©1998 Burr-Brown Corporation PDS-1449A Printed in U.S.A. June, 1998
XTR106
4-20mA CURRENT TRANSMITTER
with Bridge Excitation and Linearization
®
FEATURES
●LOW TOTAL UNADJUSTED ERROR
●2.5V, 5V BRIDGE EXCITATION REFERENCE
●5.1V REGULATOR OUTPUT
●LOW SPAN DRIFT: ±25ppm/°C max
●LOW OFFSET DRIFT: 0.25µV/°C
●HIGH PSR: 110dB min
●HIGH CMR: 86dB min
●WIDE SUPPLY RANGE: 7.5V to 36V
●14-PIN DIP AND SO-14 SURFACE-MOUNT
APPLICATIONS
●PRESSURE BRIDGE TRANSMITTER
●STRAIN GAGE TRANSMITTER
●TEMPERATURE BRIDGE TRANSMITTER
●INDUSTRIAL PROCESS CONTROL
●SCADA REMOTE DATA ACQUISITION
●REMOTE TRANSDUCERS
●WEIGHING SYSTEMS
●ACCELEROMETERS
DESCRIPTION
The XTR106 is a low cost, monolithic 4-20mA, two-
wire current transmitter designed for bridge sensors. It
provides complete bridge excitation (2.5V or 5V refer-
ence), instrumentation amplifier, sensor linearization,
and current output circuitry. Current for powering ad-
ditional external input circuitry is available from the
VREG pin.
The instrumentation amplifier can be used over a wide
range of gain, accommodating a variety of input signal
types and sensors. Total unadjusted error of the com-
plete current transmitter, including the linearized bridge,
is low enough to permit use without adjustment in many
applications. The XTR106 operates on loop power sup-
ply voltages down to 7.5V.
Linearization circuitry provides second-order correction
to the transfer function by controlling bridge excitation
voltage. It provides up to a 20:1 improvement in
nonlinearity, even with low cost transducers.
The XTR106 is available in 14-pin plastic DIP and
SO-14 surface-mount packages and is specified for the
–40°C to +85°C temperature range. Operation is from
–55°C to +125°C.
XTR106
XTR106
XTR106
–
R
L
I
OUT
I
RET
V
O
4-20mA
V
PS
+7.5V to 36V
V
REF
2.5V
V
REF
5
5V
R
LIN
Lin
Polarity
V
REG
(5.1V)
R
G
BRIDGE NONLINEARITY CORRECTION
USING XTR106
0mV Bridge Output 10mV
2.0
1.5
1.0
0.5
0
–0.5
Uncorrected
Bridge Output
Corrected
5mV
Nonlinearity (%)
2
®
XTR106
IO = VIN • (40/RG) + 4mA, VIN in Volts, RG in Ω
SPECIFICATIONS
At TA = +25°C, V+ = 24V, and TIP29C external transistor, unless otherwise noted.
XTR106P, U XTR106PA, UA
PARAMETER CONDITIONS MIN TYP MAX MIN TYP MAX UNITS
OUTPUT
Output Current Equation IOA
Output Current, Specified Range 4 20 ✻✻mA
Over-Scale Limit IOVER 24 28 30 ✻✻✻ mA
Under-Scale Limit IUNDER IREG = 0, IREF = 0 1 1.6 2.2 ✻✻✻ mA
IREF + IREG = 2.5mA 2.9 3.4 4 ✻✻✻ mA
ZERO OUTPUT(1) IZERO VIN = 0V, RG = ∞4✻mA
Initial Error ±5±25 ✻±50 µA
vs Temperature TA = –40°C to +85°C±0.07 ±0.9 ✻✻ µA/°C
vs Supply Voltage, V+ V+ = 7.5V to 36V 0.04 0.2 ✻✻ µA/V
vs Common-Mode Voltage
(CMRR)
VCM = 1.1V to 3.5V(5) 0.02 ✻µA/V
vs VREG (IO)0.8 ✻ µA/mA
Noise: 0.1Hz to 10Hz in0.035 ✻ µAp-p
SPAN
Span Equation (Transconductance) S
S = 40/R
G
✻A/V
Untrimmed Error Full Scale (VIN) = 50mV ±0.05 ±0.2 ✻±0.4 %
vs Temper ature(2) TA = –40°C to +85°C±3±25 ✻✻ ppm/°C
Nonlinearity: Ideal Input(3) Full Scale (VIN) = 50mV ±0.001 ±0.01 ✻✻ %
INPUT(4)
Offset Voltage VOS VCM = 2.5V ±50 ±100 ✻±250 µV
vs Temperature TA = –40°C to +85°C±0.25 ±1.5 ✻±3µV/°C
vs Supply Voltage, V+ V+ = 7.5V to 36V ±0.1 ±3✻✻ µV/V
vs Common-Mode Voltage, RTI CMRR VCM = 1.1V to 3.5V(5) ±10 ±50 ✻±100 µV/V
Common-Mode Range(5) VCM 1.1 3.5 ✻✻V
Input Bias Current IB525 ✻50 nA
