High-precision active isolation type 4-20mA current loop circuit and HART protocol transparent transmission method thereof

Abstract

The invention discloses a high-precision active isolation type 4-20 mA current loop circuit and an HART protocol transparent transmission method thereof, and relates to the technical field of industrial automation signal transmission, the high-precision active isolation type 4-20 mA current loop circuit comprises an input side, an output side and an isolation layer, the input end is connected with the transient protection circuit, the transient protection circuit is connected with the LC pi-type filter circuit, and the transient protection circuit and the LC pi-type filter circuit are respectively connected with the current reverse connection protection circuit. The influence of ground loop interference and common-mode voltage on the system is avoided, the safety of signal transmission is ensured, and the 3000Vrms withstand voltage requirement of an industrial scene is met. The interference of high-frequency noise on HART signals and 4-20mA analog signals is suppressed by optimizing the filter circuit and the EM I optimization layout.

Description

Technical Field

[0001] The present invention relates to the technical field of industrial automation signal transmission, and in particular to a high-precision active isolated 4-20mA current loop circuit and a HART protocol transparent transmission method thereof. Background Art

[0002] With the proliferation and widespread adoption of industrial automation products, electronic devices, switching power supplies, computers, and programmable controllers (PLCs), while bringing convenience to industrial automation, are also both targets and sources of noise interference. This is particularly true of transient noise, characterized by its rapid rise rate, short duration, large voltage amplitude, and strong randomness. Automation systems also rely on the acquisition, transmission, and conversion of production process information. When both ends of a signal transmission circuit are grounded, a potential difference between the two points creates a ground current loop. This ground current, passing through the resistance of the signal line, becomes an interference voltage, disrupting the measurement circuit. Therefore, in industrial automation control systems, signal isolators are often installed between control rooms, computer rooms, and PLCs, and on-site electronic measuring instruments and sensors, depending on the control method. This filter eliminates external interference, prevents noise from being transmitted to the outside world, and suppresses both series-mode and common-mode interference, thereby improving the anti-interference capabilities of electronic measuring instruments, sensors, and PLCs and enhancing system reliability. To ensure proper signal transmission, when HART-enabled smart instruments are installed on-site, a signal isolator with HART transparent transmission is required. This allows the HART handheld communicator or collector on the system side to communicate with the field instrument through the signal isolator after the smart transmitter is connected to the signal isolator. Signal isolators without HART transparent transmission cannot achieve this. Traditional 4-20mA isolation circuits are incompatible with the transparent transmission of HART high-frequency signals (1.2kHz / 2.2kHz) and the high-precision isolation of 4-20mA, preventing remote parameter configuration and diagnosis of field instruments and resulting in a loss of remote diagnostic functionality. Existing HART transparent transmission solutions suffer from issues such as high communication bit error rates due to signal attenuation, insufficient common-mode interference suppression, and low isolation power supply efficiency. Furthermore, discrete isolation solutions are complex, require multiple power supplies, are bulky, and consume high power. This makes it difficult to meet the requirements of miniaturization and low power consumption. Summary of the Invention

[0003] In view of the above existing problems, the present invention is proposed.

[0004] Therefore, the purpose of the present invention is to provide a high-precision active isolated 4-20mA current loop circuit and its HART protocol transparent transmission method, which solves the problem of industrial automation signal transmission.

[0005] In order to achieve the above object, the present invention provides the following technical solutions:

[0006] A high-precision active isolated 4-20mA current loop circuit, comprising an input side, an output side, and an isolation layer, wherein the input side includes the following:

[0007] The input end is connected to the transient protection circuit, the transient protection circuit is connected to the LCπ type filter circuit, the transient protection circuit and the LCπ type filter circuit are respectively connected to the current reverse protection circuit, the current reverse protection circuit is connected to the RC filter circuit, the RC filter circuit is connected to the current sampling circuit, the current sampling circuit is connected to the constant current drive amplifier circuit, the constant current drive amplifier circuit is connected to the constant current modulation circuit, the constant current modulation circuit is connected to the isolated power distribution output circuit, and the constant current modulation circuit is connected to the transformer coupling isolation circuit.

