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: 0.25

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 TRANSMITTERS

STRAIN GAGE TRANSMITTERS

TEMPERATURE BRIDGE TRANSMITTERS

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
V

REG

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

C to +125

XTR106

XTR

106

XTR106

OUT

RET

4-20mA

7.5V to 36V

REF

2.5V

REF

LIN

Lin

Polarity

REG

(5.1V)

BRIDGE NONLINEARITY CORRECTION

USING XTR106

Bridge Output (mV)

2.0

1.5

1.0

0.5

Uncorrected
Bridge Output

Corrected

Nonlinearity (%)

SBOS092A JUNE 1998 REVISED NOVEMBER 2003

www.ti.com

PRODUCTION DATA information is current as of publication date.
Products conform to specifications per the terms of Texas Instruments
standard warranty. Production processing does not necessarily include
testing of all parameters.

Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of
Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.

All trademarks are the property of their respective owners.

XTR106

SBOS092A

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= V

· (40/R

) + 4mA, V

in Volts, R

SPECIFICATIONS

At T

= +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

Output Current, Specified Range

Over-Scale Limit

OVER

Under-Scale Limit

UNDER

REG

= 0, I

REF

= 0

1.6

2.2

REF

+ I

REG

= 2.5mA

2.9

3.4

ZERO OUTPUT

(1)

ZERO

= 0V, R

Initial Error

vs Temperature

= 40

C to +85

0.07

0.9

vs Supply Voltage, V+

V+ = 7.5V to 36V

0.04

0.2

A/V

vs Common-Mode Voltage (CMRR)

= 1.1V to 3.5V

(5)

0.02

A/V

vs V

REG

)

0.8

A/mA

Noise: 0.1Hz to 10Hz

0.035

Ap-p

SPAN
Span Equation (Transconductance)

S = 40/R

A/V

Untrimmed Error

Full Scale (V

) = 50mV

0.05

0.2

0.4

vs Temperature

(2)

= 40

C to +85

ppm/

Nonlinearity: Ideal Input

(3)

Full Scale (V

) = 50mV

0.001

0.01

INPUT

(4)

Offset Voltage

= 2.5V

100

250

vs Temperature

= 40

C to +85

0.25

1.5

vs Supply Voltage, V+

V+ = 7.5V to 36V

0.1

V/V

vs Common-Mode Voltage, RTI

CMRR

= 1.1V to 3.5V

(5)

100

V/V

Common-Mode Range

(5)

1.1

3.5

Input Bias Current

vs Temperature

= 40

C to +85

pA/

Input Offset Current

0.2

vs Temperature

= 40

C to +85

pA/

Impedance: Differential

0.1 || 1

|| pF

Common-Mode

5 || 10

|| pF

Noise: 0.1Hz to 10Hz

0.6

Vp-p

VOLTAGE REFERENCES

(5)

Lin Polarity Connected

to V

REG

, R

LIN

= 0

Initial: 2.5V Reference

REF

2.5

5V Reference

REF

Accuracy

REF

= 2.5V or 5V

0.05

0.25

0.5

vs Temperature

= 40

C to +85

ppm/

vs Supply Voltage, V+

V+ = 7.5V to 36V

ppm/V

vs Load

REF

= 0mA to 2.5mA

ppm/mA

Noise: 0.1Hz to 10Hz

Vp-p

REG

(5)

REG

5.1

Accuracy

0.02

0.1

vs Temperature

= 40

C to +85

0.3

mV/

vs Supply Voltage, V+

V+ = 7.5V to 36V

mV/V

Output Current

REG

See Typical Curves

Output Impedance

REG

= 0mA to 2.5mA

LINEARIZATION

(6)

