# Texas Instruments | Low-Voltage Current Loop Transmitter | Application notes | Texas Instruments Low-Voltage Current Loop Transmitter Application notes

```ADC101S021,ADC121S021,LM94022,LMP8270,
LMV951,LP2951
Low-Voltage Current Loop Transmitter
Literature Number: SNOA865
8208Signal_Path_108
12/12/06
9:40 AM
Page 1
SIGNAL PATH designer
®
Tips, tricks, and techniques from the analog signal-path experts
No. 108
Low-Voltage Current Loop Transmitter
Feature Article....1-7
— By Walt Bacharowski, Applications Manager
Pressure Force
Current Loop
Factory
Automation
Solutions..............4-5
Design Tools............8
Current Loop
Transmitter
+
VS
W R1
Sensor
VT
-
Loop
Power
Supply
I L =4 to 20 mA
WR2
RR
VOUT
Figure 1. Current Loop Components and Connection
T
he 4 to 20 mA current loop, which is used extensively in industrial
and process control systems, creates challenges for maximizing the
operating loop length. In some cases, a very long loop is required and
the combination of limited loop-power supply voltage and excessive loop
wire resistance prevents it use. This article discusses the use of low-voltage
amplifiers to minimize the transmitter’s operating voltage requirements,
which will maximize the operating loop length.
Typically, the current loop is powered from the receiver side while the
transmitter controls the current flowing in the loop to indicate the value of
the physical parameter being measured by the sensor. Figure 1 shows the
basic components and connection of a current loop.
The maximum distance between the transmitter and receiver is dependent
on the power supply voltage (VS), and the sum of the loop drops, which are
the minimum transmitter voltage (VT), the voltage drops across the wire
resistance (WR1 and WR2), and receiver resistor (RR). In equation form:
EQ1
VS = VWR1 + VT + VWR2 + VRR
NEXT ISSUE:
Generating Precision Clocks
8208Signal_Path_108
12/12/06
9:40 AM
Page 2
Solutions for Pressure Force Load Testing
Bridge Sensor Application
+5V
3
6 SYNC
5 SCLK
4
DIN
1
DAC
2
+V
5V
3
+ 5 1
A1
4
2
-V
AV = 141
180
470 pF
0.2 pF
140K
2
2K
470 pF
+V
A1 = LMP2012
4 - 5
1
A1
3
+ 2
0.2 pF
140K
3
+V
8 7 SCLK
6 DOUT
5
4
CS
1
To µC
DAC = DAC081S101
180
-V
LMP2012 Precision Op Amp
• Auto zero dual op amp
• 12-bit analog-to-digital converter
• Input offset voltage, VOS, (36 µV MAX) minimizes
signal amplification errors of original input
• True differential inputs
• TCVos of 15 nV/°C maintains a stable VOS over time
and temperature
• Reference voltage between 500 mV and 2.5V
• CMRR and PSRR greater than 120 dB ensures
accuracy over various common mode voltages and
across its entire operating voltage range
• Gain bandwidth product and slew rate are best in
class at 3 MHz and 4 V/µs
• Also available:
• LMP2011 (single) in SOIC-8 and SOT23-5 packaging
• LMP2014 (quad) in TSSOP-14 packaging
• Guaranteed performance from 50 kSPS to 200 kSPS
• Binary 2’s compliant
• SPI™/QSPI™/MICROWIRE™/DSP compatible
DAC081S101 8-Bit D/A Converter
• Low power, 8-bit digital-to-analog converter
• ±0.75 LSB INL
• 3 µsec settling time
• Rail-to-rail voltage output
• SPI™/QSPI™/MICROWIRE™/DSP compatible
amplifiers.national.com
2
8208Signal_Path_108
12/12/06
9:40 AM
Page 3
SIGNAL PATH designer
Low-Voltage Current Loop Transmitter
Substituting the loop current and loop resistances
into EQ1:
In this example, a temperature sensor, such as the
LM94022, provides a signal for the transmitter.