[0008] Furthermore, the isolation layer includes the following contents:

[0009] The isolation layer HART superimposed 4-20mA signal is coupled and transmitted to the output side through the T2 isolation transformer, and a shared isolated power supply is provided. The shared isolated power supply is a single power input and output dual-channel isolation voltage that independently supplies power to the signal input and output ends, and a shared power winding is used to provide a synchronous modulation and demodulation drive signal to the signal modulation and demodulation circuit.

[0010] Furthermore, the output side includes the following contents:

[0011] A mirror constant current demodulation circuit is obtained by mirroring the constant current demodulation circuit on the input side, and the mirror constant current demodulation circuit is connected to a low-pass filter circuit, the low-pass filter circuit is connected to a band-pass filter circuit, the band-pass filter circuit is connected to a precision voltage reference circuit, the precision voltage reference circuit is connected to a zero-point full-scale feedback compensation circuit, the zero-point full-scale feedback compensation circuit is connected to a constant current output drive circuit, the constant current output drive circuit is connected to an impedance conversion circuit, the impedance conversion circuit is connected to a transient suppression protection circuit, and the transient suppression protection circuit is connected to an LCπ-type filter circuit.

[0012] A HART protocol transparent transmission method for a high-precision active isolated 4-20mA current loop circuit includes the following:

[0013] S1. Modulate the HART digital signal (1200 bps) using frequency shift keying (FSK) technology into a 1.2 kHz sine wave (logic "0") and a 2.2 kHz sine wave (logic "1"). The signal amplitude is controlled within ±0.5 mA to ensure that the superposition does not affect the 4-20 mA baseband signal. The HART modulated high-frequency signal is directly superimposed on the 4-20 mA low-frequency analog current signal to form a composite current signal. The total current is maintained within the 4-20 mA range through the current loop drive circuit to avoid exceeding the limit and causing misjudgment at the receiving end.

[0014] S2, on the input side, converts the 4-20mA DC signal into a high-frequency carrier signal (such as a 100kHz square wave) through a constant current modulation circuit, and drives the HART high-frequency FSK signal through MOSFET chopping. It is directly coupled to the isolation transformer and adopts the mirror current loop FET modulation technology. The input and output side MOSFETs are controlled by synchronous square wave pulses to achieve timing matching of signal chopping and demodulation. The broadband isolation transformer transmission transmits two types of signals synchronously through a miniature high-frequency core transformer (T2): low-frequency path: the 4-20mA modulated carrier signal is transmitted through the low-frequency winding of the transformer and the high-frequency path is coupled through the high-frequency winding;

[0015] S3. Output side signal separation is achieved through low-pass filtering, using an RC low-pass filter with a cutoff frequency of 10Hz to extract the 4-20mA baseband signal and suppress high-frequency noise. Band-pass filtering uses a 2-4kHz band-pass filter to extract the HART FSK signal and filter out low-frequency and ultra-high-frequency interference. When restoring the signal, the high-frequency carrier is restored to a DC current signal through a mirror constant current demodulation circuit, that is, the band-pass filtered signal is FSK demodulated using the HART Modem chip to restore the original digital signal. The MOSFET switches on the input and output sides of the synchronous chopper drive are driven by synchronous square wave pulses generated by the same controller, with frequency matching to ensure phase synchronization between modulation and demodulation, so that the power supply noise and signal noise are in the same frequency band, which is convenient for subsequent filtering and elimination.

[0016] S4. The output side current signal is monitored in real time through the zero-point full-scale feedback compensation circuit, and the signal deviation caused by the line resistance voltage drop (such as the loss of 100-meter transmission wire) and the ambient temperature drift is dynamically compensated. Zero point calibration: When the input side current is 4mA, the output side reference voltage is adjusted through the feedback circuit to eliminate the zero point offset error. Full-scale calibration: When the input side current is 20mA, the gain of the constant current output drive circuit is adjusted to ensure that the full-scale output error is ≤±0.05%.

