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Keywords:
subsystem reference design, Industrial Automation, 4-20mA Loop Sensor
SUBSYSTEM BOARD 5683
Monterey (MAXREFDES15#): Ultra-Low Power, High-Accuracy 4–
20mA 2-Wire Current-Loop Sensor
Muhammad
Qazi
Carmelo
Morello
Oct 18, 2013
By:
Abstract: The Monterey (MAXREFDES15#) subsystem reference design is a high-accuracy industrial loop powered sensor transmitter that
connects to any standard PT1000 resistance sensor and converts the linearized temperature to a 4–20mA current signal, which is immune
to noise and remains constant over long distances.
A similar version of this article appears on
EDN,
March 13, 2014.
Introduction
The 4–20mA current loop is widely used as an analog
communication interface in industrial applications for transmitting
the data from remote sensors to a programmable logic controller
(PLC) in a central control center over a twisted pair cable. Here,
4mA represents the lowest temperature value, and 20mA
represents the highest measured temperature. There are four main
advantages of the current loop. First, the accuracy of the signal is
not affected by the voltage drop in the loop, as long as the power-
supply voltage is greater than the total voltage drop across the
loop. Secondly, it uses two wires for power as well as data
communication over the entire loop. Thirdly, it is more immune to
More detailed image
(JPG)
noise. And lastly, it is offered at a low cost and easy installation.
The Monterey design is based on a complete Maxim solution which combines ultra-low power, high accuracy, and high precision.
1.
2.
3.
4.
5.
6.
MAX44248 low-noise,. zero-drift op amplifier
MAX11200
high-accuracy, low-power, 24-bit delta-sigma ADC
MAXQ615
low-power microcontroller
MAX5216 low-noise, high-accuracy, low-power 16-bit DAC
MAX9620 low-power op amplifier
MAX15007, wide Input voltage low-power LDO
This loop-powered sensor transmitter is targeted for industrial sensors, industrial automation, and process control, but it can be used in
any application requiring high-accuracy conversion.
Simplicity, reliability, good noise immunity, long distance, and low cost make the Monterey (MAXREFDES15#) (Figure
1)
interface well
suited for industrial process control and automation of remote objects.
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More detailed image
(GIF)
Figure 1. The Monterey subsystem design block diagram.
Features
Ultra-low power
-100°C to +100°C interval
High accuracy
High precision
Simple power supply and wide input range
System current consumption less than 2.1mA
Resistance temperature detector (RTD) PT1000
Resolution 10,000 counts or 0.1%
Accuracy 0.25%
Equipment Required
10V to 40V power supply
High-accuracy current meter
RTD PT1000 sensor or a temperature calibrator
Quick Start
Connect an industrial standard power supply across
LOOP+
and
LOOP-
terminals of J1 to power up the board.
Connect the sensor across J3. The current design of the board is configured for 2-wire RTD application. Minor modifications at the
sensor side can be performed to use a 3/4-wire RTD application.
Jumper Descriptions
Table 1. Connector Description and Default Jumper Positions
Jumper
J1
J2
J3
J4
Default Position
Installed
Installed
Installed
Installed across pins
1 and 2
Connect RTD across IN+ and IN-.
Using a jumper across pins 1 and 2 of J4 uses the
MAX44248.
This case is used to gain the
input signal.
Function
Power pin for Monterey Board. Connect across LOOP+ and LOOP- for power.
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J5
J6
J7
J8
J9
Not installed
Not installed
Installed
Installed
Installed
Using a jumper across pins 2 and 3 of J5 bypass the MAX44248. The input signal range is
high and amplifier is not required.
When not installed, this jumper’s subsequent pins can be used to measure the loop-
generated current.
Test Procedure
Relation Between Current Output (mA) and Temperature (°C)
The simplified correspondence between the generated output current and the temperature can be expressed by the following equation
below:
I
OUT
= 16mA[T°C/200°C] + 12mA
where T is the temperature sensed by the RTD and I
OUT
is the current loop.
If RTD Used
If RTD is used, test it at different temperatures, and observe the associated loop current.
At -100°C, the expected current is 4mA
At -50°C, the expected current is 8mA
At 0°C, the expected current is 12mA
At +50°C, the expected current is 16mA
At +100°C, the expected current is 20mA
If RTD Calibrator Used
If calibrator is used, test by varying the temperature value to change the resistance across the RTD calibrator terminals.
Observe:
At -100°C, the expected current is 4mA
At -50°C, the expected current is 8mA
At 0°C, the expected current is 12mA
At +50°C, the expected current is 16mA
At +100°C the expected current is 20mA
Detailed Description
The entire 4–20mA loop application consists of:
Smart sensor transmitter (process measurement field instrument)
Receiver (PLC)
The MAXREFDES15# contains the sensor analog front-end, the microcontroller, and the transmitter (Figure
2).