LIN

(external) Equation

LIN

Linearization Factor

LIN

REF

= 5V

6.645

REF

= 2.5V

9.905

Accuracy

vs Temperature

= 40

C to +85

100

ppm/

Max Correctable Sensor Nonlinearity

REF

= 5V

% of V

REF

= 2.5V

2.5, +5

% of V

POWER SUPPLY

V+

Specified

+24

Voltage Range

+7.5

+36

TEMPERATURE RANGE
Specification

+85

Operating

+125

Storage

+125

Thermal Resistance

14-Pin DIP

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, R

. (3) Increasing the full-scale input range improves nonlinearity. (4) Does not include Zero Output initial error. (5) Voltage

measured with respect to I

RET

pin. (6) See "Linearization" text for detailed explanation. V

= full-scale V

LIN

= K

LIN

· , K

LIN

, B is nonlinearity relative to V

1 2B

XTR106

SBOS092A

www.ti.com

REG

RET

REF

2.5

Lin Polarity

LIN

V+

B (Base)

E (Emitter)

Power Supply, V+ (referenced to I

pin) .......................................... 40V

Input Voltage, V

, V

(referenced to I

RET

pin) ......................... 0V to V+

Storage Temperature Range ....................................... 55

C to +125

Lead Temperature (soldering, 10s) .............................................. +300

Output Current Limit ............................................................... Continuous
Junction Temperature ................................................................... +165

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

PACKAGE/ORDERING INFORMATION

ELECTROSTATIC
DISCHARGE SENSITIVITY

This integrated circuit can be damaged by ESD. Texas Instru-
ments 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 degrada-
tion 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.

For the most current package and ordering information, see
the Package Option Addendum at the end of this data sheet.

XTR106

SBOS092A

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TYPICAL PERFORMANCE CURVES

At T

= +25

C, V+ = 24V, unless otherwise noted.

1.0

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

1.0

1.5

2.0

2.5

(

vs I

REG

vs I

REF

20mA

STEP RESPONSE

s/div

A/div

4mA

= 1k

OUT

= 0.01

= 50

100

10k

100k

Frequency (Hz)

TRANSCONDUCTANCE vs FREQUENCY

Transconductance (20 log mA/V)

OUT

= 0.01µF

= 1k

= 250

= 50

OUT

= 0.01µF

OUT

= 0.033µF

OUT

connected

between V+ and I

100

10k

100k

Frequency (Hz)

COMMON-MODE REJECTION vs FREQUENCY

110

100

Common-Mode Rejection (dB)

= 1k

= 50

100

10k

100k

Frequency (Hz)

POWER SUPPLY REJECTION vs FREQUENCY

160

140

120

100

Power Supply Rejection (dB)

= 1k

OUT

= 0

= 50

INPUT OFFSET VOLTAGE DRIFT

PRODUCTION DISTRIBUTION

Percent of Units (%)

Offset Voltage Drift (µV/°C)

Typical production
distribution of
packaged units.

0.25

0.5

0.75

1.0

1.25

1.5

1.75

2.0

2.25

2.5

2.75

3.0

XTR106

SBOS092A

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TYPICAL PERFORMANCE CURVES

(CONT)

At T

= +25

C, V+

= 24V, unless otherwise noted.

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

1.0

Zero Output Error (

ZERO

Error vs I

REG

ZERO

Error vs I

REF

100

Temperature (°C)

ZERO OUTPUT CURRENT ERROR

vs TEMPERATURE

125

Zero Output Current Error (µA)

100

Temperature (°C)

UNDER-SCALE CURRENT vs TEMPERATURE

125

2.5

2.0

1.5

1.0

0.5

Under-Scale Current (mA)

V+ = 7.5V to 36V

0.5

1.0

1.5

2.0

2.5

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

Under-Scale Current (mA)

= +125°C

= +25°C

= 55°C

100

Temperature (°C)

OVER-SCALE CURRENT vs TEMPERATURE

125

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)

Typical production
distribution of
packaged units.

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

XTR106

SBOS092A

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TYPICAL PERFORMANCE CURVES

(CONT)

At T

= +25

C, V+

= 24V, unless otherwise noted.