[0017] Furthermore, the zero-point full-scale feedback compensation circuit calibrates the output side current signal in real time and dynamically compensates for the line resistance voltage drop and environmental temperature drift, wherein: when the input side current is 4mA, the output side reference voltage is adjusted to eliminate the zero point offset; when the input side current is 20mA, the constant current output drive circuit gain is adjusted to make the full-scale error ≤±0.05%; the temperature drift compensation coefficient is <10ppm / ℃, supporting kilometer-level transmission line resistance compensation.

[0018] Furthermore, triple protection is set at the input / output end: TVS tube suppresses surges greater than 10kV / μs, LCπ-type filter cuts off noise above 1MHz, and magnetic coupling isolation blocks ground loop interference; the core circuit is encapsulated with epoxy resin, withstanding vibrations of ≥5g and IP67 protection level; the shared isolated power supply is a full-bridge DC-DC topology, with an input of 3~30V, an output of dual-channel isolated voltage, an efficiency of greater than 85%, and reuses the power winding to generate a synchronous drive signal.

[0019] In the above technical solution, the technical effects and advantages provided by the present invention are:

[0020] 1. The present invention adopts a DC-DC isolated switching power supply integrated controller with a full-bridge topology. The bridge drive mode has fewer transformer windings, high integration and fewer components, small board space, low cost, high power conversion efficiency, and low standby power consumption. It meets the application requirements of 3V to 30V and has strong compatibility. One single power supply input and output dual-channel isolated voltage independently supplies power to the signal input and output ends, with an efficiency of >85%. The shared power supply winding provides a synchronous modulation and demodulation drive signal to the signal modulation and demodulation circuit. Using mixed signal isolation technology, the analog current signal and the HART digital modulation wave are synchronously transmitted in a single isolation channel to avoid the timing mismatch problem caused by discrete channels. The chopping action of the power supply and the signal is synchronized through the same controller to simplify the complexity of system management. Through the unified chopping frequency design, the noise of the power supply and signal paths can be synchronously filtered to avoid mutual interference caused by frequency differences.

[0021] 2. This invention utilizes dual isolation of power and signal: By isolating the power supply (DC-DC module) and signal isolator, conflicts between different power supply circuits are effectively resolved, preventing ground loop interference and the impact of common-mode voltage on the system, ensuring signal transmission security and meeting the 3000Vrms withstand voltage requirement for industrial scenarios. By optimizing the filtering circuit (such as RC low-pass filtering) and EMI optimization layout, high-frequency noise interference on HART signals (1200Hz / 2200Hz FSK modulation) and 4-20mA analog signals is suppressed.

[0022] 3. The present invention adopts seamless compatibility with traditional systems: through transformer coupling technology, HART digital signals (±0.5mA sine wave) are superimposed on the 4-20mA DC signal to achieve the coexistence of analog transmission and digital communication. It is compatible with traditional analog equipment and intelligent digital systems without the need to modify existing wiring.

[0023] 4. The present invention adopts the mirror current loop FET modulation and demodulation transformer coupling isolation technology to always meet the conditions of HART signal superposition on 4-20mA current loop coupling transmission, realize HART signal two-way communication, support remote equipment diagnosis, parameter configuration and calibration, and improve the intelligent management level of industrial site. Bandpass filtering (such as RC network) and transformer coupling technology are used to suppress high-frequency noise interference, maintain the amplitude-frequency characteristics of HART signal, and the bit error rate is as low as 10 -6 . BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments described in the present invention. For ordinary technicians in this field, other drawings can also be obtained based on these drawings.

[0025] Figure 1 It is a principle block diagram of the present invention;

[0026] Figure 2 is a signal channel circuit diagram of the present invention;

[0027] Figure 3 This is a circuit diagram of a DC-DC isolated power supply according to the present invention. DETAILED DESCRIPTION

[0028] In order to enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings.