EQ2
The components A1, Q1, and R1 through R5
form a voltage-to-current converter. The noninverting input of A1, pin 3, is the summing node for
three signals, the loop current, offset current, and
sensor signal voltage. The resistor R2 is the current
shunt that measures the current flowing in the loop
and is fed back through R3. The total loop current
is the sum of the currents flowing in resistor R2
and R3, IL=IR2+IR3. The amplifier, A1, forces the
voltages at its inputs, pins 3 and 4, to be equal by
forcing more or less current through R2. The result
is that R2 and R3 have the same voltage across
them. The ratio of the currents in R2 and R3 is the
inverse of the resistor ratio:
VS = ILWR1 + VT + ILWR2 + ILRR
Given the wire’s resistance in X Ohms per foot, the
maximum loop current of 20 mA, the value of RR
equal to 10Ω, and the equal lengths of wire, EQ2
can be rearranged to calculate the maximum loop
distance in terms the loop parameters:
EQ3
VS − VT − 0. 2
0.04 (X Ω/ft)
ft =
EQ3 illustrates three ways to increase the
maximum loop length: (1) increase the loop power
supply voltage, (2) increase the wire gage, which
will reduce the wire’s ohms per foot, or (3) reduce
the minimum voltage required for the current loop
transmitter operation, which is the focus of the
following section.
The use of low voltage amplifiers, such as the
LMV951, and low drop out voltage regulators,
such as the LP2951, can reduce the minimum voltage required for the current loop transmitter.
Figure 2 shows the schematic of a loop-powered
4 to 20 mA transmitter, which will function with a
minimum of 1.9V, and a 4 to 20 mA receiver.
1
C3
R5
10K
C4
4.7 µF
1 GS0
5 GS1
V IN
3
TS
R5
100K
4
TS = LM94022
A1 = LMV951
VREG = LP2951
R2
)
+5
V3
SD
3
+
C2
1 µF
Loop
Power
Supply
VS
–
W R1
4
6
VREF
3
5
Q1
2N3904
R1
4.7K
C1
4.096V
10K
4 to 20 mA
Current Loop
Transmitter
R2
10
1
6
+
5
A2
W R2
1 µF
IR2
1
8
–
3
100nF
7,8
1,2
+5
RR
10
V2
R3
R3
This highlights that the current in R3 is also part of
the voltage-to-current conversion and is not an
error current. An error source that will affect the
loop current is the offset voltage of amplifier A1,
which will add an error current to the loop current.
At the minimum loop current of 4 mA, the voltage
V2 is very close to 0.040V.
GND
4
1
A1
–
2
IR3
IR3
=
6
+
GND
2
I R2
(
8
R7
25.5K
R4
402K
VOUT 3
VIN
VREG
10nF
R6
20K
4
VDD
VOUT
EQ4
180
4
3
4
CS
5
SDATA
2
2
470pF
100nF
IL
V1
4 to 20 mA
Current Loop
VREF = LM4140ACM-4.1
A2 = LMP8270
Figure 2. Loop-Powered Transmitter Schematic
signalpath.national.com/designer
SCLK
6
03
To
µP
8208Signal_Path_108
12/12/06
9:40 AM
Page 4
Solutions for Factory Automation
Resistance Temperature Detector Application
+V
4 3
1
W1
W2
3
+
A1
2
-
5
4
6
LM4140A-2.500
1,2 7,8
3
-V
4
RTD
+
A2
-
+V
5
R2
10K
1
2
-V
R1
10K
R5
10K
+V
W3
3
R7
3.205K
W4
R8
2.5K
4
3
A1, A2, A3, and A4 = LMP7701
or one LMP7704 (quad op amp)
A3
+
R6
10K
+V
5
1
2
-V
4
R4
10K
+
A4
-
+V
5
1
1