The 4–20mA receiver can
be implemented by Santa Fe (MAXREFDES5#) and Campbell (MAXREFDES4#). Information about the Santa Fe and Campbell can be
found at
Santa Fe (MAXREFDES5#)
Campbell (MAXREFDES4#)
This document describes the operation of the smart sensor enabling the 4–20mA loop process.
The Smart Sensor Transmitter
The smart sensor transmitter block (see Figure 2) consists of:
Analog front-end
Microcontroller
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Transmitter
The analog front-end combines the following:
RTD sensor PT1000
Ultra-precision, low-noise, zero-drift operational amplifier (MAX44248)
Ultra-low-power, high-accuracy delta-sigma ADC (MAX11200)
The low-power microcontroller (MAXQ615) is used to implement calibration and linearization.
The sensor used in the Monterey board is a platinum resistance thermometer (PT1000). The entire system offers excellent accuracy of
wide temperature range from -100°C to +100°C. The basic operation of this sensor block is to measure the temperature, which is
subsequently converted into current by the 4–20mA loop current source.
The variation in temperature creates a change in galvanic resistance of the wheatstone bridge, due to change in the resistivity of the RTD
element. As the temperature ranges from -100°C to +100°C, a 150mV differential voltage swing is observed across the wheatstone bridge
to INPUT A and INPUT B of the 50x amplifier.
The gain stage is used to amplify very minor differential voltage arising from the wheat-stone bridge. Jumpers J4 and J5 provide options to
use the amplifier stage or bypass it. When used, the differential voltage is gained 50 times by using the MAX44248 before digitized by the
MAX11200 ADC. A dual amplifier stage (MAX44248) is used as a 50x differential amplifier. The gain can be changed by changing the
resistors to match the desired signal range from the wheatstone bridge, thereby maintaining proper input signal range and output swing
from the amplifier driving the ADC.
Figure 2. Amplifier gain stage sensor block.
The MAXQ615 microcontroller then maps for the voltage represented to the calibrated temperature reading that the PT1000 is expected to
show. The MAXQ615 has a transfer function that compensates for the nonlinear function of the PT1000. The below equation represents
the compensation function:
T = 0.8462dV
2
- 48.6623dV - 0.1519
where T is the temperature and dV is differential voltage
The above information only supports the PT1000 and for this specified wheatstone bridge configuration, sensor front-end component
selection. This mapped information is then sent to the transmitter through the SPI interface for loop-current generation.
The Transmitter Block
The transmitter block description and step-by-step design guide is discussed in the
MAX5216LPT data sheet.
Evaluation of 4–20mA Loop Application
To evaluate the complete 4–20mA loop application, the following is required.
The Monterey Board (MAXREFDES15#), consisting of the smart sensor block and low-power transmitter
An analog front-end receiver (Santa Fe, Campbell, Fresno)
An interface board such as
Munich USB2PMB1#
can be used to facilitate the evaluation of this application.
The following step-by-step user guide explains testing the entire application using the
RTD Fluke 724 calibrator at the sensor end
Monterey Board MAXREFDES15#
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Santa Fe Board MAXREFDES5# AFE as receiver
Munich USB2PMB1# adaptor board
1. Download the most recent version of the Munich USB2PMB1# board software,
PMOD_SPI.ZIP.
Save the software to a temporary
folder and decompress the ZIP file.
2. Connect the USB cable between the Munich USB2PMB1# board and the PC; the USB driver is installed automatically.
3. Connect the Munich USB2PMB1# board’s 2 x 6-pin right-angle connector to the Santa Fe's 2 x 6-pin right-angle header.
4. With the power supply off, connect the positive lead of 12V DC power supply to
LOOP+
on the Monterey board.
5. Connect
LOOP-
on the Monterey board to
AIN2
on the Santa Fe board.
6. Connect
GND2
on the Santa Fe board to the negative lead of the power supply.
7. Connect
RTD
connector of the Fluke 724 to
IN+
of the Monterey board.
8. Connect
COM
connector of the Fluke 724 to
IN-
of the Monterey board.
9. Push the
RTD
button on the Fluke 724 to select PT1000.
10. Turn on the power supply.
11. Open the Munich USB2PMB1# GUI and select the Santa Fe tab.
12. Click on the Scan_Devices tab and make sure that Hardware: Santa Fe Connected is shown on the GUI.
13. Select 0V to +6V input range on the GUI.
14. Select PlotCurrent on the GUI.
15. Push the Measure/Source button on the Fluke 724 to select Source.
16. Adjust the arrow button on the Fluke 724 to set the temperature to -100°C. Observe that the current display on the GUI is 4mA.
17. Similarly, adjust the arrow button on the Fluke 724 to set the temperature to +100°C. Observe that the current display on the GUI is
20mA.
Lab Measurements
Figure 3. Error change vs. temperature at 12V.
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