vs Temperature TA = –40°C to +85°C20 ✻pA/°C
Input Offset Current IOS ±0.2 ±3✻±10 nA
vs Temperature TA = –40°C to +85°C5 ✻pA/°C
Impedance: Differential ZIN 0.1 || 1 ✻GΩ || pF
Common-Mode 5 || 10 ✻GΩ || pF
Noise: 0.1Hz to 10Hz Vn0.6 ✻µVp-p
VOLTAGE REFERENCES(5) Lin Polarity Connected
to VREG, RLIN = 0
Initial: 2.5V Reference VREF2.5 2.5 ✻V
5V Reference VREF55✻V
Accuracy VREF = 2.5V or 5V ±0.05 ±0.25 ✻±0.5 %
vs Temperature TA = –40°C to +85°C±20 ±35 ✻±75 ppm/°C
vs Supply Voltage, V+ V+ = 7.5V to 36V ±5±20 ✻✻ ppm/V
vs Load IREF = 0mA to 2.5mA 60 ✻ppm/mA
Noise: 0.1Hz to 10Hz 10 ✻µVp-p
VREG(5) VREG 5.1 ✻V
Accuracy ±0.02 ±0.1 ✻✻ V
vs Temperature TA = –40°C to +85°C±0.3 ✻mV/°C
vs Supply Voltage, V+ V+ = 7.5V to 36V 1 ✻mV/V
Output Current IREG See Typical Curves ✻mA
Output Impedance IREG = 0mA to 2.5mA 80 ✻Ω
LINEARIZATION(6)
RLIN (external) Equation RLIN Ω
KLIN Linearization Factor KLIN VREF = 5V 6.645 ✻kΩ
VREF = 2.5V 9.905 ✻kΩ
Accuracy ±1±5✻✻ %
vs Temperature TA = –40°C to +85°C±50 ±100 ✻✻ ppm/°C
Max Correctable Sensor Nonlinearity B VREF = 5V ±5✻% of VFS
VREF = 2.5V –2.5, +5 ✻% of VFS
POWER SUPPLY V+
Specified +24 ✻V
Voltage Range +7.5 +36 ✻✻V
TEMPERATURE RANGE
Specification –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
✻ Specification same as XTR106P, XTR106U.
NOTES: (1) Describes accuracy of the 4mA low-scale offset current. Does not include input amplifier effects. Can be trimmed to zero. (2) Does not include initial
error or TCR of gain-setting resistor, RG. (3) Increasing the full-scale input range improves nonlinearity. (4) Does not include Zero Output initial error. (5) Voltage
measured with respect to IRET pin. (6) See “Linearization” text for detailed explanation. VFS = full-scale VIN.
RLIN = KLIN • , KLIN in Ω, B is nonlinearity relative to VFS
4B
1 – 2B
3
®
XTR106
V
REG
V
IN
R
G
R
G
V
IN
I
RET
I
O
V
REF
5
V
REF
2.5
Lin Polarity
R
LIN
V+
B (Base)
E (Emitter)
1
2
3
4
5
6
7
14
13
12
11
10
9
8
–
+
Power Supply, V+ (referenced to IO pin) ..........................................40V
Input Voltage, VIN, VIN (referenced to IRET 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.
Exposure to absolute maximum conditions for extended periods may degrade
device reliability.
ABSOLUTE MAXIMUM RATINGS(1)
Top View DIP and SOIC
PIN CONFIGURATION
+–
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/ORDERING INFORMATION
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.
PACKAGE SPECIFIED
DRAWING TEMPERATURE PACKAGE ORDERING TRANSPORT
PRODUCT PACKAGE NUMBER(1) RANGE MARKING NUMBER(2) MEDIA
XTR106P 14-Pin DIP 010 –40°C to +85°C XTR106P XTR106P Rails
XTR106PA 14-Pin DIP 010 –40°C to +85°C XTR106PA XTR106PA Rails
XTR106U SO-14 Surface Mount 235 –40°C to +85°C XTR106U XTR106U Rails
"""""XTR106U/2K5 Tape and Reel
XTR106UA SO-14 Surface Mount 235 –40°C to +85°C XTR106UA XTR106UA Rails
"""""XTR106UA/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 “XTR106U/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.
4
®
XTR106
FUNCTIONAL DIAGRAM
REF
Amp
Lin
Amp
Current
Direction
Switch
Bandgap
V
REF
V
REF
5
V
REF
2.5
V
IN
R
G
14
13
I
RET
+
V
IN
–
5.1V
11 110
25
Ω
975
Ω
I = 100µA + V
IN
R
G
5
4
3
2
100µA
R
LIN
V
REG
Lin
Polarity
V+
12
B
9
E
8
67
I
O
= 4mA + V
IN
• ( )
40
R
G
5
®
XTR106
TYPICAL PERFORMANCE CURVES
At TA = +25°C, V+ = 24V, unless otherwise noted.