[0029] The embodiment of the present invention discloses a high-precision active isolated 4-20mA current loop circuit and a HART protocol transparent transmission method thereof.

[0030] The present invention provides Figure 1-3 A high-precision active isolated 4-20mA current loop circuit and its HART protocol transparent transmission method are shown, comprising an input side, an output side, and an isolation layer. The input side is characterized by including the following:

[0031] The input end is connected to the transient protection circuit, the transient protection circuit is connected to the LCπ type filter circuit, the transient protection circuit and the LCπ type filter circuit are respectively connected to the current reverse protection circuit, the current reverse protection circuit is connected to the RC filter circuit, the RC filter circuit is connected to the current sampling circuit, the current sampling circuit is connected to the constant current drive amplifier circuit, the constant current drive amplifier circuit is connected to the constant current modulation circuit, the constant current modulation circuit is connected to the isolated power distribution output circuit, and the modulation circuit is connected to the transformer coupling isolation circuit, which is similar to the LCπ type filter circuit on the input side and is used to filter out high-frequency noise and interference in the output signal and improve signal quality.

[0032] Wherein, the isolation layer includes the following contents:

[0033] The isolation layer HART superimposed 4-20mA signal is coupled and transmitted to the output side through the T2 isolation transformer, and a shared isolated power supply is provided. The shared isolated power supply is a single power input and output dual-channel isolated voltage that independently supplies power to the signal input and output ends, and a shared power winding is used to provide a synchronous modulation and demodulation drive signal to the signal modulation and demodulation circuit. The superimposed composite current signal is isolated and transmitted through the transformer T2 to ensure electrical isolation between the input and output side circuits.

[0034] The output side includes the following:

[0035] A mirror constant current demodulation circuit is obtained by mirroring the constant current demodulation circuit on the input side, and the mirror constant current demodulation circuit is connected to a low-pass filter circuit, the low-pass filter circuit is connected to a band-pass filter circuit, the band-pass filter circuit is connected to a precision voltage reference circuit, the precision voltage reference circuit is connected to a zero-point full-scale feedback compensation circuit, the zero-point full-scale feedback compensation circuit is connected to a constant current output drive circuit, the constant current output drive circuit is connected to an impedance conversion circuit, the impedance conversion circuit is connected to a transient suppression protection circuit, and the transient suppression protection circuit is connected to an LCπ-type filter circuit.

[0036] A HART protocol transparent transmission method for a high-precision active isolated 4-20mA current loop circuit includes the following:

[0037] S1. Modulate the HART digital signal (1200 bps) using frequency shift keying (FSK) technology into a 1.2 kHz sine wave (logic "0") and a 2.2 kHz sine wave (logic "1"). The signal amplitude is controlled within ±0.5 mA to ensure that the superposition does not affect the 4-20 mA baseband signal. The HART modulated high-frequency signal is directly superimposed on the 4-20 mA low-frequency analog current signal to form a composite current signal. The total current is maintained within the 4-20 mA range through the current loop drive circuit to avoid exceeding the limit and causing misjudgment at the receiving end.

[0038] S2, on the input side, converts the 4-20mA DC signal into a high-frequency carrier signal (such as a 100kHz square wave) through a constant current modulation circuit, and drives the HART high-frequency FSK signal through MOSFET chopping. It is directly coupled to the isolation transformer and adopts the mirror current loop FET modulation technology. The input and output side MOSFETs are controlled by synchronous square wave pulses to achieve timing matching of signal chopping and demodulation. The broadband isolation transformer transmission transmits two types of signals synchronously through a miniature high-frequency core transformer (T2): low-frequency path: the 4-20mA modulated carrier signal is transmitted through the low-frequency winding of the transformer and the high-frequency path is coupled through the high-frequency winding;

[0039] S3. Output side signal separation is achieved through low-pass filtering, using an RC low-pass filter with a cutoff frequency of 10Hz to extract the 4-20mA baseband signal and suppress high-frequency noise. Band-pass filtering uses a 2-4kHz band-pass filter to extract the HART FSK signal and filter out low-frequency and ultra-high-frequency interference. When restoring the signal, the high-frequency carrier is restored to a DC current signal through a mirror constant current demodulation circuit, that is, the band-pass filtered signal is FSK demodulated using the HART Modem chip to restore the original digital signal. The MOSFET switches on the input and output sides of the synchronous chopper drive are driven by synchronous square wave pulses generated by the same controller, with frequency matching to ensure phase synchronization between modulation and demodulation, so that the power supply noise and signal noise are in the same frequency band, which is convenient for subsequent filtering and elimination.