100

125

Temperature (

INPUT BIAS and OFFSET CURRENT

vs TEMPERATURE

Input Bias and Offset Current (nA)

1.0

0.5

1.0

1.5

2.0

2.5

REG

Output Current (mA)

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

REG

Output Current (V)

= +125

= +25

C, 55

s/div

REFERENCE TRANSIENT RESPONSE

REF

= 5V

500

A/div

50m

/div

1mA

Reference

Output

1.0

0.5

1.0

1.5

2.0

2.5

REG

Current (mA)

REF

5 vs V

REG

OUTPUT CURRENT

5.008

5.004

5.000

4.996

4.992

4.988

REF

5 (V)

= +25°C

= +125°C

= 55°C

100

10k

100k

Frequency (Hz)

REFERENCE AC LINE REJECTION vs FREQUENCY

120

100

Line Rejection (dB)

REF

2.5

REF

100

10k

Frequency (Hz)

INPUT VOLTAGE, INPUT CURRENT, and ZERO

OUTPUT CURRENT NOISE DENSITY vs FREQUENCY

100k

10k

100

Input Voltage Noise (nV/

Hz)

10k

100

Input Current Noise (fA/

Hz)

Zero Output Current Noise (pA/

Hz)

Input Current Noise

Input Voltage Noise

Zero Output Noise

XTR106

SBOS092A

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APPLICATIONS INFORMATION

Figure 1 shows the basic connection diagram for the XTR106.
The loop power supply, V

, provides power for all circuitry.

Output loop current is measured as a voltage across the series
load resistor, R

. A 0.01

F to 0.03

F supply bypass capacitor

connected between V+ and I

is recommended. For applica-

tions where fault and/or overload conditions might saturate
the inputs, a 0.03

F capacitor is recommended.

A 2.5V or 5V reference is available to excite a bridge sensor.
For 5V excitation, pin 14 (V

REF

5) should be connected to the

bridge as shown in Figure 1. For 2.5V excitation, connect
pin 13 (V

REF

2.5) to pin 14 as shown in Figure 3b. The output

terminals of the bridge are connected to the instrumentation
amplifier inputs, V

and V

. A 0.01

F capacitor is shown

connected between the inputs and is recommended for high
impedance bridges (> 10k

). The resistor R

sets the gain

of the instrumentation amplifier as required by the full-scale
bridge voltage, V

Lin Polarity and R

LIN

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 I

RET

or V

REG

. Lin Polarity should be con-

nected to V

REG

even if linearity correction is not desired.

LIN

is chosen according to the equation in Figure 1 and is

dependent on K

LIN

(linearization constant) and the bridge's

nonlinearity relative to V

(see "Linearization" section).

The transfer function for the complete current transmitter is:

= 4mA + V

· (40/R

)

(1)

in Volts, R

in Ohms

where V

is the differential input voltage. As evident from

the transfer function, if no R

is used (R

), the gain is

zero and the output is simply the XTR106's zero current.

A negative input voltage, V

, will cause the output current

to be less than 4mA. Increasingly negative V

will cause the

output current to limit at approximately 1.6mA. If current is
being sourced from the reference and/or V

REG

, the current

limit value may increase. Refer to the Typical Performance
Curves, "Under-Scale Current vs I

REF

+ I

REG

" and "Under-

Scale Current vs Temperature."

Increasingly positive input voltage (greater than the full-
scale input, V

) 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 I

RET

pin is the return path for all current from the

references and V

REG

. I

RET

also serves as a local ground and

is the reference point for V

REG

and the on-board voltage

references. The I

RET

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.

Bridge

Sensor

XTR106

I = 4mA + V

· ( )

LIN

(3)

REG

REF

2.5

(4)

(5)

LIN

REG

V+

RET

Lin

(1)

Polarity

4-20 mA

OUT

0.01

(2)

7.5V to 36V

REF

For 2.5V excitation, connect

pin 13 to pin 14

Possible choices for Q

(see text).

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

REG

if linearity correction is not desired. Refer to "Linearization"

section and Figure 3.

= (V

/400

A) ·

(4)

(2) Recommended for bridge impedances > 10k

(5) R

and R

form bridge trim circuit to compensate for the initial

accuracy of the bridge. See "Bridge Balance" text.