–1.0 –0.5 0 0.5 1.0 1.5 2.0 2.5
Current (mA)
INPUT OFFSET VOLTAGE CHANGE
vs V
REG
and V
REF
CURRENTS
1.5
1.0
0.5
0
–0.5
–1.0
–1.5
–2.0
–2.5
∆ V
OS
(µV)
V
OS
vs I
REG
V
OS
vs I
REF
20mA
STEP RESPONSE
50µs/div
4mA/div
4mA
RG = 1kΩCOUT = 0.01µF
RG = 50Ω
100 1k 10k 100k 1M
Frequency (Hz)
TRANSCONDUCTANCE vs FREQUENCY
60
50
40
30
20
10
0
Transconductance (20 log mA/V)
C
OUT
= 0.01µF
R
G
= 1kΩ
R
L
= 250Ω
R
G
= 50Ω
C
OUT
= 0.01µF
C
OUT
= 0.033µF
C
OUT
connected
between V+ and I
O
10 1k100 10k 100k 1M
Frequency (Hz)
COMMON-MODE REJECTION vs FREQUENCY
110
100
90
80
70
60
50
40
30
Common-Mode Rejection (dB)
R
G
= 1kΩ
R
G
= 50Ω
10 1k100 10k 100k 1M
Frequency (Hz)
POWER SUPPLY REJECTION vs FREQUENCY
160
140
120
100
80
60
40
20
0
Power Supply Rejection (dB)
R
G
= 1kΩ
C
OUT
= 0
R
G
= 50Ω
INPUT OFFSET VOLTAGE DRIFT
PRODUCTION DISTRIBUTION
Percent of Units (%)
Offset Voltage Drift (µV/°C)
90
80
70
60
50
40
30
20
10
0
Typical production
distribution of
packaged units.
0
0.25
0.5
0.75
1.0
1.25
1.5
1.75
2.0
2.25
2.5
2.75
3.0
6
®
XTR106
TYPICAL PERFORMANCE CURVES (CONT)
At TA = +25°C, V+ = 24V, unless otherwise noted.
–1 –0.5 0 0.5 1.0 1.5 2.0
Current (mA)
ZERO OUTPUT ERROR
vs V
REF
and V
REG
CURRENTS
2.5
3.0
2.5
2.0
1.5
1.0
0.5
0
–0.5
–1.0
Zero Output Error (µA)
I
ZERO
Error vs I
REG
I
ZERO
Error vs I
REF
–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)
–75 –50 –25 0 25 50 75 100
Temperature (°C)
UNDER-SCALE CURRENT vs TEMPERATURE
125
2.5
2.0
1.5
1.0
0.5
0
Under-Scale Current (mA)
V+ = 7.5V to 36V
0 0.5 1.0 1.5 2.0 2.5
I
REF
+ I
REG
(mA)
UNDER-SCALE CURRENT vs I
REF
+ I
REG
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
Under-Scale Current (mA)
T
A
= +125°C
T
A
= +25°C
T
A
= –55°C
–75 –50 –25 0 25 50 75 100
Temperature (°C)
OVER-SCALE CURRENT vs TEMPERATURE
125
30
29
28
27
26
25
24
Over-Scale Current (mA)
V+ = 7.5V
V+ = 36V
V+ = 24V
With External Transistor
ZERO OUTPUT DRIFT
PRODUCTION DISTRIBUTION
Percent of Units (%)
Zero Output Drift (µA/°C)
70
60
50
40
30
20
10
0
Typical production
distribution of
packaged units.
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8
0.85
0.9
7
®
XTR106
TYPICAL PERFORMANCE CURVES (CONT)
At TA = +25°C, V+ = 24V, unless otherwise noted.
