Two-wire sensor principle

1. A number title

In industry, it is common to measure various non-electrical physical quantities, such as temperature, pressure, speed, and angle. These quantities need to be converted into analog electrical signals before they can be transmitted to control rooms or display devices hundreds of meters away. Such devices that convert physical quantities into electrical signals are called transmitters. The most widely used method in industry is to transmit analog quantities using a 4-20mA current.

The reason for using current signals is that they are less susceptible to interference. Furthermore, the current source has infinite internal resistance, and the resistance of the conductor in series in the loop does not affect accuracy; it can transmit over hundreds of meters on ordinary twisted-pair cable. The upper limit of 20mA is chosen because of explosion-proof requirements: the spark energy caused by switching a 20mA current is insufficient to ignite methane gas. The lower limit is not set to 0mA to detect open circuits: under normal operation, the current will not fall below 4mA; when the transmission line is broken due to a fault, the loop current drops to 0. 2mA is commonly used as the alarm value for open circuits.

Current-type transmitters convert physical quantities into a 4-20mA current output, requiring an external power supply. Typically, a transmitter needs two power lines and two current output lines, totaling four wires, hence the name four-wire transmitter. Alternatively, the current output can share a single wire with the power supply (VCC or GND), saving one wire, resulting in a three-wire transmitter. In fact, you might notice that the 4-20mA current itself can power the transmitter, as shown in Figure 1C.

In a circuit, the transmitter functions as a special load, characterized by its current consumption varying between 4 and 20 mA depending on the sensor output. The display instrument simply needs to be connected in series in the circuit. This type of transmitter requires only two external wires and is therefore called a two-wire transmitter. The lower limit of the industrial current loop standard is 4 mA, ensuring the transmitter receives at least 4 mA of power within its measurement range. This makes the design of a two-wire sensor possible.

In industrial applications, measurement points are typically located on-site, while display or control devices are usually housed in a control room or cabinet. The distance between the two can range from tens to hundreds of meters. For a distance of 100 meters, eliminating two wires translates to a cost reduction of nearly 100 yuan! Therefore, two-wire sensors are undoubtedly the preferred choice in these applications.

2. Structure and Principle of Two-Wire Transmitters

Two-wire transmitters operate on the principle of using a 4-20mA signal to power themselves. If the transmitter's own power consumption exceeds 4mA, it will be impossible to output the lower limit of 4mA. Therefore, it is generally required that the power consumption of a two-wire transmitter (including all circuitry, including the sensor) does not exceed 3.5mA. This is one of the fundamental design principles of two-wire transmitters.

From an overall structural perspective, a two-wire transmitter consists of three main parts: a sensor, a conditioning circuit, and a two-wire V/I converter. The sensor converts physical quantities such as temperature and pressure into electrical parameters. The conditioning circuit amplifies, conditions, and converts the weak or nonlinear electrical signal output by the sensor into a linear voltage output. The two-wire V/I converter controls the overall power consumption current based on the output of the signal conditioning circuit; simultaneously, it obtains and stabilizes the voltage from the loop, supplying power to the conditioning circuit and the sensor.

Besides the V/I conversion circuit, each part of the circuit has its own power consumption current. The core design idea of ​​the two-wire transmitter is to include all currents within the feedback loop of the V/I conversion. As shown in the figure, the sampling resistor Rs is connected in series at the low end of the circuit, and all currents will flow back to the negative terminal of the power supply through Rs. The feedback signal taken from Rs contains the power consumption of all circuits.

In a two-wire transmitter, the total power consumption of all circuits must not exceed 3.5mA; therefore, low power consumption becomes the main design challenge. The principles and design considerations of each circuit section will be analyzed below.

3 Two-wire V/I converter

A V/I converter is a circuit that uses a voltage signal to control the output current. The difference between a two-wire V/I converter and a general V/I converter is that the voltage signal does not directly control the output current, but rather controls the current consumed by the entire circuit itself. Simultaneously, a stable voltage is extracted from the current loop to power the conditioning circuit and the sensor. The attached diagram shows the basic principle of a two-wire V/I converter circuit:

In the diagram, OP1, Q1, R1, R2, and Rs constitute a V/I converter. Analyzing the negative feedback process: If point A is higher than 0V for some reason, the output of op-amp OP1 increases, the voltage across Re increases, and the current through Re increases. This is equivalent to an increase in overall power consumption, and the current through the sampling resistor Rs also increases, causing the voltage at point B to decrease (more negative).

The result is that the voltage at point A is pulled down through R2. Conversely, if point A drops below 0V for some reason, it will be raised back to 0V by negative feedback. In short, the result of negative feedback is a virtual short of op-amp OP1, and the voltage at point A = 0V. The following analyzes the control principle of Vo on the total power consumption:

Assuming the output voltage of the conditioning circuit is Vo, then the current flowing through R1 is...

I1=Vo/R1

Since the op-amp input cannot sink current, I1 flows entirely through R2. Therefore, the voltage at point B...

VB = -I1 * R2 = -Vo * R2 / R1

When R1=R2, we have VB=-Vo

There are only two resistors, Rs and R2, between the power supply negative and the entire transmitter circuit. Therefore, all current flows through Rs and R2. The upper end of R2 is virtual ground (0V), and the upper end of Rs is GND. Therefore, the voltage across R2 and Rs is exactly the same, both equal to VB. This is equivalent to Rs and R2 being connected in parallel as a current sampling resistor. Therefore, the total circuit current is:

Is = Vo/(Rs//R2)

If we take R2 >> Rs, then Is = Vo/Rs

Therefore, in Figure 3, Rs = 100 ohms is selected. When the conditioning circuit outputs 0.4~2V, the total power consumption is 4~20mA.

