XTR112UA [BB]
4-20mA CURRENT TRANSMITTERS with Sensor Excitation and Linearization; 4-20mA电流变送器传感器激励和线性
| 型号： | XTR112UA |
| 厂家： | 
BURR-BROWN CORPORATION |
| 描述： | 4-20mA CURRENT TRANSMITTERS with Sensor Excitation and Linearization |
| 文件： | 总16页 (文件大小：257K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
®
XTR114
XTR112
XTR114
XTR112
4-20mA CURRENT TRANSMITTERS
with Sensor Excitation and Linearization
FEATURES
APPLICATIONS
● LOW UNADJUSTED ERROR
● INDUSTRIAL PROCESS CONTROL
● PRECISION CURRENT SOURCES
XTR112: Two 250µA
● FACTORY AUTOMATION
● SCADA REMOTE DATA ACQUISITION
XTR114: Two 100µA
● REMOTE TEMPERATURE AND PRESSURE
● RTD OR BRIDGE EXCITATION
● LINEARIZATION
TRANSDUCERS
● TWO OR THREE-WIRE RTD OPERATION
● LOW OFFSET DRIFT: 0.4µV/°C
● LOW OUTPUT CURRENT NOISE: 30nAp-p
● HIGH PSR: 110dB min
Pt1000 NONLINEARITY CORRECTION
USING XTR112 and XTR114
5
4
● HIGH CMR: 86dB min
3
● WIDE SUPPLY RANGE: 7.5V TO 36V
● SO-14 SOIC PACKAGE
Uncorrected
RTD Nonlinearity
2
Corrected
Nonlinearity
1
0
DESCRIPTION
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 am-
plifier, and current output circuitry on a single inte-
grated circuit. The XTR112 has two 250µA current
sources while the XTR114 has two 100µA sources for
RTD excitation.
–1
–200°C
+850°C
Process Temperature (°C)
IR
IR
VLIN
VREG
Versatile linearization circuitry provides a 2nd-order
correction to the RTD, typically achieving a 40:1
improvement in linearity.
7.5V to 36V
VPS
+
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 XTR112
and XTR114 operate on loop power supply voltages
down to 7.5V.
4-20 mA
XTR112
XTR114
VO
RL
RG
RTD
–
XTR112: IR = 250µA
XTR114: IR = 100µA
Both are available in an SO-14 surface-mount pack-
age and are specified for the –40°C to +85°C indus-
trial temperature range.
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
Printed in U.S.A. December, 1998
SPECIFICATIONS
At TA = +25°C, V+ = 24V, and TIP29C external transistor, unless otherwise noted.
XTR112U
XTR114U
XTR112UA
XTR114UA
PARAMETER
CONDITIONS
MIN
TYP
MAX
MIN
TYP
MAX
UNITS
OUTPUT
Output Current Equation
Output Current, Specified Range
Over-Scale Limit
Under-Scale Limit: XTR112
XTR114
A
IO = VIN • (40/RG) + 4mA, VIN in Volts, RG in Ω
4
20
30
1.7
1.4
✻
✻
✻
✻
✻
✻
✻
✻
mA
mA
mA
mA
24
0.9
0.6
27
1.3
1
✻
✻
✻
IREG = 0
ZERO OUTPUT(1)
Initial Error
vs Temperature
vs Supply Voltage, V+
vs Common-Mode Voltage
vs VREG Output Current
Noise: 0.1Hz to 10Hz
VIN = 0V, RG = ∞
4
±5
±0.07
0.04
0.02
0.3
✻
✻
✻
✻
✻
✻
✻
mA
µA
±25
±0.5
0.2
±50
±0.9
✻
µA/°C
µA/V
µA/V
µA/mA
µAp-p
V+ = 7.5V to 36V
VCM = 1.25V to 3.5V(2)
0.03
SPAN
Span Equation (transconductance)
Initial Error(3)
vs Temperature(3)
Nonlinearity: Ideal Input(4)
S = 40/RG
±0.05
±3
✻
✻
✻
✻
A/V
%
ppm/°C
%
Full Scale (VIN) = 50mV