180Ω 3
4 SCLK
6 /CS
5 SDATA
2
V
-V OUT 470pF
To μP
2
R3
10K
Thermocouple Temperature Detector Application
180Ω 3
+V
4
1
5
VDD
GS0
3
VOUT
Cold Junction
Reference
2
+5
4
3
+V
Copper
3
Copper
4
5K
5
+
A1
2
1
180Ω 3
Amplified
Thermocouple
Output
4.096V
6
1,2
To
µP
100nF
7,8
1
4 SCLK
6 /CS
5 SDATA
470 pF
2
1M
TCJR= LM94022
A1 = LMP7701
www.amplifiers.national.com
4
470 pF 1
2
Type K
Thermocouple
LM4140ACM-4.1
5K
GS1
T CJR
Av = 200
Full Scale ~ 500°C
1M
Cold Junction
Temperature
4 SCLK
6 /CS
5 SDATA
8208Signal_Path_108
12/12/06
9:40 AM
Page 5
Precision Op Amps
PSRR
(dB)
GBWP
(MHz)
Voltage Noise
(nV/ Hz)
IBIAS
Room
Temp (pA)
130
3
35
-3
130
130
2.5
9
0.2
100
110
17
5.8
0.1
100
110
17
5.8
0.1
Product ID
Max VOS
Room Temp (µV)
TCVOS
(µV/°C)
Specified Supply
Voltage Range (V)
CMRR
(dB)
LMP2011/12/14
25
0.015
2.7 to 5.25
120
130
LMP7701/02/04
200
1
2.7 to 12
100
LMP7711/12
150
-1
1.8 to 5.5
100
LMP7715/16
150
-1
1.8 to 5.5
100
Gain
(dB)
Precision Current Sense Amps
Product ID
Input Voltage Range
LMP8275
-2 to 16
TCVOS (µV/°C)
30
Fixed Gain (V/V)
20
Supply Voltage (V)
4.75 to 5.5
CMRR (dB)
80
Packaging
SOIC-8
LMP8276
-2 to 16
30
20
4.75 to 5.5
80
SOIC-8
LMP8277
-2 to 16
30
14
4.75 to 5.5
80
SOIC-8
Low-Voltage Op Amps
Product ID
Typ Is/
Channel (µA)
Total Specified
Supply Range (V)
Max VOS
(mV)
Max IBIAS Over
Temperature
Typ
CMVR (V)
GBW
(MHz)
Packaging
LMV651
110
2.7 to 5.5
1
80 nA (typ)
0 to 4.0
12
SC70-5, TSSOP-14
LMV791
1150/0.14
1.8 to 5.5
1.35
100 pA
-0.3 to 4.0
17
TSOT23-6, MSOP-10
LMV796
1150
1.8 to 5.5
1.35
100 pA
-0.3 to 4.0
17
SOT23-5, MSOP-8
LMV716
1600
2.7 to 5.0
5
130 pA
-0.3 to 2.2
5
MSOP-8
LPV531
425
2.7 to 5.5
4.5
10 pA
-0.3 to 3.8
4.6
TSOT23-6
Res
# of
Inputs
Input Type
Max Power
5V/3V (mW)
Supply (V)
Max INL
(LBS)
(dB)
12
1
Packaging
500 to 1000
Single ended
16/4.5
2.7 to 5.25
±1.1
70
SOT23-6, LLP-6
12
12
1
200 to 500
Single ended
15.8/4.7
2.7 to 5.25
±1.0
70.3
SOT23-6, LLP-6
1
50 to 200
Single ended
14.7/4.3
2.7 to 5.25
±1.0
70
SOT23-6, LLP-6
10
1
500 to 1000
Single ended
16/4.5
2.7 to 5.25
±0.7
61
SOT23-6
10
1
200 to 500
Single ended
13.7/4.3
2.7 to 5.25
±0.7
60.8
SOT23-6
10
1
50 to 200
Single ended
12.6/4
2.7 to 5.25
±0.6
60.7
SOT23-6
8
1
500 to 1000
Single ended
16/4.5
2.7 to 5.25
±0.3
49
SOT23-6
8
1
200 to 500
Single ended
12.6/3.6
2.7 to 5.25
±0.3
49
SOT23-6
8
1
50 to 200
Single ended
11.6/3.24
2.7 to 5.25
±0.3
49
SOT23-6
12
1
50 to 200
Differential
2.8
4.5 to 5.5
±1.0
68.5
MSOP-8
12
1
500 to 1000
Differential
16.5
4.5 to 5.5
±.95
69.5
MSOP-8
Product ID
Pin/Function Throughput
Compatible Rate (kSPS)
*Differential input, 200 to 500 kSPS thruput rate forthcoming
05
8208Signal_Path_108
12/12/06
9:40 AM
Page 6
SIGNAL PATH designer
Low-Voltage Current Loop Transmitter
An offset voltage of 1 mV in A1 will cause an error
EQ5
0 . 001V
0 . 040 V
× 100 = 2 . 5 %
Because the ratio of IR2 to IR3 is 1000 to 1, an error
of 2.5% in IR3 results in a 0.0025% error in the
loop current.
