Fully-isolated data acquisition circuit

Abstract

The utility model relates to the technical field of industrial automation control, in particular to a fully-isolated data acquisition circuit. The data acquisition circuit comprises an isolation power supply module which is used for converting 8-36V DC input voltage into four paths of mutually isolated low-ripple output power supplies and respectively supplying power to a sensor signal conditioning circuit, a microcontroller module, a communication interface module and an analog transmitting module. According to the utility model, through the designed Flyback-PFM / PWM hybrid modulation isolation power supply, four paths of mutually isolated low ripple output (the ripple suppression ratio reaches 60dB at 100kHz) are realized under the wide voltage input condition of 8-36V, and a secondary side feedback network is combined with a Pi-type filter circuit to effectively suppress the interference of high-frequency switching noise on the reference voltage of the sigma-delta ADC. And the common-mode rejection ratio of the signal conditioning module is improved to 120dB.

Description

Technical Field

[0001] This utility model relates to the field of industrial automation control technology, specifically a fully isolated data acquisition circuit. Background Technology

[0002] In industrial automation control systems, the reliability and anti-interference capability of high-precision data acquisition terminals directly determine the overall performance of the closed-loop control system. In existing technologies, data acquisition equipment generally adopts discrete isolation schemes, with its isolation power supply modules often based on a single topology design. This leads to incompatibility between a wide input voltage range and low ripple output. Especially in wide-voltage input scenarios of 8-36V, traditional Flyback architectures are prone to secondary-side voltage fluctuations due to loop response hysteresis, causing a deterioration in the common-mode noise rejection ratio (CMRR) of the sensor signal conditioning module, directly affecting the quantization accuracy of Σ-Δ ADCs. Furthermore, the calibration mechanisms of existing equipment mostly rely on external calibration instruments to complete zero-point and full-scale corrections, requiring frequent disassembly of the equipment to connect to a standard source. This results in long calibration cycles, high operational complexity, and an inability to compensate in real time for transmitter signal offsets caused by temperature drift or device aging. More importantly, the signal interaction between the analog transmitter module and the digital control unit often uses optical isolation, which limits its transmission rate and noise suppression capability, making it difficult to meet the timing synchronization requirements under the high-speed SPI bus. This causes the PID control algorithm to exhibit a lag effect during the closed-loop calibration process, ultimately affecting the steady-state accuracy of the 4-20mA output signal. Utility Model Content

[0003] This disclosure proposes a fully isolated data acquisition circuit, which aims to overcome at least one of the defects in the prior art.

[0004] To achieve the above objectives, the technical solution disclosed in this utility model is as follows:

[0005] According to one aspect of this disclosure, a fully isolated data acquisition circuit is provided, the data acquisition circuit comprising:

[0006] The isolated power supply module is used to convert the 8-36V DC input voltage into four mutually isolated low-ripple output power supplies, which power the sensor signal conditioning circuit, microcontroller module, communication interface module and analog transmitter module respectively.

[0007] The signal conditioning module includes a differential signal input interface, a programmable gain amplifier, and a 32-bit Σ-Δ analog-to-digital converter (ADC). The ADC is connected to the microcontroller module's SPI bus via a capacitive gate isolator.

[0008] The microcontroller module integrates an ARM Cortex M0 core and is used for control signal acquisition, storing calibration parameters, and parsing communication protocols.

[0009] The analog transmitter module includes a 16-bit DAC chip, a mirror current source, and a PID control circuit, used to generate a 4-20mA isolated output current signal.

[0010] The communication interface module includes an RS485 bus interface circuit and a CAN bus interface circuit, and interacts with the microcontroller module through a high-speed capacitor gate isolator.

[0011] Furthermore, the isolated power supply module includes:

[0012] The power control chip U1 supports Flyback topology and secondary side feedback. Its input is connected to the primary winding of the high-frequency transformer T1, and its output is rectified and filtered by Schottky diodes D1-D5.

