°C
700°C
800°C
900°C
1000°C
–200°C
187/107
121000
133000
604/102
221000
243000
187/215
78700
95300
604/200
124000
150000
187/316
64900
84500
604/301
84500
110000
187/422
53600
76800
604/402
61900
86600
187/523
45300
68100
604/511
48700
73200
187/634
38300
68100
604/604
40200
63400
187/732
32400
56200
604/715
33200
57600
187/845
28000
52300
604/806
28700
52300
187/953
24900
47500
604/909
24900
47500
187/1050
21500
45300
1000/97.6
261000
287000
1370/95.3
255000
280000
1000/196
130000
154000
1370/191
124000
147000
1000/294
84500
107000
1370/287
80600
105000
1000/392
61900
84500
1370/383
59000
82500
1000/487
47500
71500
1370/475
46400
68100
1000/590
39200
61900
1370/576
37400
59000
1000/681
32400
54900
1370/665
31600
52300
1000/787
27400
49900
1740/90.9
249000
267000
2100/88.9
237000
261000
1740/182
121000
143000
2100/178
118000
137000
1740/274
78700
100000
2100/267
75000
95300
1740/365
57600
78700
2100/357
54900
75000
1740/464
44200
64900
2100/348
43200
61900
1740/549
36500
56200
2490/86.6
232000
249000
2800/82.5
221000
243000
2490/174
113000
133000
2800/165
110000
127000
2490/261
73200
93100
2800/49
69800
88700
2490/249
53600
71500
3160/80.6
215000
215000
3480/76.8
205000
221000
3160/162
105000
121000
–100°C
0°C
100°C
200°C
300°C
400°C
500°C
600°C
700°C
800°C
3740/75
200000
215000
RZ /RG
RLIN1
RLIN2
NOTE: The values listed in the table are 1% resistors (in Ω).
Exact values may be calculated from the following equations:
RZ = RTD resistance at minimum measured temperature, TMIN.
RG =
0.25 • (R 2 – R Z ) (R1 – R Z )
(R 2 – R1)
RLIN1 =
4 • RLIN (R 2 – R1)
(2R1 – R 2 – R Z )
RLIN2 =
4 • (RLIN + RG ) (R 2 – R1)
(2R1 – R 2 – R Z )
where R1 = RTD resistance at the midpoint measured temperature, (TMIN + TMAX)/2
R2 = RTD resistance at TMAX
RLIN = 1kΩ (Internal)
TABLE III. XTR114 RZ, RG, RLIN1, and RLIN2 Standard 1% Resistor Values for Three-Wire Pt1000 RTD Connection with Linearization.
®
11
XTR112, XTR114
A typical two-wire RTD application with linearization is
shown in Figure 1. Resistor RLIN1 provides positive feedback and controls linearity correction. RLIN1 is chosen according to the desired temperature range. An equation is
given in Figure 1.
RCM can be adjusted to provide an additional voltage drop to
bias the inputs of the XTR112 and XTR114 within their
common-mode input range.
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 resistance 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 commonmode signal which is rejected by the XTR112 and XTR114.
The nearest standard 1% resistor values for RLIN1 and RLIN2
should be adequate for most applications. Tables II and III
provide the 1% resistor values for a three-wire Pt1000 RTD
connection.
Table IV shows how to calculate the effect various error
sources have on circuit accuracy. A sample error calculation
for a typical RTD measurement circuit (Pt1000 RTD, 200°C
measurement span) is provided. The results reveal the
XTR112’s and XTR114’s excellent accuracy, in this case 1%
unadjusted for the XTR112, 1.16% for the XTR114. Adjusting
resistors RG and RZ for gain and offset errors improves the
XTR112’s accuracy to 0.28% (0.31% for the XTR114). Note
that these are worst-case errors; guaranteed maximum values
were used in the calculations and all errors were assumed to be
positive (additive). The XTR112 and XTR114 achieve performance which is difficult to obtain with discrete circuitry and
requires less space.
ERROR ANALYSIS
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.
OPEN-CIRCUIT PROTECTION
The optional transistor Q2 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 XTR’s
output current will go to either its high current limit (≈ 27mA)
or low current limit (≈ 1.3mA for XTR112 and ≈ 1mA for
XTR114). This is easily detected as an out-of-range condition.
RTDs
The text and figures thus far have assumed a Pt1000 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,
RLIN1(1)
12
1
VLIN
IR1
13
RLIN2(1)
+
VIN
4
IO
14
IR2
11
10
VREG
V+
RG
XTR112
XTR114
(1)
RG
3
2
B 9
E
RG
Q1
0.01µF
8
IO
–
VIN
7
IRET
EQUAL line resistances here
creates a small common-mode
voltage which is rejected by
XTR112 and XTR114.
RZ(1)
1
2
RCM
(RLINE2)
RTD
NOTES: (1) See Table I for resistor equations and
1% values. (2) Q2 optional. Provides predictable
output current if any one RTD connection is
broken:
Q2(2)
2N2222
OPEN RTD
TERMINAL
1
2
3
3
FIGURE 3. Three-Wire Connection for Remotely Located RTDs.