Full Scale (VIN) = 50mV
±0.2
±25
0.01
±0.4
✻
✻
0.003
INPUT(5)
Offset Voltage
vs Temperature
vs Supply Voltage, V+
vs Common-Mode Voltage,
RTI (CMRR)
VCM = 2V
±50
±0.4
±0.3
±10
±100
±1.5
±3
✻
✻
✻
✻
±250
±3
✻
µV
µV/°C
µV/V
µV/V
V+ = 7.5V to 36V
VCM = 1.25V to 3.5V(2)
±50
±100
Common-Mode Input Range(2)
Input Bias Current
vs Temperature
Input Offset Current
vs Temperature
Impedance: Differential
Common-Mode
Noise: 0.1Hz to 10Hz
1.25
3.5
25
✻
✻
50
V
nA
5
20
±0.2
5
0.1 || 1
5 || 10
0.6
✻
✻
✻
✻
✻
✻
✻
pA/°C
nA
pA/°C
GΩ || pF
GΩ || pF
µVp-p
±3
±10
CURRENT SOURCES
Current: XTR112
XTR114
VO = 2V(6)
250
100
±0.05
±15
±10
±0.02
±3
✻
✻
✻
✻
✻
✻
✻
✻
✻
✻
✻
✻
✻
✻
µA
µA
%
ppm/°C
ppm/V
%
ppm/°C
ppm/V
V
V
MΩ
GΩ
µAp-p
µAp-p
Accuracy
±0.2
±35
±25
±0.1
±15
10
±0.4
±75
✻
±0.2
±30
✻
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
V+ = 7.5V to 36V
V+ = 7.5V to 36V
1
(V+) –3 (V+) –2.5
✻
✻
0
–0.2
500
1.2
0.001
0.0004
(2)
VREG
5.1
±0.02
±0.2
✻
✻
✻
✻
✻
✻
✻
V
V
Accuracy
±0.1
✻
vs Temperature
vs Supply Voltage, V+
Output Current: XTR112
XTR114
mV/°C
mV/V
mA
mA
Ω
1
–1, +2.1
–1, +2.4
75
Output Impedance
LINEARIZATION
RLIN (internal)
Accuracy
1
±0.2
±25
✻
✻
✻
kΩ
%
ppm/°C
±0.5
±100
±1
✻
vs Temperature
POWER SUPPLY
Specified Voltage
Operating Voltage Range
+24
✻
V
V
+7.5
+36
✻
✻
TEMPERATURE RANGE
Specification, TMIN to TMAX
Operating/Storage Range
Thermal Resistance, θJA
SO-14 Surface-Mount
–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.
®
2
XTR112, XTR114
PIN CONFIGURATION
ABSOLUTE MAXIMUM RATINGS(1)
Power Supply, V+ (referenced to IO pin) .......................................... 40V
Input Voltage, VI+N, VI–N (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
Top View
SO-14
XTR112 and XTR114
1
2
3
4
5
6
7
IR1
VI–N
RG
RG
NC
IRET
IO
14 IR2
13 VI+N
12 VLIN
11 VREG
10 V+
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.
9
8
B (Base)
E (Emitter)
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
DRAWING
NUMBER(1)
SPECIFIED
TEMPERATURE
RANGE
CURRENT
SOURCES
ORDERING
NUMBER(2)
TRANSPORT
MEDIA
PRODUCT
PACKAGE
XTR112U
2 x 250µA
SO-14 Surface Mount
235
"
235
"
–40°C to +85°C
XTR112U
XTR112U/2K5
XTR112UA
Rails
Tape and Reel
Rails
"
"
"
"
XTR112UA
"
2 x 250µA
SO-14 Surface Mount
"
–40°C to +85°C
"
"
XTR112UA/2K5
Tape and Reel
XTR114U
2 x 100µA
SO-14 Surface Mount
235
"
235
"
–40°C to +85°C
XTR114U
XTR114U/2K5
XTR114UA
Rails
Tape and Reel
Rails
"
"
"
"
XTR114UA
"
2 x 100µA
SO-14 Surface Mount
"
–40°C to +85°C
"
"
XTR114UA/2K5
Tape and Reel
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
IR2
1
14
VREG
V+
IR1
IR2
11
10
13
VI+N
5.1V
4
B
9
RLIN
1kΩ
Q1
100µA
RG
3
E
8
VIN
RG
I = 100µA +
2
VI–N
975Ω
25Ω
7
40
IO = 4mA + VIN
•
( R )
G
6
IRET
®
4
XTR112, XTR114
TYPICAL PERFORMANCE CURVES
At TA = +25°C, and V+ = 24V, unless otherwise noted.