[0013] The secondary winding of the high-frequency transformer T1 outputs four isolation voltages, providing 9V to the sensor signal conditioning circuit, 3.3V to the microcontroller module, 8V to the communication interface module, and 15V to the analog transmitter module.

[0014] A π-type filter circuit is set at each output terminal. The π-type filter circuit consists of resistor R20, capacitors C36-C40 and inductor L1, which is used to suppress the output ripple to below 40mVp-p.

[0015] Furthermore, the signal conditioning module includes:

[0016] The precision reference voltage source U4 generates a 5V analog power supply AVCC after being amplified by the transistor Q1, and provides common-mode voltage bias for the differential input signal;

[0017] The overcurrent protection circuit includes transistor Q2, resistor R8 and capacitor C12, which cuts off the signal path when the input current exceeds 10mA.

[0018] Furthermore, in the analog transmitter module:

[0019] The 16-bit DAC chip is connected to the SPI bus of the microcontroller module through a capacitor gate isolator U12, and its output is connected to the non-inverting input of the operational amplifier U10.

[0020] The mirror current source includes an operational amplifier U10, transistors Q3-Q4, and a sampling resistor R37. The voltage across the sampling resistor R37 is fed back to the microcontroller module via a 24-bit ADC.

[0021] Furthermore, in the communication interface module:

[0022] The RS485 bus interface includes an isolator NIS8241 and a transceiver U20, and resistors R47-R48 are connected in parallel at the bus end to absorb reflected signals.

[0023] The CAN bus interface includes an isolator NSI8021 and a transceiver U18, and suppresses transient surges through an ESD protection chip U16.

[0024] Furthermore, the microcontroller module integrates a non-volatile memory to store zero-point offset, full-scale error value, and nonlinear correction curve parameters, and corrects the ADC sampling data in real time through an interpolation algorithm.

[0025] Furthermore, the capacitive gate isolator includes a four-channel digital isolation chip NSi8241, which supports an insulation withstand voltage of 5kVrms and a data transmission rate of 150Mbps, and is used to achieve signal isolation of the SPI bus and communication interface module.

[0026] Furthermore, the differential signal input interface uses a shielded twisted-pair connector, and TVS diodes D6-D7 are connected in parallel at the input end to suppress electrostatic pulse interference.

[0027] Furthermore, a low-pass filter network consisting of resistor R35 and capacitor C28 is provided between the mirrored current source and the 16-bit DAC chip to suppress the influence of high-frequency noise on the current output.

[0028] The beneficial effects of this utility model are:

[0029] This invention utilizes a designed Flyback-PFM / PWM hybrid modulation isolated power supply to achieve four mutually isolated low-ripple outputs (ripple rejection ratio of 60dB@100kHz) under a wide input voltage range of 8-36V. Its secondary-side feedback network, combined with a π-type filter circuit, effectively suppresses high-frequency switching noise interference with the Σ-Δ ADC reference voltage, increasing the common-mode rejection ratio of the signal conditioning module to 120dB. A capacitor-gate isolator replaces the traditional optocoupler, constructing a high-speed bidirectional isolated channel for the SPI bus. This ensures synchronous processing of the ADC raw data and DAC control commands by the microcontroller. Combined with a fast PID regulation mechanism using a built-in nonlinear correction algorithm, the steady-state error of the 4-20mA transmitter signal is compressed to within 0.1%FS.

[0030] Furthermore, by integrating a one-click triggered self-calibration program, the output current is fed back to the 24-bit high-precision ADC in real time using a mirror current source, forming a closed-loop calibration circuit. Zero-point drift compensation and full-scale linearity correction can be completed without an external standard source, significantly reducing the complexity of on-site maintenance. This invention is the first to achieve fully isolated collaboration of the three functional domains of sensor signal acquisition, transmission output, and communication control, fundamentally eliminating the impact of ground loop interference and common-mode noise on system accuracy. This allows the data acquisition terminal to maintain an effective resolution of 0.02% even in environments with strong electromagnetic interference. At the same time, it supports seamless networking of multiple devices using the MODBUS / CAN2.0 protocol, meeting the stringent requirements of high reliability and maintenance-free operation in Industry 4.0 scenarios. Attached Figure Description

[0031] Figure 1 This is a flowchart of the fully isolated data acquisition method of this utility model.

