简单、可靠、良好抗噪性、长距离以及低成本等特性使Monterey (MAXREFDES15#) (图1)接口非常适合于工业过程控制和远端物体自动化。
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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 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.
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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Figure 1. The Monterey subsystem design block diagram.
|Table 1. Connector Description and Default Jumper Positions|
|J1||Installed||Power pin for Monterey Board. Connect across LOOP+ and LOOP- for power.|
|J3||Installed||Connect RTD across IN+ and IN-.|
|J4||Installed across pins 1 and 2||Using a jumper across pins 1 and 2 of J4 uses the MAX44248. This case is used to gain the input signal.|
|J5||Not 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.|
|J9||Installed||When not installed, this jumper’s subsequent pins can be used to measure the loop-generated current.|
The simplified correspondence between the generated output current and the temperature can be expressed by the following equation below:
IOUT = 16mA[T°C/200°C] + 12mA
where T is the temperature sensed by the RTD and IOUT is the current loop.
The entire 4–20mA loop application consists of:
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
This document describes the operation of the smart sensor enabling the 4–20mA loop process.
The smart sensor transmitter block (see Figure 2) consists of:
The analog front-end combines the following:
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 = 0.8462dV2 - 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 description and step-by-step design guide is discussed in the MAX5216LPT data sheet.
To evaluate the complete 4–20mA loop application, the following is required.
The following step-by-step user guide explains testing the entire application using the
Figure 3. Error change vs. temperature at 12V.
Figure 4. Error change vs. temperature at 24V.
Figure 5. Current limit vs. loop voltage with 1.6kΩ sense resistor.
Figure 6. Current limit vs. temperature with 1.6kΩ sense resistor.