The voltage supply requirements for the components in transmitter must be evaluated in order to
determine the minimum operating voltage
required by the transmitter. For this example, a
full-scale sensor input signal of 1.6V is used and
results in a 10 mA per volt scale factor:
EQ6
I L MAX − I L MIN 20 mA − 4 mA 16 mA
=
=
= 10 mA /V
1 . 6V
1 .6V − 0V
V IN MAX − V IN MIN
The minimum voltage required for the transmitter (V3 – V1) is the highest voltage requirement
of the two paths from V3 to V1. Path one is from
V3 to Q1 and R2 to V1. At the maximum loop
current of 20 mA, the voltage drop across R2 is
0.2V (V2) and a collector emitter voltage of about
0.5V to stay out of saturation is a total of 0.7V.
The second path is V2 plus the output voltage of
the voltage regulator and its dropout voltage. The
full-scale sensor input signal of 1.6V requires
about a 1.65V output from the regulator and the
dropout voltage of the voltage regulator is less
then 50 mV. The path has a minimum voltage
requirement of 1.9V (0.2 + 1.65 + 0.05). Note
that the minimum operating voltage of the
LMV951 is 0.9V so the minimum transmitter
voltage could be reduced to about 1.3V by
increasing the scale factor to 18 mA per volt. This
is supported by the voltage regulator, VR, which
can be adjusted down to 1.25V, and with a drop
out voltage of 50 mV, the loop transmitter can
work down to 1.3V. The current loop transmitter
functions by summing three signals: the loop
current (R3), the offset current (R4), and the
sensor (R5).
6
The loop current generates a voltage drop across
resistor R2 such that V1 is negative with respect to
V2 and then fed back through R3.
EQ7
V1=V2-R2(IL)
The 4 to 20 mA current loop uses the offset current
level of 4 mA to represent zero signal input. This is
used as an open loop fault condition since zero
current is a broken wire, transmitter failure, or
another fault. The resistor R4 is connected to the
output of the adjustable low drop-out voltage
regulator to create the 4 mA offset current. Resistor
R4, at 402 kΩ, sets approximately a 4 mA offset
current when the output of the voltage regulator is
1.65V. The variable resistor R6 is used to set the
loop current to 4 mA when the input signal is at
zero volts. This adjustment compensates for error in
the voltage regulator’s output and resistor tolerance
in R4, R5, and R7. The offset can be calibrated to
4 mA by measuring the voltage across RR and
adjusting R6 until the voltage across RR is equal to
0.04V. The value of resistor R4 can be calculated
for other supply voltages by equating the voltages
at the amplifier’s input pins and rearranging to
solve for R4:
EQ8
R4 =
R3 x VOUT
R2 x IR2
R3
The resistor R5 is used to scale the signal input
voltage to the 16 mA span of the loop current, and
in this example, it is assumed the input signal span
is 1.6V. The equation for calculating R5 can be
developed by equating the voltages at the amplifier’s input pins and rearranging to solve for R5.
VIN is the maximum signal input, 1.6V for this
example, and IR2 is the change in output current,
16mA:
EQ9
R5 =
R3 x VIN
R2 x IR2
This equation also indicates that changing the
value of R5 can change the full-scale input voltage.
A low resistance variable resistor could be used in
12/12/06
9:40 AM
Page 7
series with R5 to add a full-scale calibration as
shown in the following schematic (Figure 3).
R5
VIN 95K
R8
10K
R4
402K
3 + 6
1
4 A1
5
2
R3
10 KΩ
Figure 3. Input Calibration
In this example, a silicon temperature sensor is
used as a signal source. The LM94022 is a low
voltage, programmable gain temperature sensor
that can be used to measure temperature from
–50°C to 150°C. The schematic in Figure 2 shows
the LM94022’s gain select pins connected to
ground, or the lowest gain. With this gain, the sensor’s output ranges from 1.299V for a temperature
of –50° C to 0.183V for a temperature of 150°C.