®
XTR112, XTR114
0.01µF
(RLINE1)
(RLINE3)
Resistance in this line causes
a small common-mode voltage
which is rejected by XTR112
and XTR114.
IO
6
12
XTR112
XTR114
IO
IO
≈ 1.3mA
≈ 27mA
≈ 1.3mA
≈ 1mA
≈ 27mA
≈ 1mA
SAMPLE ERROR CALCULATION FOR XTR112(1)
RTD value at 4mA Output (RRTD MIN)
RTD Measurement Range
Ambient Temperature Range (∆TA)
Supply Voltage Change (∆V+)
Common-Mode Voltage Change (∆CM)
ERROR SOURCE
UNADJ.
ADJUST.
526
26
100
16
668
0
26
0
0
26
0.2%/100% • 106
25ppm/V • 5V
0.1%/100% • 250µA • 1000Ω/(250µA • 3.8Ω/°C • 200°C) • 106
2000
125
1316
0
125
0
10ppm/V • 5V • 250µA • 1000Ω/(250µA • 3.8Ω/°C • 200°C)
66
66
Total Excitation Error:
3507
191
Total Gain Error:
2000
100
2100
0
100
100
Total Output Error:
1563
63
1626
0
63
63
158
2
0.5
700
395
500
626
2382
158
2
0.5
700
395
500
626
2382
3
16
2
21
3
16
2
21
10304
(1.03%)
2783
(0.28%)
VOS/(VIN MAX) • 106
CMRR • ∆CM/(VIN MAX) • 106
IB/IREF • 106
IOS • RRTD MIN/(VIN MAX) • 106
EXCITATION
Current Reference Accuracy
vs Supply
Current Reference Matching
IREF Accuracy (%)/100% • 106
(IREF vs V+) • ∆V+
IREF Matching (%)/100% • IREF •
RRTD MIN/(VIN MAX) • 106
(IREF matching vs V+) • ∆V+ •
RRTD MIN/(VIN MAX)
vs Supply
GAIN
Span
Nonlinearity
ERROR
(ppm of Full Scale)
SAMPLE
ERROR CALCULATION(2)
ERROR EQUATION
INPUT
Input Offset Voltage
vs Common-Mode
Input Bias Current
Input Offset Current
OUTPUT
Zero Output
vs Supply
1000Ω
200°C
20°C
5V
0.1V
100µV/(250µA • 3.8Ω/°C • 200°C) • 106
50µV/V • 0.1V/(250µA • 3.8Ω/°C • 200°C) • 106
0.025µA/250µA • 106
3nA • 1000Ω/(250µA • 3.8Ω/°C • 200°C) • 106
Total Input Error:
Span Error (%)/100% • 106
Nonlinearity (%)/100% • 106
0.2%/100% • 106
0.01%/100% • 106
(IZERO - 4mA)/16000µA • 106
(IZERO vs V+) • ∆V+/16000µA • 106
25µA/16000µA • 106
0.2µA/V • 5V/16000µA • 106
DRIFT (∆TA = 20°C)
Input Offset Voltage
Input Bias Current (typical)
Input Offset Current (typical)
Current Reference Accuracy
Current Reference Matching
Span
Zero Output
Drift • ∆TA/(VIN MAX) • 106
Drift • ∆TA/IREF • 106
Drift • ∆TA • RRTD MIN/(VIN MAX) • 106
Drift • ∆TA
Drift • ∆TA • IREF • RRTD MIN/(VIN MAX)
Drift • ∆TA
Drift • ∆TA/16000µA • 106
1.5µV/°C • 20°C/(250µA • 3.8Ω/°C • 200°C) • 106
20pA/°C • 20°C/250µA • 106
5pA/°C • 20°C • 1000Ω/(250µA • 3.8Ω/°C • 200°C) • 106
35ppm/°C • 20°C
15ppm/°C • 20°C • 250µA • 1000Ω/(250µA • 3.8Ω/°C • 200°C)
25ppm/°C • 20°C
0.5µA/°C • 20°C/16000µA • 106
Total Drift Error:
NOISE (0.1Hz to 10Hz, typ)
Input Offset Voltage
Current Reference
Zero Output
vn/(VIN MAX) • 106
IREF Noise • RRTD MIN/(VIN MAX) • 106
IZERO Noise/16000µA • 106
0.6µV/(250µA • 3.8Ω/°C • 200°C) • 106
3nA • 1000Ω/(250µA • 3.8Ω/°C • 200°C) • 106
0.03µA/16000µA • 106
Total Noise Error:
TOTAL ERROR:
NOTES: (1) For XTR114, IREF = 100µA. Total unadjusted error is 1.16%, adjusted error 0.31%. (2) All errors are min/max and referred to input, unless
otherwise stated.
TABLE IV. Error Calculation.