[0040] S4, monitor the output side current signal in real time through the zero-full-scale feedback compensation circuit, dynamically compensate for the signal deviation caused by line resistance voltage drop (such as 100-meter transmission wire loss) and environmental temperature drift, zero point calibration: when the input side current is 4mA, adjust the output side reference voltage through the feedback circuit to eliminate the zero offset error, full-scale calibration: when the input side current is 20mA, adjust the gain of the constant current output drive circuit to ensure that the full-scale output error is ≤±0.05%, use synchronous rectification technology to reduce switching loss, improve power conversion efficiency, add overvoltage and overcurrent protection circuits, and adopt Use protection devices with fast response and precise protection thresholds, adopt redundant design, and adopt redundant configuration for key circuits or components to improve circuit reliability and fault tolerance. Zero point calibration: When the input current is 4mA, the output side reference voltage is adjusted through the feedback circuit to eliminate the zero offset error. When the input current is 20mA, the gain of the constant current output drive circuit is adjusted to ensure that the full-scale output error is ≤±0.05%. By monitoring the circuit temperature in real time, the compensation circuit parameters are adjusted to offset the impact of temperature changes on the output current, ensuring that the temperature drift compensation coefficient is <10ppm / ℃.

[0041] Among them, the zero-point full-scale feedback compensation circuit calibrates the output side current signal in real time and dynamically compensates for the line resistance voltage drop and environmental temperature drift. Among them: when the input side current is 4mA, the output side reference voltage is adjusted to eliminate the zero point offset; when the input side current is 20mA, the constant current output drive circuit gain is adjusted to make the full-scale error ≤±0.05%; the temperature drift compensation coefficient is less than 10ppm / ℃, supporting kilometer-level transmission wire resistance compensation, real-time monitoring of the output side current signal, and dynamic compensation for the signal deviation caused by the line resistance voltage drop and environmental temperature drift.