As shown in Figure 1, the current loop transmitter
accounts for only part of the voltage drop in the
loop. The current loop receiver frequently uses a
resistor, RR in Figure 1, to generate a voltage drop
that is used to measure the loop current. The
measurement of the voltage across RR can present
some problems such as high common mode
voltages, due to the loop power supply, as well as
induced voltages from the environment.
To overcome these measurement problems a
differential amplifier, such as the LMP8270, can be
used. The LMP8270 is a high common mode
voltage differential amplifier with a fixed gain
of 20. The gain of 20 also reduces the resistance of
RR, which reduces the loop voltage drop.
Referring to Figure 2, the voltage across resistor RR
is recovered from whatever common mode voltage
exists on the current loop, up to 28V, and is
amplified and drives the input to an Analog-toDigital Converter (ADC). Internal to the
LMP8270 is a differential amplifier with a gain of
10 followed by an amplifier with a gain of two. The
internal connection between the two amplifiers is
signalpath.national.com/designer
brought out to pins 3 and 4. Also internal to the
LMP8270 is a 100 kΩ resistor in series with
the output of the first amplifier. A low pass filter is
easily implemented by connecting a capacitor from
pins 3 and 4 to ground.
Figure 2 shows a 4.096V reference being used
by the ADC, representing the full-scale input. The
differential input voltage to the LMP8270 for a
4.096V output is 4.096/20 = 0.2048V. The value
of RR for a voltage drop of 0.2048V at a current of
20 mA is 0.2048/20 = 10.24Ω. A 10Ω resistor is
used because it is a standard precision value. The
result is an output voltage from A2 of 0.8V to 4.0V
for a loop current of 4 mA to 20 mA.
The current loop transmitter was calibrated using
the end points, 0V and 1.6V, as the input voltages
while measuring the voltage across the RR resistor.
With 0.0V applied to the input the resistor R6 is
adjusted until 40 mV is across RR. With 1.6V on
the input, resistor R8, see Figure 3, is adjusted until
200 mV is across RR. Figure 4 is the measured
transfer function using a calibrated voltage source.
The worst case deviation from a straight line was
–8 µA, which is not observable on the graph in
Figure 4.
Input Voltage (V)
8208Signal_Path_108
1.70
1.60
1.50
1.40
1.30
1.20
1.10
1.00
0.90
0.80
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0.00
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Output Current (mA)
Figure 4. Output Current vs Input Voltage
In summary, by using a selection of components
that function with very low supply voltages a
current loop transmitter can be designed that
operates with as little as 1.3V.
View over 50 design seminars by industry experts. Log onto
www.national.com/onlineseminars
07
8208Signal_Path_108
12/12/06
9:40 AM
Page 8
Design Tools
WEBENCH® Signal Path Designer® Tools
Design, simulate, and optimize amplifier circuits in this FREE online
design and prototyping environment allowing you to:
I
Synthesize an anti-alias filter
I Make trade-offs based on SNR, SFDR, supply voltage
I Simulate real-world operating conditions using SPICE
I Receive samples in 24 hours
webench.national.com
I
WaveVision 4.1 Evaluation Board
Test and evaluate A/D converters with National’s easy-to use
WaveVision 4.1 evaluation board. Each evaluation board comes
complete with USB cable and support software.
Features and benefits:
• USB 2.0 interface to PC
• PC-based data capture
• Easy data capture and evaluation
• Highlighted harmonic and SFDR frequencies
• Easy waveform examination
• Produces and displays FFT plots
• Dynamic performance parameter readout with FFT
• Produces and displays histograms
National Semiconductor
2900 Semiconductor Drive
Santa Clara, CA 95051
1 800 272 9959
PO Box 58090
Santa Clara, CA 95052
signalpath.national.com
send email to:
new.feedback@nsc.com
Don't miss a single issue!
Signal Path Designer® are available:
signalpath.national.com/designer
Also, be sure to check out our
Power Designer! View online today at:
power.national.com/designer
© National Semiconductor Corporation, 2007. National Semiconductor, , LMH, LLP, Signal Path Designer, and WEBENCH are registered trademarks of
National Semiconductor. All other brand or product names are trademarks or registered trademarks of their respective holders.