REVERSE-VOLTAGE PROTECTION
SURGE PROTECTION
The XTR112’s and XTR114’s low compliance rating (7.5V)
permits the use of various voltage protection methods without compromising operating range. Figure 4 shows a diode
bridge circuit which allows normal operation even when the
voltage connection lines are reversed. The bridge causes a
two diode drop (approximately 1.4V) loss in loop supply
voltage. 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.
Remote connections to current transmitters can sometimes be
subjected to voltage surges. It is prudent to limit the maximum
surge voltage applied to the XTR 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 XTR112 or
XTR114 within loop supply voltages up to 65V.
®
13
XTR112, XTR114
Most surge protection zener diodes have a diode characteristic 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.
If the RTD sensor is remotely located, the interference may
enter at the input terminals. For integrated transmitter assemblies with short connection to the sensor, the interference 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.
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 XTR112 and XTR114 causing errors. This
generally appears as an unstable output current that varies
with the position of loop supply or input wiring.
NOTE: (1) Zener Diode 36V: 1N4753A or General
Semiconductor TransorbTM 1N6286A. Use lower
voltage zener diodes with loop power supply
voltages less than 30V for increased protection.
See “Over-Voltage Surge Protection.”
10
V+
XTR112
XTR114
0.01µF
B
E
D1(1)
9
1N4148
Diodes
RL
8
VPS
IO
Maximum VPS must be
less than minimum
voltage rating of zener
diode.
The diode bridge causes
a 1.4V loss in loop supply
voltage.
7
IRET
6
FIGURE 4. Reverse Voltage Operation and Over-Voltage Surge Protection.
12
1kΩ
13
4
RLIN1
VLIN
+
VIN
1
IR1
14
11
IR2
VREG
RG
RLIN2
XTR112
XTR114
RG
3
10
V+
B
E
RG
9
8
IO
1kΩ
2
–
VIN
7
IRET
RZ
0.01µF
6
0.01µF
RTD
(1)
RCM
0.01µF
NOTE: (1) Bypass capacitors can be connected
to either the IRET pin or the IO pin.
FIGURE 5. Input Bypassing Technique with Linearization.
®
XTR112, XTR114
14
0.01µF
IREG < 2mA
Isothermal
Block
5V
12
V+
VLIN
13
OPA277
Type J
+
VIN
1
IR1
14
IR2
11
VREG
V–
4
1MΩ(1)
10
V+
RG
1MΩ
RG
1250Ω
0.01µF
3
1N4148
XTR112
XTR114
E
RG
9
8
IO
20kΩ
2
1kΩ
–
VIN
7
IRET
6
50Ω
25Ω
B
+
–
IO = 4mA + (VIN –VIN) 40
RG
RCM(2)
NOTES: (1) For burn-out indication.
(2) XTR112, RCM = 3.3kΩ
XTR114, RCM = 8.2kΩ
FIGURE 6. Thermocouple Low Offset, Low Drift Loop Measurement with Diode Cold-Junction Compensation.
12
13
4
VLIN
+
VIN
3
1N4148
14
11
IR2
10
VREG
V+
+12V
1µF
RG
RG
1270Ω
RLIN1
18.7kΩ
1
IR1
XTR112
RG
B 9
Q1
0.01µF
16
2
RTD
RZ
1370Ω
6
12
RCV420
2
7
IO = 4mA – 20mA
VO = 0 to 5V
14
13
IRET
Pt1000
100°C to
600°C
11
15
IO
–
VIN
10
3
E 8
5
4
1µF
–12V
RCM
0.01µF
NOTE: A two-wire RTD connection is shown. For remotely
located RTDs, a three-wire RTD conection is recommended. RG
becomes 1180Ω, RLIN2 is 40.2kΩ. See Figure 3 and Table II.
FIGURE 7. ±12V Powered Transmitter/Receiver Loop.
®
15
XTR112, XTR114
12
RLIN1
RLIN2
13
4
1
IR1
VLIN
+
VIN
1N4148
14
11
IR2
10
VREG
V+
0
RG
1µF
XTR112
XTR114
RG
3
+15V
1µF
B
E
RG
9
Q1
–15V
0.01µF
16
10
3
8
11
12
–
2 VIN
IRET
2
7
6
13
4
IO = 4mA – 20mA
14
V+
1
15
RCV420
IO
RZ
Isolated Power
from PWS740
9
15
ISO124
5
10
7
8
VO
0 – 5V
2
16
RTD
V–
NOTE: A three-wire RTD connection is shown.
For a two-wire RTD connection, eliminate RLIN2.
RCM
0.01µF
FIGURE 8. Isolated Transmitter/Receiver Loop.
200µA (XTR114)
500µA (XTR112)
12
VLIN
+
13 VIN
4
1
IR1
14
IR2
11
VREG 10
V+
RG
XTR112
XTR114
RG
3
2
B
E
RG
–
VIN
9
8
7
IRET
6
RCM
(1)
NOTE: (1) Use RCM to adjust the
common-mode voltage to within
1.25V to 3.5V.
FIGURE 9. Bridge Input, Current Excitation.
®
XTR112, XTR114
16