[0032] Figure 2 This is a block diagram of the overall hardware structure of this utility model.

[0033] Figure 3 This is the circuit diagram of the isolated power supply of this utility model.

[0034] Figure 4 This is a circuit diagram of the strain gauge sensor signal conditioning circuit of this utility model.

[0035] Figure 5 This is a circuit diagram of the CAN bus and RS485 bus interface of this utility model.

[0036] Figure 6 This is a circuit diagram of the three-wire analog transmitter signal output circuit of this utility model.

[0037] Figure 7 This is a circuit diagram of the MCU, touch keyboard, and OLED display interface of this utility model. Detailed Implementation

[0038] The technical solutions of the present utility model will be clearly and completely described below with reference to the accompanying drawings of the embodiments. Obviously, the described embodiments are only some embodiments of the present utility model, not all embodiments. Based on the embodiments of the present utility model, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of the present utility model.

[0039] In this embodiment of the invention, the terms "exemplary" or "for example" are used to indicate that something is an example, illustration, or description. Any embodiment or design described as "exemplary" or "for example" in this embodiment of the invention should not be construed as being more preferred or advantageous than other embodiments or designs. Specifically, the use of the terms "exemplary" or "for example" is intended to present the relevant concepts in a specific manner.

[0040] The present invention provides the following preferred embodiments:

[0041] Example 1

[0042] To address the degradation of sensor signal-to-noise ratio caused by power supply ripple coupling in industrial data acquisition terminals under wide voltage input scenarios, and the low maintenance efficiency resulting from the reliance on external equipment in traditional calibration processes, this embodiment provides a fully isolated data acquisition method based on a capacitive gate fully isolated architecture. For example... Figure 1 As shown, the method employs a multi-level signal chain collaborative design, achieving high-precision signal acquisition and transmission output through precise power management, high-speed isolated transmission, and closed-loop self-calibration algorithms. Specific steps include:

[0043] S100: Acquires analog signals of non-electrical quantities through sensors and preprocesses the analog signals, including low-pass filtering, common-mode noise suppression, and programmable gain amplification.

[0044] S200: The pre-processed analog signal is transmitted to the analog-to-digital converter module through a capacitor gate isolator. The capacitor gate isolator uses a bidirectional SPI bus protocol to achieve signal isolation transmission.

[0045] S300: After analog-to-digital conversion is completed, the microcontroller performs online self-calibration of the digital signal based on preset calibration parameters. The self-calibration includes zero-point drift compensation, sensitivity correction, and nonlinear error correction.

[0046] S400: The analog transmitter module is controlled by a microcontroller to generate an isolated 4-20mA standard transmitter signal. The current output value of the analog transmitter module is adjusted in real time by a fast PID regulation algorithm and closed-loop control is performed based on the calibration sampling value fed back by the microcontroller.

[0047] S500: Transmits calibrated data and status information to the communication interface module through a high-speed capacitive gate isolator, and selects RS485 bus or CAN bus protocol for data interaction according to external commands. It also supports multi-device networking communication using MODBUS and CAN2.0 protocols.

[0048] Specifically, the non-electrical analog signal output by the sensor first enters the signal conditioning circuit of the differential input structure. The differential input structure is composed of an AD8421 instrumentation amplifier, whose common-mode rejection ratio (CMRR) reaches 120dB at 50Hz, effectively suppressing common-mode interference introduced by the environment. A second-order Butterworth active low-pass filter is used in the signal conditioning circuit, with the cutoff frequency set to 1.5 times the effective bandwidth of the sensor; for example, for a strain gauge sensor, the cutoff frequency is set to 2kHz to filter out high-frequency noise. The programmable gain amplifier (PGA) uses an AD8251 chip, whose gain range can be configured from 1 to 1000 times via SPI commands, dynamically adjusting the amplitude of the sensor output signal to adapt to the input range of the 32-bit Σ-Δ ADC (ADS1263). It is important to understand that the PGA's gain switching response time is less than 10μs, ensuring that the linearity of the signal chain is maintained even during sudden changes in the sensor's measurement range.