[0042] The system features triple protection at the input and output terminals: a TVS diode suppresses surges >10kV / μs, an LC π-type filter cuts off noise above 1MHz, and magnetic coupling isolation blocks ground loop interference. The core circuit is epoxy-encapsulated, withstands ≥5g vibration, and maintains IP67 protection. The shared isolated power supply utilizes a full-bridge DC-DC topology, with a 3-30V input and dual isolated output voltages. The power windings are reused to generate a synchronous drive signal. The input circuit filters and conditions the 4-20mA signal, converting the DC component to an AC signal through a modulation circuit. This signal is then fed into transformer T2 along with the HART signal. T2 utilizes a high-frequency core transformer, with optimized primary and secondary winding parameters designed through core material and winding processes. The transformer supports bidirectional transmission of low-frequency (for the 4-20mA modulated AC signal) and high-frequency (for the HART signal) signals, minimizing signal attenuation and ensuring simultaneous transmission of the 4-20mA modulated signal and the HART signal. Magnetic isolation eliminates ground loop interference and improves system stability. The secondary side demodulates the signal, recovering a 4-20mA DC signal and outputting it through a current loop. The HART signal is filtered and directly transmitted to the receiver, maintaining communication integrity. The 4-20mA current signal's constant current transmission characteristics effectively suppress voltage noise interference in industrial environments. The current source's internal resistance approaches infinity, minimizing the impact of wire resistance on accuracy and enabling transmission over hundreds of meters. Transient voltage suppressors (TVS) are integrated at the signal input and output terminals. They withstand common-mode surges of 3000V / μs, protecting against transient interference caused by lightning strikes or equipment startup and shutdown. They also suppress common-mode transients and protect downstream circuitry. An LC π-type filter circuit filters and purifies the input signal, blocking ground loop interference and common-mode noise, preventing signal distortion and improving signal integrity and accuracy. The constant current loop driver utilizes bidirectional symmetrical chopper switching field-effect transistors to form a current loop, modulating the input signal at a chopping frequency matched to the power supply. The constant current driver circuit provides a stable bias current, ensuring that the current amplitude remains stable with load fluctuations during signal transmission. To enable bidirectional communication, the HART digital signal is superimposed on a 4-20mA low-frequency analog signal, sharing the same isolated transmission path. Mirror current loop FET modulation and demodulation transformer-coupled isolation technology is employed. Through a symmetrical current loop design at the input and output ends, lossless signal replication and synchronous transmission are achieved. The high-frequency switching characteristics of the FET simultaneously meet the requirements of power modulation and signal modulation, eliminating the need for separate switching circuits for different isolation scenarios. A unified chopping frequency design allows for simultaneous noise filtering in both the power and signal paths, preventing mutual interference caused by frequency differences. The current signal (4-20mA) inherently has strong electromagnetic interference resistance, and combined with the electrical isolation of the transformer, it blocks ground loop noise and common-mode interference, improving signal integrity. The HART digital signal is modulated using frequency shift keying (FSK), allowing for stable decoding even after isolation, preventing increased bit error rates.At the same time, a high-efficiency DC-DC isolated power supply is integrated to power the input and output sides of the circuit, realizing signal input and signal output, and meeting the power supply 3000VRMS full isolation design requirements.

[0043] The core circuit components are fixed using a potting process to ensure stable electrical connections even in industrial vibration environments.

[0044] Working principle of the present invention:

[0045] The three-terminal isolation architecture independently isolates the input, output, and power ports, blocking common-mode noise coupling paths and ensuring interference cannot be coupled into the signal loop. This ensures 4-20mA DC signal accuracy (≤±0.05%). The mixed-signal isolation mirror current loop technology simultaneously transmits the analog current signal and the HART digital modulation signal within a single isolation channel, eliminating timing mismatches caused by separate channels. The HART digital signal is modulated into a high-frequency waveform of 1.2kHz / 2.2kHz using frequency-shift keying (FSK) and superimposed on the 4-20mA low-frequency analog signal, sharing the same transmission path. A self-wound toroidal micro-transformer is used as the isolation medium, supporting wideband signal transmission (covering both HART high-frequency and analog low-frequency signals) with an isolation withstand voltage of over 3000 to 10,000Vrms. This utilizes the frequency domain differences between the high- and low-frequency signals to enable co-linear transmission without mutual interference. It also blocks ground loop interference and common-mode noise, preventing signal distortion and maintaining HART signal integrity (amplitude ±0.5mA, phase shift <1μs). Dual-signal isolation is achieved through a single transformer, reducing the number of discrete isolation components used in traditional solutions, reducing circuit complexity and space usage. The HART protocol's FSK digital signal (1200bps) and the 4-20mA analog signal are superimposed and transmitted on a shared signal channel without interfering with each other. This prevents signal distortion after isolation and supports two-way communication (such as device diagnostics and parameter configuration) while maintaining the real-time nature of the analog signal to meet process control requirements.

[0046] Low-pass filter design: A low-pass filter with a cutoff frequency of 10Hz is configured in the analog signal path. This 10Hz low-pass filter extracts the 4-20mA analog signal and suppresses high-frequency electromagnetic interference (such as inverter harmonics), preserving the pure 4-20mA baseband signal. A 2-4kHz bandpass filter is designed to extract the HART digital signal and achieve frequency domain separation.