570088-008
IMPORTANT NOTICE
Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, modifications, enhancements, improvements,
and other changes to its products and services at any time and to discontinue any product or service without notice. Customers should
obtain the latest relevant information before placing orders and should verify that such information is current and complete. All products are
sold subject to TI’s terms and conditions of sale supplied at the time of order acknowledgment.
TI warrants performance of its hardware products to the specifications applicable at the time of sale in accordance with TI’s standard
warranty. Testing and other quality control techniques are used to the extent TI deems necessary to support this warranty. Except where
mandated by government requirements, testing of all parameters of each product is not necessarily performed.
TI assumes no liability for applications assistance or customer product design. Customers are responsible for their products and
applications using TI components. To minimize the risks associated with customer products and applications, customers should provide
TI does not warrant or represent that any license, either express or implied, is granted under any TI patent right, copyright, mask work right,
or other TI intellectual property right relating to any combination, machine, or process in which TI products or services are used. Information
published by TI regarding third-party products or services does not constitute a license from TI to use such products or services or a
warranty or endorsement thereof. Use of such information may require a license from a third party under the patents or other intellectual
property of the third party, or a license from TI under the patents or other intellectual property of TI.
Reproduction of TI information in TI data books or data sheets is permissible only if reproduction is without alteration and is accompanied
by all associated warranties, conditions, limitations, and notices. Reproduction of this information with alteration is an unfair and deceptive
business practice. TI is not responsible or liable for such altered documentation. Information of third parties may be subject to additional
restrictions.
Resale of TI products or services with statements different from or beyond the parameters stated by TI for that product or service voids all
express and any implied warranties for the associated TI product or service and is an unfair and deceptive business practice. TI is not
responsible or liable for any such statements.
TI products are not authorized for use in safety-critical applications (such as life support) where a failure of the TI product would reasonably
be expected to cause severe personal injury or death, unless officers of the parties have executed an agreement specifically governing
such use. Buyers represent that they have all necessary expertise in the safety and regulatory ramifications of their applications, and
acknowledge and agree that they are solely responsible for all legal, regulatory and safety-related requirements concerning their products
and any use of TI products in such safety-critical applications, notwithstanding any applications-related information or support that may be
provided by TI. Further, Buyers must fully indemnify TI and its representatives against any damages arising out of the use of TI products in
such safety-critical applications.
TI products are neither designed nor intended for use in military/aerospace applications or environments unless the TI products are
specifically designated by TI as military-grade or "enhanced plastic." Only products designated by TI as military-grade meet military
specifications. Buyers acknowledge and agree that any such use of TI products which TI has not designated as military-grade is solely at
the Buyer's risk, and that they are solely responsible for compliance with all legal and regulatory requirements in connection with such use.
TI products are neither designed nor intended for use in automotive applications or environments unless the specific TI products are
designated by TI as compliant with ISO/TS 16949 requirements. Buyers acknowledge and agree that, if they use any non-designated
products in automotive applications, TI will not be responsible for any failure to meet such requirements.
Following are URLs where you can obtain information on other Texas Instruments products and application solutions:
Products
Applications
Audio
www.ti.com/audio
Communications and Telecom www.ti.com/communications
Amplifiers
amplifier.ti.com
Computers and Peripherals
www.ti.com/computers
Data Converters
dataconverter.ti.com
Consumer Electronics
www.ti.com/consumer-apps
DLP® Products
www.dlp.com
Energy and Lighting
www.ti.com/energy
DSP
dsp.ti.com
Industrial
www.ti.com/industrial
Clocks and Timers
www.ti.com/clocks
Medical
www.ti.com/medical
Interface
interface.ti.com
Security
www.ti.com/security
Logic
logic.ti.com
Space, Avionics and Defense
www.ti.com/space-avionics-defense
Power Mgmt
power.ti.com
Transportation and Automotive www.ti.com/automotive
Microcontrollers
microcontroller.ti.com
Video and Imaging
RFID
www.ti-rfid.com
OMAP Mobile Processors
www.ti.com/omap
Wireless Connectivity
www.ti.com/wirelessconnectivity