[0049] Furthermore, the preprocessed analog signal undergoes analog-to-digital conversion via ADS1263 and is then transmitted to the microcontroller through a capacitive gate isolator (NSi8241). The capacitive gate isolator employs a dual-channel SPI protocol with a transmission rate configured at 20MHz, a transmission delay of less than 5ns, and an isolation withstand voltage of 5kVrms. Understandably, compared to traditional optocoupler isolation schemes, the capacitive gate isolator can achieve bidirectional synchronous data transmission without external driver circuitry, thereby reducing signal chain complexity and improving electromagnetic interference immunity.

[0050] Furthermore, the microcontroller (ARM Cortex-M0 core) executes an online self-calibration procedure based on preset calibration parameters. When zero-point calibration is triggered, the microcontroller sends a 4mA output command to the analog transmitter module via the SPI bus and simultaneously acquires the conversion results of the ADS1263 to calculate the zero-point offset ΔZ. During full-scale calibration, a full-scale load is applied through a built-in weight calibration module (e.g., the HX711's built-in excitation circuit) to obtain the full-scale value FS_act of the ADC output, which is then compared with the theoretical full-scale value FS_ref to calculate the gain error ΔG. Based on ΔZ and ΔG, a piecewise cubic spline interpolation method is used to generate a nonlinear correction curve, and the correction parameters are stored in the microcontroller's Flash memory. It is important to understand that the node spacing of the interpolation algorithm is dynamically adjusted according to the sensor's nonlinearity distribution. For example, dense nodes are set in the 0–20% range where the nonlinearity is high to ensure that the linearity error is less than 0.01%FS across the entire range.

[0051] Furthermore, the 4-20mA output of the analog transmitter module is generated collaboratively by a 16-bit DAC (DAC8563) and a mirror current source. The output voltage of the DAC8563 drives a bipolar transistor (TIP31C) to generate load current via a voltage-to-current conversion circuit composed of an operational amplifier OPA2188. The sampling resistor (R37) of the mirror current source is a low-temperature drift alloy resistor (5ppm / ℃), and its voltage is sampled in real time by a 24-bit ADC (ADS1256) and fed back to the microcontroller. The microcontroller dynamically adjusts the DAC output using an incremental PID algorithm based on the difference between the target current I_target and the feedback current I_fb. Specifically, the proportional coefficient Kp of the PID controller is set segmentally according to the absolute value of the error: when |I_target-I_fb|>1mA, Kp=2.0 to speed up the response; when the error drops below 0.5mA, Kp=0.5 to suppress overshoot. The integration time Ti and the derivative time Td are set to 10ms and 2ms respectively, so that the steady-state error of the output current is less than 0.1%FS.

[0052] Furthermore, the calibrated data and device status information are transmitted to the communication interface module via a high-speed capacitive gate isolator NSi8061. The isolator is configured with a transmission rate of 50Mbps and supports seamless switching between RS485 and CAN bus protocols. The RS485 interface uses an SN65HVD3082E transceiver, with a 120Ω terminating resistor and a 22pF capacitor connected in parallel at its bus end to suppress signal reflection; the CAN interface uses a TJA1050 transceiver and integrates a dedicated ESD protection chip (SRV05-4) to resist ±15kV contact discharge interference. In the communication protocol stack, the MODBUS protocol uses RTU transmission mode with a data frame interval of 3.5 character times; the CAN2.0 protocol is configured with a rate of 500kbps and uses an extended frame format to support multi-node arbitration.