TRANSCONDUCTANCE vs FREQUENCY
50
STEP RESPONSE
RG = 500Ω
RG = 125Ω
RG = 2kΩ
40
30
20
10
0
20mA
RG = 125Ω
RG = 2kΩ
4mA
25µs/div
100
1k
10k
100k
1M
Frequency (Hz)
COMMON-MODE REJECTION RATIO vs FREQUENCY
POWER-SUPPLY REJECTION RATIO vs FREQUENCY
110
100
90
80
70
60
50
40
30
20
140
120
100
80
RG = 125Ω
R
G = 125Ω
60
RG = 2kΩ
RG = 2kΩ
40
20
0
10
100
1k
10k
100k
1M
Frequency (Hz)
10
100
1k
10k
100k
1M
Frequency (Hz)
OVER-SCALE CURRENT vs TEMPERATURE
With External Transistor
UNDER-SCALE CURRENT vs TEMPERATURE
29
28
27
26
25
24
23
1.45
1.4
1.35
1.3
XTR112
1.25
1.2
V+ = 36V
V+ = 7.5V
V+ = 24V
1.15
1.1
1.05
1
XTR114
0.95
–75
–50
–25
0
25
50
75
100
125
–75
–50
–25
0
25
50
75
100
125
Temperature (°C)
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
1k
10k
1k
10k
1k
Zero Output Current
Current Noise
100
10
100
10
100
10
XTR112
Reference Current
Voltage Noise
XTR114
1
10
100
1k
10k
100k
1
10
100
1k
10k
100k
Frequency (Hz)
Frequency (Hz)
ZERO OUTPUT CURRENT ERROR
vs TEMPERATURE
INPUT BIAS AND OFFSET CURRENT
vs TEMPERATURE
4
2
25
20
15
10
5
0
–2
–4
–6
–8
–10
–12
+IB
–IB
IOS
0
–75
–50
–25
0
25
50
75
100
125
–75
–50
–25
0
25
50
75
100
125
Temperature (°C)
Temperature (°C)
INPUT OFFSET VOLTAGE DRIFT
PRODUCTION DISTRIBUTION
ZERO OUTPUT DRIFT
PRODUCTION DISTRIBUTION
80
70
60
50
40
30
20
10
0
40
35
30
25
20
15
10
5
Typical production distribution
of packaged units. XTR112 and
XTR114 included.
Typical production distribution
of packaged units. XTR112
and XTR114 included.
0
Input Offset Voltage Drift (µV/°C)
Zero Output Drift (µA/°C)
®
6
XTR112, XTR114
TYPICAL PERFORMANCE CURVES (CONT)
At TA = +25°C, and V+ = 24V, unless otherwise noted.
CURRENT SOURCE DRIFT
CURRENT SOURCE MATCHING
PRODUCTION DISTRIBUTION
DRIFT PRODUCTION DISTRIBUTION
40
35
30
25
20
15
10
5
90
80
70
60
50
40
30
20
10
0
Typical production distribution
of packaged units.
XTR112 and XTR114 included.
Typical production distribution
of packaged units. XTR112 and
XTR114 included.
0
Current Source Drift (ppm/°C)
Current Source Matching Drift (ppm/°C)
XTR114 VREG OUTPUT VOLTAGE
XTR112 VREG OUTPUT VOLTAGE
vs VREG OUTPUT CURRENT
vs VREG OUTPUT CURRENT
5.35
5.30
5.25
5.20
5.15
5.10
5.05
5.00
5.35
5.30
5.25
5.20
5.15
5.10
5.05
5.00
125°C
125°C
25°C
25°C
–55°C
–55°C
NOTE: Above 2.1mA,
zero output degrades
NOTE: Above 2.4mA,
zero output degrades
–1
–0.5
0
0.5
1
1.5
2
2.5
3
–1
–0.5
0
0.5
1
1.5
2
2.5
3
VREG Output Current (mA)
VREG Output Current (mA)
REFERENCE CURRENT ERROR
vs TEMPERATURE
+0.05
0
–0.05
–0.10
–0.15
–0.20
–75
–50
–25
0
25
50
75
100
125
Temperature (°C)
®
7
XTR112, XTR114
The transfer function through the complete instrumentation
amplifier and voltage-to-current converter is:
APPLICATION INFORMATION
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 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).
Two matched current sources drive the RTD and zero-
setting 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) measure-
ment 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 tem-
perature range. RLIN1 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 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 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
IR1
Possible choices for Q1 (see text).