–75 –50 –25 0 25 50 75 100 125
Temperature (°C)
INPUT BIAS and OFFSET CURRENT
vs TEMPERATURE
10
8
6
4
2
0
–2
Input Bias and Offset Current (nA)
I
B
I
OS
–1.0 –0.5 0 0.5 1.0 1.5 2.0 2.5
V
REG
Output Current (mA)
V
REG
OUTPUT VOLTAGE vs V
REG
OUTPUT CURRENT
5.6
5.5
5.4
5.3
5.2
5.1
5.0
4.9
4.8
V
REG
Output Current (V)
T
A
= +125°C
T
A
= +25°C, –55°C
10µs/div
REFERENCE TRANSIENT RESPONSE
VREF = 5V
500µA/div 50mV/div
0
1mA
Reference
Output
–1.0 –0.5 0 0.5 1.0 1.5 2.0 2.5
V
REG
Current (mA)
V
REF
5 vs V
REG
OUTPUT CURRENT
5.008
5.004
5.000
4.996
4.992
4.988
V
REF
5 (V)
T
A
= +25°C
T
A
= +125°C
T
A
= –55°C
10 100 1k 10k 100k 1M
Frequency (Hz)
REFERENCE AC LINE REJECTION vs FREQUENCY
120
100
80
60
40
20
0
Line Rejection (dB)
V
REF
2.5
V
REF
5
1 10 100 1k 10k
Frequency (Hz)
INPUT VOLTAGE, INPUT CURRENT, and ZERO
OUTPUT CURRENT NOISE DENSITY vs FREQUENCY
100k
10k
1k
100
10
Input Voltage Noise (nV/√Hz)
10k
1k
100
10
Input Current Noise (fA/√Hz)
Zero Output Current Noise (pA/√Hz)
Input Current Noise
Input Voltage Noise
Zero Output Noise
8
®
XTR106
TYPICAL PERFORMANCE CURVES (CONT)
At TA = +25°C, V+ = 24V, unless otherwise noted.
REFERENCE VOLTAGE DRIFT
PRODUCTION DISTRIBUTION
Percent of Units (%)
Reference Voltage Drift (ppm/°C)
40
35
30
25
20
15
10
5
0
Typical production
distribution of
packaged units.
0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
32
34
36
38
40
–75 –50 –25 0 25 50 75 100 125
Temperature (°C)
REFERENCE VOLTAGE DEVIATION
vs TEMPERATURE
0.1
0
–0.1
–0.2
–0.3
–0.4
–0.5
Reference Voltage Deviation (%)
V
REF
= 5V
V
REF
= 2.5V
9
®
XTR106
APPLICATIONS INFORMATION
Figure 1 shows the basic connection diagram for the XTR106.
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. A 0.01µF to 0.03µF supply
bypass capacitor connected between V+ and IO is recom-
mended. For applications where fault and/or overload con-
ditions might saturate the inputs, a 0.03µF capacitor is
recommended.
A 2.5V or 5V reference is available to excite a bridge sensor.
For 5V excitation, pin 14 (VREF5) should be connected to the
bridge as shown in Figure 1. For 2.5V excitation, connect
pin 13 (VREF2.5) to pin 14 as shown in Figure 3b. The output
terminals of the bridge are connected to the instrumentation
amplifier inputs, VIN and VIN. A 0.01µF capacitor is shown
connected between the inputs and is recommended for high
impedance bridges (> 10kΩ). The resistor RG sets the gain
of the instrumentation amplifier as required by the full-scale
bridge voltage, VFS.
Lin Polarity and RLIN provide second-order linearization
correction to the bridge, achieving up to a 20:1 improvement
in linearity. Connections to Lin Polarity (pin 12) determine
the polarity of nonlinearity correction and should be con-
nected either to IRET or VREG. Lin Polarity should be con-
nected to VREG even if linearity correction is not desired.
RLIN is chosen according to the equation in Figure 1 and is
dependent on KLIN (linearization constant) and the bridge’s
nonlinearity relative to VFS (see “Linearization” section).
The transfer function for the complete current transmitter is:
IO = 4mA + VIN • (40/RG) (1)
VIN in Volts, RG in Ohms
where VIN is the differential input voltage. As evident from
the transfer function, if no RG is used (RG = ∞), the gain is
zero and the output is simply the XTR106’s zero current.
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 1.6mA. If current is
being sourced from the reference and/or VREG, the current
limit value may increase. Refer to the Typical Performance
Curves, “Under-Scale Current vs IREF + IREG” and “Under-
Scale Current vs Temperature.”
Increasingly positive input voltage (greater than the full-
scale input, VFS) will produce increasing output current
according to the transfer function, up to the output current
limit of approximately 28mA. Refer to the Typical Perfor-
mance Curve, “Over-Scale Current vs Temperature.”
The IRET pin is the return path for all current from the
references and VREG. IRET also serves as a local ground and
is the reference point for VREG and the on-board voltage
references. The IRET pin allows any current used in external
circuitry to be sensed by the XTR106 and to be included in
the output current without causing error. The input voltage
range of the XTR106 is referred to this pin.
FIGURE 1. Basic Bridge Measurement Circuit with Linearization.
+–
11 1
14
5
5V
Bridge
Sensor
4
3
2
R
G
XTR106
7
13
I = 4mA + V
IN
• ( )
O
40
R
G
R
LIN(3)
V
REG
V
REF
2.5
6
or
(4)
R
2(5)
R
1
R
B
(5)
R
G
R
G
V
IN
–
V
IN
+
R
LIN
V
REG
V+
I
RET
Lin
(1)
Polarity I
O
E
B
V
PS
4-20 mA
I
O
C
OUT
0.01µF
C
IN
0.01µF
(2)
7.5V to 36V
–
+
9
8
10
12
R
L
V
O
Q
1
V
REF
5
For 2.5V excitation, connect
pin 13 to pin 14
Possible choices for Q
1
(see text).