LIN

= K

LIN

where K

LIN

= 9.905k

for 2.5V reference

LIN

= 6.645k

for 5V reference

B is the bridge nonlinearity relative to V

is the full-scale input voltage

1 2B

( 3)

LIN

)

in V)

1 + 2B

1 2B

2N4922
TIP29C
TIP31C

TYPE

TO-225
TO-220
TO-220

PACKAGE

XTR106

SBOS092A

www.ti.com

TRIM

EXTERNAL TRANSISTOR

External pass transistor, Q

, 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: V

CEO

= 45V

min,

= 40 min and P

= 800mW. Power dissipation require-

ments may be lower if the loop power supply voltage is less
than 36V. Some possible choices for Q

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 Q

is not recommended for extended temperature ranges. A
resistor (R = 3.3k

) connected between the I

RET

pin and the

E (emitter) pin may be needed for operation below 0

without Q

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 (R

, R

). 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 R

and R

depend on the impedance of the

bridge, and the trim range required. This trim circuit places
an additional load on the V

REF

output. Be sure the additional

load on V

REF

does not affect zero output. See the Typical

Performance Curve, "Under-Scale Current vs I

REF

+ I

REG

The effective load of the trim circuit is nearly equal to R

An approximate value for R

can be calculated:

(3)

where, R

is the resistance of the bridge.

TRIM

is the desired

voltage trim range (in V).

Make R

equal or lower in value to R

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 I

RET

(pin 6) as

shown in Figure 3a. This causes V

REF

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 V

REG

(pin 1) as shown in Figure

3b. This causes V

REF

to decrease with bridge output. The Lin

Polarity pin is a high impedance node.

If no linearity correction is desired, both the R

LIN

and Lin

Polarity pins should be connected to V

REG

(Figure 3c). This

results in a constant reference voltage independent of input
signal. R

LIN

or Lin Polarity pins should not be left open

or connected to another potential.

LIN

is the external linearization resistor and is connected

between pin 11 and pin 1 (V

REG

) as shown in Figures 3a and

3b. To determine the value of R

LIN

, 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 I

connection, pin 7. V+ can range from 7.5V

to 36V. The loop supply voltage, V

, will differ from the

voltage applied to the XTR106 according to the voltage drop
on the current sensing resistor, R

(plus any other voltage

drop in the line).

If a low loop supply voltage is used, R

(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%.

max

) 7. 5V

20mA

WIRING

XTR106

0.01µF

RET

V+

= 3.3k

For operation without external
transistor, connect a 3.3k

resistor between pin 6 and
pin 8. See text for discussion
of performance.

FIGURE 2. Operation without External Transistor.

XTR106

SBOS092A

www.ti.com

LIN

(9905

) (4) ( 0.025)

1 (2) ( 0.025)

943

15k

100k

RET

REG

Lin

Polarity

Open R

for negative bridge nonlinearity

Open R

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 R

LIN

is chosen according to Equation 4 shown

in Figure 3. R

LIN

is dependent on a linearization factor,

LIN

, 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.

XTR106

REF

2.5

REG

LIN

RET

Lin

Polarity

REF

2.5V

XTR106

REG

LIN

RET

Lin

Polarity

REF

2.5

REF

XTR106

REF

2.5

REG

LIN

RET

Lin

Polarity

REF

3a. Connection for Positive Bridge Nonlinearity, V

REF

= 5V

3b. Connection for Negative Bridge Nonlinearity, V

REF

= 2.5V

3c. Connection if no linearity correction is desired, V

REF

= 5V

LIN

1 2B

400

1 2B

REF (Adj)

REF (Initial)

1 2B

EQUATIONS

Linearization Resistor:

(4)

Gain-Set Resistor:

(5)

Adjusted Excitation Voltage at Full-Scale Output:

(6)

where, K

LIN

is the linearization factor (in

)

LIN

= 9905

for the 2.5V reference

LIN

= 6645

for the 5V reference

B is the sensor nonlinearity relative to V

(for 2.5% nonlinearity, B = 0.025)

is the full-scale bridge output without

linearization (in V)

Example:

Calculate R

LIN

and the resulting R

for a bridge sensor with

2.5% downward bow nonlinearity relative to V

and determine

if the input common-mode range is valid.