TYPE
PACKAGE
2N4922
TIP29C
TIP31C
TO-225
TO-220
TO-220
12
1
IR1
7.5V to 36V
VLIN
14
13
11
VI+N
IR2
10
V+
VREG
IO
4
RG
4-20 mA
9
8
R(G2)
B
E
0.01µF
Q1
XTR112
XTR114
VO
+
3
2
RG
VI–N
(3)
RLIN1
RL
VPS
–
IO
7
IRET
(1)
6
40
RG
RTD
RZ
IO = 4mA + VIN • (
)
NOTES: (1) RZ = RTD resistance at minimum measured temperature.
2.5 • IREF [R1(R2 + RZ) – 2(R2RZ)]
RCM
(2)
RG
=
R2 – R1
0.4 • RLIN(R2 – R1)
IREF (2R1 – R2 – RZ)
(3)
0.01µF
RLIN1 =
where R1 = RTD Resistance at (TMIN + TMAX)/2
R2 = RTD Resistance at TMAX
RLIN = 1kΩ (Internal)
XTR112: IR1 = IR2 = 250µA, RCM = 3.3kΩ
XTR114: IR1 = IR2 = 100µA, RCM = 8.2kΩ
IREF = 0.25 for XTR112
IREF = 0.1 for XTR114
FIGURE 1. Basic Two-Wire RTD Temperature Measurement Circuit with Linearization.
®
8
XTR112, XTR114
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 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.”
(V+) – 7.5V
RL max =
– RWIRING
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-of-
range 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 cor-
rected 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 intro-
duce error. This error can be partially corrected by adjusting
LOOP POWER SUPPLY
The voltage applied to the XTR112 and XTR114, V+, is
measured with respect to the IO connection, pin 7. V+ can
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 XTR112 and XTR114. A second
resistor, RLIN2, is required for linearization.
10
V+
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:
8
E
XTR112
0.01µF
XTR114
2.5• IREF R (R + R ) – 2(R R )
[
]
1
2
Z
2
Z
R
=
Two-wire:
G
R2 – R1
IO
7
2.5• IREF (R2 – RZ )(R1 – RZ )
R =
Three-wire:
IRET
G
R2 – R1
6
For operation without external
where RZ = RTD resistance at TMIN
R1 = RTD resistance at (TMIN + TMAX)/2
R2 = RTD resistance at TMAX
IREF = 0.25 for XTR112
transistor, connect a 3.3kΩ
resistor between pin 6 and
pin 8. See text for discussion
of performance.
RQ = 3.3kΩ
IREF = 0.1 for XTR114
FIGURE 2. Operation Without External Transistor.
®
9
XTR112, XTR114
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.
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.
TWO-WIRE
THREE-WIRE
RG
RLIN1
RG
RLIN1
RLIN2
0.4 • (RLIN + RG)(R2 – R1)
IREF • (2R1 – R2 – RZ)
0.4 • RLIN (R2 – R1)
REF • (2R1 – R2 – RZ)
0.4 • RLIN (R2 – R1)
IREF • 2.5 (R2 – RZ) (R1 – RZ)]
(R2 – R1)
IREF • 2.5 [R1 (R2 + RZ) – 2 (R2RZ)]
(R2 – R1)
=
=
General Equations
=
=
=
I
I
REF • (2R1 – R2 – RZ)
1.6 • (RLIN + RG)(R2 – R1)
(2R1 – R2 – RZ)
1.6 • RLIN (R2 – R1)
(2R1 – R2 – RZ)
1.6 • RLIN (R2 – R1)
(2R1 – R2 – RZ)
0.625 • (R2 – RZ) (R1 – RZ)]
(R2 – R1)
0.625 • [R1 (R2 + RZ) – 2 (R2RZ)]
(R2 – R1)
XTR112 (IREF = 0.25)
(see Table II)
=
=
=
=
=
=
=
=
=
=
4 • (RLIN + RG)(R2 – R1)
(2R1 – R2 – RZ)
4 • RLIN (R2 – R1)
(2R1 – R2 – RZ)
4 • RLIN (R2 – R1)
(2R1 – R2 – RZ)
0.25 • (R2 – RZ) (R1 – RZ)]
(R2 – R1)
0.25 • [R1 (R2 + RZ) – 2 (R2RZ)]
(R2 – R1)
XTR114 (IREF = 0.1)
(see Table III)
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:
Calculation of Pt1000 Resistance Values
(according to DIN IEC 751)
RZ = 604Ω
RG = 750Ω
RLIN1 = 33.2kΩ
RLIN2 = 59kΩ
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).