V
REG(1)
+–
NOTES:
(1) Connect Lin Polarity (pin 12) to I
RET
(pin 6) to correct for positive
bridge nonlinearity or connect to V
REG
(pin 1) for negative bridge
nonlinearity. The R
LIN
pin and Lin Polarity pin must be connected to
V
REG
if linearity correction is not desired. Refer to “Linearization”
section and Figure 3.
R
G
= (V
FS
/400µA) •
(4)
(2) Recommended for bridge impedances > 10kΩ
(5) R
1
and R
2
form bridge trim circuit to compensate for the initial
accuracy of the bridge. See “Bridge Balance” text.
R
LIN
= K
LIN
•
where K
LIN
= 9.905kΩ for 2.5V reference
K
LIN
= 6.645kΩ for 5V reference
B is the bridge nonlinearity relative to V
FS
V
FS
is the full-scale input voltage
4B
1 – 2B
( 3) (K
LIN
in Ω)
(V
FS
in V)
1 + 2B
1 – 2B
2N4922
TIP29C
TIP31C
TYPE
TO-225
TO-220
TO-220
PACKAGE
10
®
XTR106
R1≈5V •RB
4•VTRIM
EXTERNAL TRANSISTOR
External pass 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 XTR106,
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 require-
ments may be lower if the loop power supply voltage is less
than 36V. Some possible choices for Q1 are listed in Figure 1.
The XTR106 can be operated without an external pass
transistor. Accuracy, however, 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.
The low operating voltage (7.5V) of the XTR106 allows
operation directly from personal computer power supplies
(12V ±5%). When used with the RCV420 Current Loop
Receiver (Figure 8), load resistor voltage drop is limited to 3V.
BRIDGE BALANCE
Figure 1 shows a bridge trim circuit (R1, R2). This adjust-
ment can be used to compensate for the initial accuracy of
the bridge and/or to trim the offset voltage of the XTR106.
The values of R1 and R2 depend on the impedance of the
bridge, and the trim range required. This trim circuit places
an additional load on the VREF output. Be sure the additional
load on VREF does not affect zero output. See the Typical
Performance Curve, “Under-Scale Current vs IREF + IREG.”
The effective load of the trim circuit is nearly equal to R2.
An approximate value for R1 can be calculated:
(3)
where, RB is the resistance of the bridge.
VTRIM is the desired ±voltage trim range (in V).
Make R2 equal or lower in value to R1.
LINEARIZATION
Many bridge sensors are inherently nonlinear. With the
addition of one external resistor, it is possible to compensate
for parabolic nonlinearity resulting in up to 20:1 improve-
ment over an uncompensated bridge output.
Linearity correction is accomplished by varying the bridge
excitation voltage. Signal-dependent variation of the bridge
excitation voltage adds a second-order term to the overall
transfer function (including the bridge). This can be tailored
to correct for bridge sensor nonlinearity.
Either positive or negative bridge non-linearity errors can be
compensated by proper connection of the Lin Polarity pin.
To correct for positive bridge nonlinearity (upward bowing),
Lin Polarity (pin 12) should be connected to IRET (pin 6) as
shown in Figure 3a. This causes VREF to increase with bridge
output which compensates for a positive bow in the bridge
response. To correct negative nonlinearity (downward bow-
ing), connect Lin Polarity to VREG (pin 1) as shown in Figure
3b. This causes VREF to decrease with bridge output. The Lin
Polarity pin is a high impedance node.
If no linearity correction is desired, both the RLIN and Lin
Polarity pins should be connected to VREG (Figure 3c). This
results in a constant reference voltage independent of input
signal. RLIN or Lin Polarity pins should not be left open
or connected to another potential.
RLIN is the external linearization resistor and is connected
between pin 11 and pin 1 (VREG) as shown in Figures 3a and
3b. To determine the value of RLIN, the nonlinearity of the
bridge sensor with constant excitation voltage must be
known. The XTR106’s linearity circuitry can only compen-
sate for the parabolic-shaped portions of a sensor’s
nonlinearity. Optimum correction occurs when maximum
deviation from linear output occurs at mid-scale (see Figure
4). Sensors with nonlinearity curves similar to that shown in
LOOP POWER SUPPLY
The voltage applied to the XTR106, V+, is measured with
respect to the IO connection, pin 7. V+ can range from 7.5V
to 36V. The loop supply voltage, VPS, will differ from the
voltage applied to the XTR106 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:
(2)
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. V+ must be at least 8V if 5V sensor
excitation is used and if correcting for bridge nonlinearity
greater than +3%.
R
L
max =(V+)–7.5V
20mA
–R
WIRING
8
XTR106 0.01µF
E
I
O
I
RET
V+
10
7
6R
Q
= 3.3kΩ
For operation without external
transistor, connect a 3.3kΩ
resistor between pin 6 and
pin 8. See text for discussion
of performance.