It doesn't matter if R2 >> Rs, since Rs and R2 are in parallel (Rs // R2), which is a fixed value. Is and Vo are still linearly related, and the error proportionality coefficient can be eliminated during calibration.

Besides the circuit being correct, the circuit also requires two other conditions to function properly: First, its own power consumption must be as low as possible, with the saved current used to power the conditioning circuit and the transmitter. Second, the operational amplifier must be able to operate with a single power supply, meaning that even without a negative power supply, the input terminal can still accept 0V and function normally.

The LM358/324 is the most common and lowest-priced single-supply op-amp, consuming 400uA per op-amp, which is generally acceptable. With a single supply, it operates normally within an input range of -0.3V to Vcc-1.5V. If a precision amplifier such as the OP07 were used instead, it would not work in this circuit because the input voltage cannot be as low as 0V.

R5 and U1 form a reference source, generating a stable 2.5V reference voltage. The LM385 is a low-cost, low-power reference that can operate at 20uA or more. The datasheet shows that its curve is flattest around 100uA, so the current is controlled to be around 100uA through R5. OP2 forms a non-inverting amplifier to amplify the reference voltage and power the conditioning circuit and sensors. Because wide-input-voltage, low-power regulators are scarce and expensive, using the reference amplifier as a regulated power supply is a cost-effective solution.

This part of the circuit can also be made using readily available integrated circuits, such as XTR115/116/105, which offer better accuracy and stability than custom-made circuits and have lower power consumption (meaning more current can be allocated to the conditioning circuit, making the conditioning section easier to design). However, the cost is more than 10 times higher than the above solutions.

4. Design of a two-wire pressure transmitter

The output signals from pressure bridges and load cells are weak, typically in the mV range. These small signals generally require a differential amplifier for initial amplification. Low offset and low temperature drift differential amplifiers are generally chosen. Furthermore, low power consumption is essential in two-wire applications. The AD623 is a commonly used low-power precision differential amplifier, often used in the preamplifier stage of differential outputs. The AD623 has a maximum offset of 200µV and a temperature drift of 1µV/degree, ensuring sufficient accuracy in general pressure transmitter applications.

R0 adds 0.4V to the REF pin (pin 5) of the AD623. With the pressure = 0, adjust R0 to output 4mA, then adjust RG to output 20.00mA to complete the calibration.

When designing the circuit, it's important to note that the pressure bridge sensor is essentially a kiloohm resistor, resulting in relatively high power consumption. Appropriately reducing the excitation voltage of the pressure bridge can decrease the current consumption. However, this also reduces the output amplitude, necessitating an increase in the gain of the AD623. The sensor shown in Figure 6 uses a constant voltage power supply. In practical applications, most semiconductor pressure sensors require a constant current power supply to achieve good temperature characteristics. An operational amplifier can be used to construct a constant current source to provide the excitation.

5. Considerations for stability and security

Industrial environments are harsh and require high reliability, so the design of two-wire transmitters needs to consider certain protection and stability enhancement measures.

1. Power protection

Reverse power connection, overvoltage, and surges are common power supply problems in industry. Reverse power connection is the most common error during equipment installation and wiring. Simply connecting a diode in series at the input port can prevent damage to the circuit when the power supply is reversed. If a full-bridge rectifier is added to the input, the circuit can still operate normally even if the power supply is reversed.

To prevent damage to the transmitter from lightning strikes, electrostatic discharge, surges, and other energy sources, a TVS diode can be installed at the transmitter inlet to absorb instantaneous overvoltage energy. Generally, the TVS voltage should be slightly lower than the op-amp's limiting voltage to provide adequate protection. If a lightning strike is possible, the TVS diode's absorption capacity may be insufficient, and a varistor is necessary; however, leakage current in the varistor itself can introduce some error.

2. Overcurrent protection

During equipment operation, errors such as sensor disconnection or short circuit may occur. Alternatively, the input quantity itself may exceed the range. The transmitter must ensure that the output does not increase indefinitely under any circumstances; otherwise, it may damage the transmitter itself, the power supply, or the remote display instrument.

In the diagram, Rb and Z1 form an overcurrent protection circuit. Regardless of the reason why the output of OP1 exceeds 6.2V (1N4735 is a 6.2V Zener diode), it will be clamped by Z1, and the base of Q1 cannot exceed 6.2V. Therefore, the voltage across Re cannot exceed 6.2 - 0.6 = 5.6V, and thus the total current will not exceed Ue/Re = 5.6V/200 = 28mA.

3. Wide voltage adaptability

Two-wire transmitters generally can adapt to a wide range of voltage variations without affecting accuracy. This makes them suitable for various power supplies and can accommodate large load resistances. The most power-sensitive component is the reference source, which is also the main component determining accuracy. In the diagram on the third floor, the reference is current-limited by R5. When the power supply voltage changes, the current across R5 also changes, significantly affecting the reference's stability. In the attached diagram, a constant current source LM334 is used to power the reference. When the voltage changes over a wide range, the current remains essentially constant, ensuring the reference's stability.

4. Decoupling capacitor

In typical circuit designs, each integrated circuit has a decoupling capacitor at its power supply terminal. When a two-wire transmitter is powered on, the charging of these capacitors can cause a large current instantaneously, potentially damaging remote instruments. Therefore, each decoupling capacitor should generally not exceed 10nF, and the total decoupling capacitance should not exceed 50nF. A 10nF capacitor at the input is necessary to ensure that the circuit does not oscillate under long-line inductive loads.

5. Two-wire V/I converter (with illustration)