Equation (2) Temperature range from 0°C to +850°C:
R(T) = 1000 (1 + 3.90802 • 10–3 • T – 0.5802 • 10–6 • T2)
R2 is the RTD resistance at the maximum measured temperature, TMAX = 200°C.
Using Equation (2) at right gives R2 = 1758.4Ω.
where: R(T) is the resistance in Ω at temperature T.
T is the temperature in °C.
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.
NOTE: Most RTD manufacturers provide reference tables for
resistance values at various temperatures.
Step 2: Calculate RG, RLIN1, and RLIN2 using equations above.
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.burr-
brown.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.
®
10
XTR112, XTR114
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
187/536
31600
48700
187/806 187/1050 187/1330 187/1580 187/1820 187/2100 187/2370 187/2670
25500
46400
21500
44200
17800
41200
15000
39200
13000
36500
11300
34800
9760
33200
8660
31600
–100°C
0°C
604/255
86600
110000
604/499
49900
75000
604/4750 604/1000 604/1270 604/1500 604/1780 604/2050 604/2260
33200
59000
24900
49900
19600
44200
15800
40200
13300
37400
11500
34800
10000
32400
1000/243 1000/487 1000/732 1000/976 1000/1210 1000/1470 1000/1740 1000/1960
105000
130000
51100
76800
33200
57600
24300
48700
19100
42200
15400
38300
13000
35700
11000
33200
100°C
200°C
300°C
400°C
500°C
600°C
700°C
800°C
1370/237 1370/475 1370/715 1370/953 1370/1180 1370/1430 1370/1690
102000
127000
49900
73200
32400
56200
23700
46400
18700
40200
15000
36500
12400
33200
RZ /RG
RLIN1
RLIN2
1740/232 1740/464 1740/698 1740/931 1740/1150 1740/1400
100000
121000
48700
69800
31600
53600
23200
44200
17800
38300
14300
34800
2100/221 2100/442 2100/665 2100/887 2100/1130
95300
118000
46400
68100
30100
51100
22100
42200
17400
36500
NOTE: The values listed in the table are 1% resistors (in Ω).
Exact values may be calculated from the following equations:
2490/215 2490/432 2490/649 2490/866
R
R
R
R
Z = RTD resistance at minimum measured temperature, TMIN
.
93100
113000
45300
64900
29400
48700
21500
40200
0.625 • (R – R ) (R – R
)
Z
2
Z
1
2800/210 2800/412 2800/619
=
G
(R – R )
2
1
887000
107000
43200
61900
28000
45300
1.6 • R (R – R )
LIN
2
1
=
LIN1
LIN2
3160/200 3160/402
(2R – R – R
)
1
2
Z
86600
102000
42200
59000
1.6 • (R
+R ) (R – R )
G
2 1
LIN
=
(2R – R – R )
Z
1
2
3480/191
82500
100000
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
187/215
78700
95300
187/316
64900
84500
187/422
53600
76800
187/523
45300
68100
187/634
38300
68100
187/732
32400
56200
187/845
28000
52300
187/953
24900
47500
187/1050
21500
45300
–100°C
0°C
604/102
221000
243000
604/200
124000
150000
604/301
84500
110000
604/402
61900
86600
604/511
48700
73200
604/604
40200
63400
604/715
33200
57600
604/806
28700
52300
604/909
24900
47500
1000/97.6 1000/196 1000/294 1000/392 1000/487 1000/590 1000/681 1000/787
261000
287000
130000
154000
84500
107000
61900
84500
47500
71500
39200
61900
32400
54900
27400
49900
100°C
200°C
300°C
400°C
500°C
600°C
700°C
800°C
1370/95.3 1370/191 1370/287 1370/383 1370/475 1370/576 1370/665
255000
280000
124000
147000
80600
105000
59000
82500
46400
68100
37400
59000
31600
52300
RZ /RG
RLIN1
RLIN2
1740/90.9 1740/182 1740/274 1740/365 1740/464 1740/549
249000
267000
121000
143000
78700
100000
57600
78700
44200
64900
36500
56200
2100/88.9 2100/178 2100/267 2100/357 2100/348
237000
261000
118000
137000
75000
95300
54900
75000
43200
61900
NOTE: The values listed in the table are 1% resistors (in Ω).