FIGURE 2. Operation without External Transistor.
11
®
XTR106
R
LIN
=(9905Ω) (4) (–0.025)
1– (2) (–0.025) =943Ω
R
Y
15kΩ
R
X
100kΩ
I
RET
V
REG
Lin
Polarity
612
1
Open R
X
for negative bridge nonlinearity
Open R
Y
for positive bridge nonlinearity
XTR106
FIGURE 3. Connections and Equations to Correct Positive and Negative Bridge Nonlinearity.
Figure 4, but not peaking exactly at mid-scale can be
substantially improved. A sensor with a “S-shaped”
nonlinearity curve (equal positive and negative nonlinearity)
cannot be improved with the XTR106’s correction circuitry.
The value of RLIN is chosen according to Equation 4 shown
in Figure 3. RLIN is dependent on a linearization factor,
KLIN, which differs for the 2.5V reference and 5V reference.
The sensor’s nonlinearity term, B (relative to full scale), is
positive or negative depending on the direction of the bow.
A maximum ±5% non-linearity can be corrected when the
5V reference is used. Sensor nonlinearity of +5%/–2.5% can
be corrected with 2.5V excitation. The trim circuit shown in
Figure 3d can be used for bridges with unknown bridge
nonlinearity polarity.
Gain is affected by the varying excitation voltage used to
correct bridge nonlinearity. The corrected value of the gain
resistor is calculated from Equation 5 given in Figure 3.
111
14
5
5V 4
3
2
R
G
XTR106
13
V
REF
2.5
V
REG
6
R
2
R
1
R
LIN
I
RET
Lin
Polarity
12
V
REF
5
+–
+
–
1
11
14
5
2.5V 4
3
2
R
G
XTR106
13
V
REG
6
R
2
R
1
R
LIN
I
RET
Lin
Polarity
12
V
REF
2.5
V
REF
5
+–
+
–
1
11
14
5
5V 4
3
2
R
G
XTR106
13
V
REF
2.5
V
REG
6
R
2
R
1
R
LIN
I
RET
Lin
Polarity
12
V
REF
5
+–
+
–
3a. Connection for Positive Bridge Nonlinearity, VREF = 5V
3b. Connection for Negative Bridge Nonlinearity, VREF = 2.5V
3c. Connection if no linearity correction is desired, VREF = 5V
R
LIN
=K
LIN
•4B
1– 2B
R
G
=V
FS
400µA•1+2B
1–2B
V
REF(Adj)
=V
REF(Initial)
•1+2B
1–2B
EQUATIONS
Linearization Resistor:
(4)
Gain-Set Resistor:
(5)
Adjusted Excitation Voltage at Full-Scale Output:
(6)
where, KLIN is the linearization factor (in Ω)
KLIN = 9905Ω for the 2.5V reference
KLIN = 6645Ω for the 5V reference
B is the sensor nonlinearity relative to VFS
(for –2.5% nonlinearity, B = –0.025)
VFS is the full-scale bridge output without
linearization (in V)
Example:
Calculate RLIN and the resulting RG for a bridge sensor with
2.5% downward bow nonlinearity relative to VFS and determine
if the input common-mode range is valid.
VREF = 2.5V and VFS = 50mV
For a 2.5% downward bow, B = –0.025
(Lin Polarity pin connected to VREG)
For VREF = 2.5V, KLIN = 9905Ω
which falls within the 1.1V to 3.5V input common-mode range.
(in Ω)
(in Ω)
(in V)
3d. On-Board Resistor Circuit for Unknown Bridge Nonlinearity Polarity
R
G
=0.05V
400µA•1+(2)(–0.025)
1– (2) (–0.025) =113Ω
VCM =VREF(Adj)
2=1
2•2.5V •1+(2) (–0.025)
1– (2) (–0.025) =1.13V
12
®
XTR106
NONLINEARITY vs STIMULUS
0
3
2
1
0
–1
–2
–3
Normalized Stimulus
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Nonlinearity (% of Full Scale)
Negative Nonlinearity
B = –0.019
Positive Nonlinearity
B = +0.025
When using linearity correction, care should be taken to
insure that the sensor’s output common-mode voltage re-
mains within the XTR106’s allowable input range of 1.1V to
3.5V. Equation 6 in Figure 3 can be used to calculate the
XTR106’s new excitation voltage. The common-mode volt-
age of the bridge output is simply half this value if no
common-mode resistor is used (refer to the example in
Figure 3). Exceeding the common-mode range may yield
unpredicatable results.
For high precision applications (errors < 1%), a two-step
calibration process can be employed. First, the nonlinearity
of the sensor bridge is measured with the initial gain resistor
and RLIN = 0 (RLIN pin connected directly to VREG). Using
the resulting sensor nonlinearity, B, values for RG and RLIN
are calculated using Equations 4 and 5 from Figure 3. A
second calibration measurement is then taken to adjust RG to
account for the offsets and mismatches in the linearization.