[0047] Common-mode transient suppression: Electromagnetic interference (such as inverter harmonics and lightning surges) in industrial scenarios can easily cause HART communication interruptions or increased bit error rates. A triple isolation protection design (TVS+LC+magnetic coupling) is used at the signal input and output terminals to suppress common-mode transient interference >10kV / μs. The isolation withstand voltage is ≥3kVrms and can withstand common-mode surges of 3000V / μs, preventing transient interference caused by lightning strikes or equipment startup and shutdown. A highly efficient DC-DC isolated power supply is designed: The sensor and host sides are independently powered, and the drive signal output is extracted through the power transformer windings. These are connected to the drive control terminals of the MOSFET modulation and demodulation circuits in the signal isolation circuit. Two sets of synchronous square wave pulse drive signals control the synchronous switching of the MOSFET modulation and demodulation circuits, achieving chopping and inversion of the input signal. The resulting current signal is output through transformer isolation coupling, while preventing power supply ripple from interfering with signal transmission. The power supply utilizes a full-bridge topology, a DC-DC isolated switching power supply with an integrated controller. This bridge drive approach features fewer transformer windings, high integration density, fewer components, a small board footprint, and low cost. It meets 3V to 30V applications and offers strong compatibility. This controller is a transformer driver specifically designed for compact, low-standby-power micropower isolated power supplies. Operating at a near 100% duty cycle, it transfers energy to the secondary side, resulting in high conversion efficiency and excellent dynamic characteristics. The peripherals require only simple input and output filter capacitors, an isolation transformer, and a rectifier circuit. After rectification, only a small output filter capacitor is required to achieve low output voltage ripple. The controller also integrates overcurrent detection and overtemperature protection to prevent damage in abnormal conditions such as output shorts.

[0048] The above description is merely illustrative of certain exemplary embodiments of the present invention. It goes without saying that those skilled in the art will be able to modify the described embodiments in various ways without departing from the spirit and scope of the present invention. Therefore, the above drawings and description are illustrative in nature and should not be construed as limiting the scope of protection of the claims.

Claims

1. A high-precision active isolated 4-20mA current loop circuit, comprising an input side, an output side, and an isolation layer, wherein the input side is characterized by: Including the following contents; The input end is connected to the transient protection circuit, the transient protection circuit is connected to the LCπ type filter circuit, the transient protection circuit and the LCπ type filter circuit are respectively connected to the current reverse protection circuit, the current reverse protection circuit is connected to the RC filter circuit, the RC filter circuit is connected to the current sampling circuit, the current sampling circuit is connected to the constant current drive amplifier circuit, the constant current drive amplifier circuit is connected to the constant current modulation circuit, the constant current modulation circuit is connected to the isolated power distribution output circuit, and at the same time, the constant current modulation circuit is connected to the transformer coupling isolation circuit.

2. The high-precision active isolation 4-20mA current loop circuit according to claim 1, characterized in that: The isolation layer includes the following contents: The isolation layer HART superimposed 4-20mA signal is coupled and transmitted to the output side through the T2 isolation transformer, and a shared isolated power supply is provided. The shared isolated power supply is a single power input and output dual-channel isolated voltage that independently supplies power to the signal input and output ends, and a shared power winding is used to provide a synchronous modulation and demodulation drive signal to the signal modulation and demodulation circuit.

3. The high-precision active isolated 4-20mA current loop circuit according to claim 2, characterized in that: The output side includes the following contents: A mirror constant current demodulation circuit is obtained by mirroring the constant current demodulation circuit on the input side, and the mirror constant current demodulation circuit is connected to a low-pass filter circuit, the low-pass filter circuit is connected to a band-pass filter circuit, the band-pass filter circuit is connected to a precision voltage reference circuit, the precision voltage reference circuit is connected to a zero-point full-scale feedback compensation circuit, the zero-point full-scale feedback compensation circuit is connected to a constant current output drive circuit, the constant current output drive circuit is connected to an impedance conversion circuit, the impedance conversion circuit is connected to a transient suppression protection circuit, and the transient suppression protection circuit is connected to an LCπ-type filter circuit.