Exact values may be calculated from the following equations:
2490/86.6 2490/174 2490/261 2490/249
232000
249000
RZ = RTD resistance at minimum measured temperature, TMIN
.
113000
133000
73200
93100
53600
71500
2800/82.5 2800/165
2800/49
69800
88700
0.25 • (R – R ) (R – R )
Z
2
Z
1
R
=
G
221000
243000
110000
127000
(R – R )
2
1
4 • R (R – R )
LIN
2
1
)
3160/80.6 3160/162
R
=
LIN1
215000
215000
105000
121000
(2R – R – R
1
2
Z
4 • (R
+R ) (R – R )
LIN
G
2
1
3480/76.8
205000
R
=
LIN2
(2R – R – R
)
1
2
Z
221000
where R1 = RTD resistance at the midpoint measured temperature, (TMIN + TMAX)/2
R2 = RTD resistance at TMAX
3740/75
200000
215000
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 feed-
back and controls linearity correction. RLIN1 is chosen ac-
cording 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.
ERROR ANALYSIS
In three-wire RTD connections, an additional resistor, RLIN2
,
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 perfor-
mance which is difficult to obtain with discrete circuitry and
requires less space.
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 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.
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,
12
IO
1
IR1
VLIN
14
11
IR2
13
VI+N
(1)
(1)
10
V+
RLIN1
RLIN2
VREG
4
RG
R(G1)
XTR112
XTR114
9
8
B
E
Q1
0.01µF
3
2
RG
VI–N
IO
7
IRET
(1)
EQUAL line resistances here
creates a small common-mode
voltage which is rejected by
XTR112 and XTR114.
RZ
IO
6
2
1
RCM
0.01µF
(RLINE2
)
(RLINE1)
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:
(2)
Q2
2N2222
RTD
XTR112
IO
XTR114
IO
(RLINE3
)
OPEN RTD
TERMINAL
Resistance in this line causes
a small common-mode voltage
which is rejected by XTR112
and XTR114.
1
2
3
≈ 1.3mA
≈ 27mA
≈ 1.3mA
≈ 1mA
≈ 27mA
≈ 1mA
3
FIGURE 3. Three-Wire Connection for Remotely Located RTDs.
®
12
XTR112, XTR114
SAMPLE ERROR CALCULATION FOR XTR112(1)
RTD value at 4mA Output (RRTD MIN
RTD Measurement Range
Ambient Temperature Range (∆TA)
Supply Voltage Change (∆V+)
)
1000Ω
200°C
20°C
5V
Common-Mode Voltage Change (∆CM)
0.1V
ERROR
(ppm of Full Scale)
SAMPLE
ERROR CALCULATION(2)
ERROR SOURCE
ERROR EQUATION
UNADJ.
ADJUST.
INPUT
Input Offset Voltage
vs Common-Mode
Input Bias Current
Input Offset Current
VOS/(VIN MAX) • 106
CMRR • ∆CM/(VIN MAX) • 106
IB/IREF • 106
100µV/(250µA • 3.8Ω/°C • 200°C) • 106
50µV/V • 0.1V/(250µA • 3.8Ω/°C • 200°C) • 106
0.025µA/250µA • 106
526
26
100
16
0
26
0
IOS • RRTD MIN/(VIN MAX) • 106
3nA • 1000Ω/(250µA • 3.8Ω/°C • 200°C) • 106
0
Total Input Error:
668
26
EXCITATION
Current Reference Accuracy
vs Supply
IREF Accuracy (%)/100% • 106
(IREF vs V+) • ∆V+
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
Current Reference Matching
IREF Matching (%)/100% • IREF
•
R
RTD MIN/(VIN MAX) • 106
vs Supply
(IREF matching vs V+) • ∆V+ •
RTD MIN/(VIN MAX
10ppm/V • 5V • 250µA • 1000Ω/(250µA • 3.8Ω/°C • 200°C)
66
66
R
)
Total Excitation Error:
3507
191
GAIN
Span
Nonlinearity
Span Error (%)/100% • 106
Nonlinearity (%)/100% • 106
0.2%/100% • 106
0.01%/100% • 106
Total Gain Error:
2000
100
2100
0
100
100
OUTPUT
Zero Output
vs Supply
(IZERO - 4mA)/16000µA • 106
(IZERO vs V+) • ∆V+/16000µA • 106
25µA/16000µA • 106
0.2µA/V • 5V/16000µA • 106
Total Output Error:
1563
63
1626
0
63
63
DRIFT (∆TA = 20°C)
Input Offset Voltage
Input Bias Current (typical)
Input Offset Current (typical)
Current Reference Accuracy
Current Reference Matching
Span
Drift • ∆TA/(VIN MAX) • 106
Drift • ∆TA/IREF • 106
Drift • ∆TA • RRTD MIN/(VIN MAX) • 106
Drift • ∆TA
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
158
2
158
2
0.5
0.5
700
395
500
626
2382
700
395
500
626
2382
Drift • ∆TA • IREF • RRTD MIN/(VIN MAX
)
15ppm/°C • 20°C • 250µA • 1000Ω/(250µA • 3.8Ω/°C • 200°C)
25ppm/°C • 20°C
Drift • ∆TA
Drift • ∆TA/16000µA • 106
Zero Output
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
3
16
2
3
16
2
Total Noise Error:
21
21
TOTAL ERROR:
10304
2783
(1.03%)
(0.28%)
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 with-
out 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 approxi-
mately 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 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.