UNDER-SCALE CURRENT
The total current being drawn from the VREF and VREG
voltage sources, as well as temperature, affect the XTR106’s
under-scale current value (see the Typical Performance
Curve, “Under-Scale Current vs IREF + IREG). This should be
considered when choosing the bridge resistance and excita-
tion voltage, especially for transducers operating over a
wide temperature range (see the Typical Performance Curve,
“Under-Scale Current vs Temperature”).
LOW IMPEDANCE BRIDGES
The XTR106’s two available excitation voltages (2.5V and
5V) allow the use of a wide variety of bridge values. Bridge
impedances as low as 1kΩ can be used without any addi-
tional circuitry. Lower impedance bridges can be used with
the XTR106 by adding a series resistance to limit excitation
current to ≤ 2.5mA (Figure 5). Resistance should be added
FIGURE 5. 350Ω Bridge with x50 Preamplifier.
BRIDGE TRANSDUCER TRANSFER FUNCTION
WITH PARABOLIC NONLINEARITY
0
10
9
8
7
6
5
4
3
2
1
0
Normalized Stimulus
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Bridge Output (mV)
Positive Nonlinearity
B = +0.025
B = –0.019
Negative Nonlinearity
Linear Response
FIGURE 4. Parabolic Nonlinearity.
1
14 13
5
3
2
XTR106
12
6
R
G
R
G
V
–IN
V
+IN
V+
I
O
I
RET
E
B
8
7
0.01µF
1N4148
9
10
4
11
R
LIN
Lin
Polarity
R
G
125Ω
I
O
= 4-20mA
1/2
OPA2277
1/2
OPA2277
5V
V
REF
2.5 V
REG
V
REF
5
350Ω
412Ω
10kΩ
10kΩ
1kΩ
3.4kΩ
3.4kΩ
I
REG
≈ 1.6mA
700µA at 5V
I
TOTAL
= 0.7mA + 1.6mA ≤ 2.5mA
Shown connected to correct positive
bridge nonlinearity. For negative bridge
nonlinearity, see Figure 3b.
Bridge excitation
voltage = 0.245V Approx. x50
amplifier
13
®
XTR106
to the upper and lower sides of the bridge to keep the bridge
output within the 1.1V to 3.5V common-mode input range.
Bridge output is reduced so a preamplifier as shown may be
needed to reduce offset voltage and drift.
OTHER SENSOR TYPES
The XTR106 can be used with a wide variety of inputs. Its
high input impedance instrumentation amplifier is versatile
and can be configured for differential input voltages from
millivolts to a maximum of 2.4V full scale. The linear range
of the inputs is from 1.1V to 3.5V, referenced to the IRET
terminal, pin 6. The linearization feature of the XTR106 can
be used with any sensor whose output is ratiometric with an
excitation voltage.
ERROR ANALYSIS
Table I shows how to calculate the effect various error
sources have on circuit accuracy. A sample error calculation
for a typical bridge sensor measurement circuit is shown
(5kΩ bridge, VREF = 5V, VFS = 50mV) is provided. The
results reveal the XTR106’s excellent accuracy, in this case
1.2% unadjusted. Adjusting gain and offset errors improves
circuit accuracy to 0.33%. Note that these are worst-case
errors; guaranteed maximum values were used in the calcu-
lations and all errors were assumed to be positive (additive).
The XTR106 achieves performance which is difficult to
obtain with discrete circuitry and requires less board space.
Bridge Impedance (RB)5kΩFull Scale Input (VFS) 50mV
Ambient Temperature Range (∆TA)20°C Excitation Voltage (VREF)5V
Supply Voltage Change (∆V+) 5V Common-Mode Voltage Change (∆CM) 25mV (= VFS/2)
ERROR
(ppm of Full Scale)
TABLE I. Error Calculation.
SAMPLE ERROR CALCULATION
NOTE (1): All errors are min/max and referred to input, unless otherwise stated.