REF

= 2.5V and V

= 50mV

For a 2.5% downward bow, B = 0.025

(Lin Polarity pin connected to V

REG

)

For V

REF

= 2.5V, K

LIN

= 9905

which falls within the 1.1V to 3.5V input common-mode range.

(in

)

(in

)

(in V)

3d. On-Board Resistor Circuit for Unknown Bridge Nonlinearity Polarity

0.05V

400

(2) ( 0.025)

1 (2) ( 0.025)

113

REF (Adj)

2. 5V

(2) ( 0.025)

1 (2) ( 0.025)

1.13V

XTR106

SBOS092A

www.ti.com

NONLINEARITY vs STIMULUS

Normalized Stimulus

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

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 R

LIN

= 0 (R

LIN

pin connected directly to V

REG

). Using

the resulting sensor nonlinearity, B, values for R

and R

LIN

are calculated using Equations 4 and 5 from Figure 3. A
second calibration measurement is then taken to adjust R

account for the offsets and mismatches in the linearization.

UNDER-SCALE CURRENT

The total current being drawn from the V

REF

and V

REG

voltage sources, as well as temperature, affect the XTR106's
under-scale current value (see the Typical Performance
Curve, "Under-Scale Current vs I

REF

+ I

REG

). 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 1k

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. 350

Bridge with x50 Preamplifier.

BRIDGE TRANSDUCER TRANSFER FUNCTION

WITH PARABOLIC NONLINEARITY

Normalized Stimulus

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Bridge Output (mV)

Positive Nonlinearity

B = +0.025

B = 0.019

Negative Nonlinearity

Linear Response

FIGURE 4. Parabolic Nonlinearity.

XTR106

V+

RET

0.01µF

1N4148

LIN

Lin

Polarity

125

= 4-20mA

1/2

OPA2277

1/2

OPA2277

REF

2.5

REG

REF

350

412

10k

3.4k

REG

1.6mA

700µA at 5V

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

XTR106

SBOS092A

www.ti.com

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 I

RET

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
(5k

bridge, V

REF

= 5V, V

= 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 (R

)

Full Scale Input (V

)

50mV

Ambient Temperature Range (

)

Excitation Voltage (V

REF

)

Supply Voltage Change (

V+)

Common-Mode Voltage Change (

CM)

25mV (= V

/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

· 10

200

V/50mV · 10

2000

vs Common-Mode

CMRR ·

CM/V

· 10

V/V · 0.025V/50mV · 10

vs Power Supply

vs V+) · (

V+)/V

· 10

V/V · 5V/50mV · 10

300

Input Bias Current

CMRR · I

· (R

/2)/ V

· 10

V/V · 25nA · 2.5k

/50mV · 10

0.1

Input Offset Current

· R

· 10

3nA · 5k

/50mV · 10

300

Total Input Error

2625

325

EXCITATION

Voltage Reference Accuracy

REF

Accuracy (%)/100% · 10

0.25%/100% · 10

2500

vs Supply

REF

vs V+) ·

(

V+) · (V

REF

)

20ppm/V · 5V (50mV/5V)

Total Excitation Error

2501

GAIN

Span

Span Error (%)/100% · 10

0.2%/100% · 10

2000

Nonlinearity

Nonlinearity (%)/100% · 10

0.01%/100% · 10

100

Total Gain Error

2100

100

OUTPUT

Zero Output

| I

ZERO

4mA | /16000

A · 10

A/16000

A · 10

1563

vs Supply

ZERO

vs V+) ·

(

V+)/16000

A · 10

0.2

A/V · 5V/16000

A · 10

62.5

Total Output Error

1626

DRIFT (

= 20

Input Offset Voltage

Drift ·

/ (V

) · 10

1.5

V /

C · 20

C / (50mV) · 10

600

Input Offset Current (typical)

Drift ·

· R

/ (V

) · 10

5pA /

C · 20

C · 5k

/ (50mV) · 10

Voltage Refrence Accuracy

35ppm/

C · 20

700

Span

225ppm/

C · 20

500

Zero Output

Drift ·

/ 16000

A · 10

0.9

A /

C · 20

C / 16000

A · 10

1125

Total Drift Error

2936

NOISE (0.1Hz to 10Hz, typ)