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.
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+
0.01µF
XTR112
XTR114
1N4148
Diodes
(1)
B
E
D1
9
8
Maximum VPS must be
less than minimum
voltage rating of zener
RL
VPS
diode.
IO
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
1
IR1
VLIN
VI+N
1kΩ
14
IR2
13
11
VREG
10
V+
4
RG
RLIN1
RLIN2
XTR112
XTR114
9
8
RG
B
E
0.01µF
3
2
RG
VI–N
IO
1kΩ
7
IRET
RZ
6
0.01µF
0.01µF
RTD
(1)
RCM
NOTE: (1) Bypass capacitors can be connected
to either the IRET pin or the IO pin.
0.01µF
FIGURE 5. Input Bypassing Technique with Linearization.
®
14
XTR112, XTR114
I
REG < 2mA
Isothermal
Block
5V
14
12
1
V+
OPA277
V–
VLIN
IR1
13
4
11
VREG
VI+N
IR2
10
V+
Type J
RG
1MΩ(1)
1MΩ
RG
1250Ω
XTR112
XTR114
9
8
B
E
0.01µF
3
RG
VI–N
1N4148
IO
20kΩ
2
7
1kΩ
25Ω
IRET
IO = 4mA + (VI+N –VI–N
)
40
RG
6
50Ω
(2)
RCM
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
1
1N4148
VLIN
14
13
IR1
+12V
11
VI+N
IR2
10
V+
VREG
1µF
4
RG
B
E
9
8
RLIN1
18.7kΩ
RG
1270Ω
XTR112
Q1
0.01µF
16
10
11
3
2
12
3
RG
VI–N
VO = 0 to 5V
15
14
IO
RCV420
2
7
13
IRET
5
4
Pt1000
100°C to
600°C
IO = 4mA – 20mA
RZ
1370Ω
6
RTD
1µF
–12V
RCM
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.
0.01µF
FIGURE 7. ±12V Powered Transmitter/Receiver Loop.
®
15
XTR112, XTR114
12
RLIN1
RLIN2
1
1N4148
VLIN
14
13
IR1
11
VI+N
IR2
+15V
10
V+
VREG
1µF
1µF
Isolated Power
from PWS740
0
4
RG
–15V
XTR112
XTR114
9
8
B
E
Q1
0.01µF
16
RG
10
11
3
2
12
3
2
RG
VI–N
V+
1
15
14
RCV420
IO
9
15
7
8
13
7
RZ
ISO124
VO
IRET
5
4
0 – 5V
10
IO = 4mA – 20mA
6
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
1
VLIN
14
IR2
IR1
11
VI+N
13
4
10
VREG
V+
RG
XTR112
XTR114
9
8
B
E
RG
3
2
RG
VI–N
7
IRET
6
(1)
RCM
NOTE: (1) Use RCM to adjust the
common-mode voltage to within
1.25V to 3.5V.
FIGURE 9. Bridge Input, Current Excitation.
®
16
XTR112, XTR114
相关型号：
XTR112UA/2K5E4
IC SPECIALTY ANALOG CIRCUIT, PDSO14, ROHS COMPLIANT, PLASTIC, SOIC-14, Analog IC:Other
TI
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