4. A HART protocol transparent transmission method for a high-precision active isolated 4-20mA current loop circuit, implemented in a high-precision active isolated 4-20mA current loop circuit as claimed in claim 3, characterized in that: Includes the following: S1. Modulate the HART digital signal (1200 bps) using frequency shift keying (FSK) technology into a 1.2 kHz sine wave (logic "0") and a 2.2 kHz sine wave (logic "1"). Control the signal amplitude within ±0.5 mA to ensure that the superposition does not affect the 4-20 mA baseband signal. Directly superimpose the HART-modulated high-frequency signal on the 4-20 mA low-frequency analog current signal to form a composite current signal. Maintain the total current within the 4-20 mA range through the current loop drive circuit to prevent over-limit current from causing misjudgment at the receiving end. S2, on the input side, converts the 4-20mA DC signal into a high-frequency carrier signal (such as a 100kHz square wave) through a constant current modulation circuit, and drives the HART high-frequency FSK signal through MOSFET chopping. It is directly coupled to the isolation transformer and adopts the mirror current loop FET modulation technology. The input and output side MOSFETs are controlled by synchronous square wave pulses to achieve timing matching of signal chopping and demodulation. The broadband isolation transformer transmission transmits two types of signals synchronously through a miniature high-frequency core transformer (T2): low-frequency path: the 4-20mA modulated carrier signal is transmitted through the low-frequency winding of the transformer and the high-frequency path is coupled through the high-frequency winding; S3. Output side signal separation is achieved through low-pass filtering, using an RC low-pass filter with a cutoff frequency of 10Hz to extract the 4-20mA baseband signal and suppress high-frequency noise. Band-pass filtering uses a 2-4kHz band-pass filter to extract the HART FSK signal and filter out low-frequency and ultra-high-frequency interference. When restoring the signal, the high-frequency carrier is restored to a DC current signal through a mirror constant current demodulation circuit, that is, the band-pass filtered signal is FSK demodulated using the HART Modem chip to restore the original digital signal. The MOSFET switches on the input and output sides of the synchronous chopper drive are driven by synchronous square wave pulses generated by the same controller, with frequency matching to ensure phase synchronization between modulation and demodulation, so that the power supply noise and signal noise are in the same frequency band, which is convenient for subsequent filtering and elimination. S4. The output side current signal is monitored in real time through the zero-point full-scale feedback compensation circuit, and the signal deviation caused by the line resistance voltage drop (such as the loss of 100-meter transmission wire) and the ambient temperature drift is dynamically compensated. Zero point calibration: When the input side current is 4mA, the output side reference voltage is adjusted through the feedback circuit to eliminate the zero point offset error. Full-scale calibration: When the input side current is 20mA, the gain of the constant current output drive circuit is adjusted to ensure that the full-scale output error is ≤±0.05%.

5. The HART protocol transparent transmission method of the high-precision active isolated 4-20mA current loop circuit according to claim 4 is characterized in that: The zero-point full-scale feedback compensation circuit calibrates the output current signal in real time and dynamically compensates for line resistance voltage drop and ambient temperature drift. Specifically, when the input current is 4mA, the output reference voltage is adjusted to eliminate zero-point offset; when the input current is 20mA, the constant current output drive circuit gain is adjusted to make the full-scale error ≤±0.05%. The temperature drift compensation coefficient is less than 10ppm / °C, supporting compensation for kilometer-level transmission line resistance.

6. The HART protocol transparent transmission method of the high-precision active isolated 4-20mA current loop circuit according to claim 4, characterized in that: The input / output ends are provided with triple protection: TVS tubes suppress surges >10kV / μs, LC π-type filters cut off noise above 1MHz, and magnetic coupling isolation blocks ground loop interference; the core circuit is encapsulated with epoxy resin, withstanding vibrations ≥5g and an IP67 protection level; the shared isolated power supply is a full-bridge DC-DC topology, with an input of 3~30V, an output of dual-channel isolated voltage, an efficiency >85%, and a multiplexing of the power supply winding to generate a synchronous drive signal.

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