Input Offset Voltage

(p-p)/ V

· 10

0.6

V / 50mV · 10

Zero Output

ZERO

Noise / 16000

A · 10

0.035

A / 16000

A · 10

2.2

Thermal R

Noise

[

2 ·

/ 2 ) / 1k

· 4nV /

Hz ·

10Hz ] / V

· 10

[

2 ·

2.5k

/ 1k

· 4nV/

Hz ·

10Hz ] / 50mV · 10

0.6

Input Current Noise

· 40.8 ·

2 · R

/ 2)/ V

· 10

(200fA/

Hz · 40.8 ·

2 · 2.5k

)/50mV· 10

0.6

Total Noise Error

TOTAL ERROR:

11803

3340

1.18%

0.33%

XTR106

SBOS092A

www.ti.com

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 I

RET

terminal as shown in Figure 6. Although the dc voltage at
the I

RET

terminal is not equal to 0V (at the loop supply, V

)

this circuit point can be considered the transmitter's "ground."
The 0.01

F capacitor connected between V+ and I

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.

0.01µF

(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

must be

less than minimum
voltage rating of zener
diode.

The diode bridge causes
a 1.4V loss in loop supply
voltage.

1N4148

Diodes

Bridge

Sensor

XTR106

REF

2.5

V+

RET

0.01µF

REF

XTR106

SBOS092A

www.ti.com

FIGURE 7. Thermocouple Low Offset, Low Drift Loop Measurement with Diode Cold-Junction Compensation.

FIGURE 8.

12V-Powered Transmitter/Receiver Loop.

XTR106

= 4mA + V

· ( )

REG

(pin 1)

40
R

REF

2.5

LIN

REG

V+

RET

Lin

Polarity

4-20 mA

OUT

0.01µF

7.5V to 36V

REF

Type K

100

5.2k

4.8k

(1)

20k

0.01µF

OPA277

See ISO124 data sheet

if isolation is needed.

1N4148

Isothermal

Block

NOTE: (1) For burn-out indication.

XTR106

V+

Lin

Polarity

RET

NOTE: Lin Polarity shown connected to correct positive bridge
nonlinearity. See Figure 3b to correct negative bridge nonlinearity.

LIN

REF

2.5

REF

2.5V

REG

Bridge

Sensor

RCV420

1µF

0.01µF

1N4148

+12V

12V

= 0V to 5V

= 4-20mA

See ISO124 data sheet
if isolation is needed.

MECHANICAL

MPDI002C JANUARY 1995 REVISED DECEMBER 20002

POST OFFICE BOX 655303

DALLAS, TEXAS 75265

N (R-PDIP-T**)

PLASTIC DUAL-IN-LINE PACKAGE

0.325 (8,26)
0.300 (7,62)

0.010 (0,25) NOM

Gauge Plane

0.015 (0,38)

0.430 (10,92) MAX

1.060

(26,92)

0.940

(23,88)

0.920

0.850

0.775

0.745

(19,69)

(18,92)

0.775

(19,69)

(18,92)

0.745

A MIN

DIM

A MAX

PINS **

(23,37)

(21,59)

Seating Plane

14/18 PIN ONLY
20 pin vendor option

4040049/E 12/2002

0.070 (1,78)

0.045 (1,14)

0.020 (0,51) MIN

0.015 (0,38)

0.021 (0,53)

0.200 (5,08) MAX

0.125 (3,18) MIN

0.240 (6,10)

0.260 (6,60)

0.010 (0,25)

0.100 (2,54)

16 PINS SHOWN

MS-100

VARIATION

0.030 (0,76)

0.045 (1,14)

NOTES: A. All linear dimensions are in inches (millimeters).

B. This drawing is subject to change without notice.

C. Falls within JEDEC MS-001, except 18 and 20 pin minimum body lrngth (Dim A).

D. The 20 pin end lead shoulder width is a vendor option, either half or full width.

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