SAMPLE
ERROR SOURCE ERROR EQUATION ERROR CALCULATION UNADJ ADJUST
INPUT
Input Offset Voltage VOS/VFS • 106200µV/50mV • 1062000 0
vs Common-Mode CMRR • ∆CM/VFS • 10650µV/V • 0.025V/50mV • 10625 25
vs Power Supply (VOS vs V+) • (∆V+)/VFS • 1063µV/V • 5V/50mV • 106300 300
Input Bias Current CMRR • IB • (RB/2)/VFS • 10650µV/V • 25nA • 2.5kΩ/50mV • 1060.1 0
Input Offset Current IOS • RB/VFS • 1063nA • 5kΩ/50mV • 106300 0
Total Input Error 2625 325
EXCITATION
Voltage Reference Accuracy VREF Accuracy (%)/100% • 1060.25%/100% • 1062500 0
vs Supply (VREF vs V+) • (∆V+) • (VFS/VREF) 20ppm/V • 5V (50mV/5V) 1 1
Total Excitation Error 2501 1
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/16000µA • 10662.5 62.5
Total Output Error 1626 63
DRIFT (∆TA = 20
°
C)
Input Offset Voltage Drift • ∆TA/(VFS) • 1061.5µV/°C • 20°C/(50mV) • 106600 600
Input Offset Current (typical) Drift • ∆TA• RB/(V
FS) • 1065pA/°C • 20°C • 5kΩ/(50mV) • 10610 10
Voltage Refrence Accuracy 35ppm/°C • 20°C 700 700
Span 225ppm/°C • 20°C 500 500
Zero Output Drift • ∆TA/16000µA • 1060.9µA/°C • 20°C/16000µA • 1061125 1125
Total Drift Error 2936 2936
NOISE (0.1Hz to 10Hz, typ)
Input Offset Voltage Vn(p-p)/VFS • 1060.6µV/50mV • 10612 12
Zero Output IZERO Noise/16000µA • 1060.035µA/16000µA • 1062.2 2.2
Thermal RB Noise
[√2 • √(RB/2)/1kΩ • 4nV/√Hz • √10Hz]/VFS • 106[√2 • √2.5kΩ/1kΩ • 4nV/√Hz • √10Hz]/50mV • 106
0.6 0.6
Input Current Noise (in • 40.8 • √2 • RB/2)/VFS •
106(200fA/√Hz • 40.8 • √2 • 2.5kΩ)/50mV• 106
0.6 0.6
Total Noise Error 15 15
TOTAL ERROR: 11803 3340
1.18% 0.33%
14
®
XTR106
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.
RADIO FREQUENCY INTERFERENCE
The long wire lengths of current loops invite radio fre-
quency interference. RF can be rectified by the sensitive
input circuitry of the XTR106 causing errors. This generally
appears as an unstable output current that varies with the
position of loop supply or input wiring.
If the bridge 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 6. 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.
REVERSE-VOLTAGE PROTECTION
The XTR106’s low compliance rating (7.5V) permits the
use of various voltage protection methods without compro-
mising operating range. Figure 6 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. A diode can be
inserted in series with the loop supply voltage and the V+
pin as shown in Figure 8 to protect against reverse output
connection lines with only a 0.7V loss in loop supply
voltage.
OVER-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 XTR106 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 XTR106 with
loop supply voltages up to 65V.
FIGURE 6. Reverse Voltage Operation and Over-Voltage Surge Protection.
V
PS
0.01µF
R
L
D
1(1)
NOTE: (1) Zener Diode 36V: 1N4753A or Motorola
P6KE39A. 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
14
5
5V
Bridge
Sensor
4
3
2
R
G
XTR106
7
13
V
REF
2.5
6
R
B
R
G
R
G
V
IN
–
V
IN
+
V+
I
RET
I
O
E
B9
8
10
Q
1
0.01µF0.01µF
V
REF
5
+–
15
®
XTR106
FIGURE 7. Thermocouple Low Offset, Low Drift Loop Measurement with Diode Cold-Junction Compensation.
FIGURE 8. ±12V-Powered Transmitter/Receiver Loop.
11 1
14
5
4
3
2
R
G
1kΩ
XTR106
7
13
I
O
= 4mA + V
IN
• ( )
V
REG
(pin 1)
40
R
G
V
REF
2.5
6
R
G
R
G
V
IN
–
V
IN
+
R
LIN
V
REG
V+
I
RET
Lin
Polarity
I
O
E
B
V
PS
4-20 mA
I
O
C
OUT
0.01µF
7.5V to 36V
–
+
9
8
10
12
R
L
V
O
Q
1
V
REF
5
Type K
100Ω
50Ω
5.2kΩ
4.8kΩ
1MΩ
(1)
20kΩ
6kΩ
2kΩ
0.01µF
1MΩ
0.01µF
OPA277
See ISO124 data sheet
if isolation is needed.
1N4148
Isothermal
Block
NOTE: (1) For burn-out indication.
4
3
2
R
G
XTR106
7
6
R
G
R
G
V+10
5
B
E
9
8
V
IN
Lin
Polarity
–
V
IN
+
I
RET
I
O
12
NOTE: Lin Polarity shown connected to correct positive bridge
nonlinearity. See Figure 3b to correct negative bridge nonlinearity.
14
11
1
13
R
LIN
V
REF
2.5
V
REF
5
2.5V
R
B
V
REG
Bridge
Sensor
–+
5
4
2
3
15
13 14
11
10
12
RCV420
16
1µF
1µF
0.01µF
1N4148 +12V
–12V
V
O
= 0V to 5V
I
O
= 4-20mA
See ISO124 data sheet
if isolation is needed.