[0053] Furthermore, at the input of the signal conditioning circuit, an overcurrent protection module is constructed using a 2N3904 transistor and a current-limiting resistor R8. When the input current exceeds 10mA, the base voltage of the 2N3904 increases, causing it to conduct and short-circuiting the input signal path to ground, thus preventing damage to subsequent circuits. In the power management module, a precision reference voltage source REF5045 generates a 5V isolated power supply, and its output is connected in series with a π-type filter circuit (R = 10Ω, C = 10μF + 100nF) to suppress the power supply ripple to below 50μVrms. It is important to understand that the cutoff frequency of the π-type filter is set to 1 / 10 of the switching power supply ripple frequency (100kHz) to maximize the attenuation of high-frequency noise.

[0054] The advantage of this embodiment lies in the deep integration of the capacitive gate isolator and the SPI bus, which achieves fully isolated data interaction between the sensor signal chain, transmitter output, and communication interface, fundamentally eliminating common-mode errors caused by ground loop interference. The use of a piecewise PID control algorithm and nonlinear interpolation correction technology enables the steady-state accuracy of the 4-20mA transmitter signal to reach 0.1%FS, and the calibration process requires no external calibration equipment. Furthermore, the wide-voltage input isolated power supply design combined with a π-type filter network allows the system to maintain an ultra-low ripple output of 40mVp-p even under 8-36V input conditions, ensuring that the effective number of bits (ENOB) of the high-resolution ADC is not less than 24 bits.

[0055] Example 2

[0056] This embodiment discloses a fully isolated data acquisition circuit, such as Figure 2 The hardware overall structure diagram shown depicts a system powered by a wide-range DC power supply of 8-36VDC. Utilizing an integrated high-performance PFM / PWM power control chip, a low-ripple four-channel isolated power supply is obtained through a high-frequency transformer and related secondary-side feedback. This power supply supplies the sensor signal conditioning circuit, MCU and keyboard display circuit, communication interface circuit, and analog transmitter signal output module. Data exchange between modules is achieved via high-speed capacitive gate isolators, thus electrically isolating the modules from each other and from the system power supply, significantly improving the system's anti-interference performance. Simultaneously, a self-calibration function is integrated into the analog transmitter signal module. By capturing real-time transmitter output and performing rapid PID adjustment, it achieves fast and accurate calibration at zero point (4mA) and full scale (20mA). The system program integrates self-calibration procedures based on sensitivity and other parameters, enabling rapid sensor calibration. The keyboard uses touch-sensitive keys, and the display is a self-generating OLED display. To broaden the module's applicability, RS485 bus interfaces and CAN interfaces based on MODBUS and CAN2.0 standard protocols are provided.

[0057] Furthermore, such as Figure 3 In the isolated power supply circuit diagram shown, U1 is a highly integrated power control chip suitable for various topologies (Flyback, Boost, Buck) and output voltage feedback methods (SSR, PSR, resistor divider). It features a built-in loop compensation circuit with fast dynamic response, giving the switching power supply high stability and dynamic response. The T1 high-frequency transformer uses an EPC17 magnetic core and employs secondary-side feedback (SSR). It has an input voltage of 8-36V and four isolated outputs that power different hardware modules in the system. Figure 3D4, R1, and C6 form a high-voltage clamping circuit, R2 and R4 provide a high-voltage detection circuit, and R3 is used for current detection. F1 is a resettable fuse, D9 is a transient voltage suppressor diode, and L1 is an EMI filter. These multiple protections ensure that the isolated power supply can operate in harsh environments. D1, D2, D3, and D5 are Schottky diodes, which, together with the subsequent capacitors, implement rectification and filtering functions. U2 is a linear optocoupler, implementing SSR feedback.

[0058] Understandably, in addition to the design of the relevant parameters of the high-frequency transformer, the design of the RC energy absorption circuit of R20 and C36 and the high-voltage ceramic capacitor circuit of C37 to C40 are also very important for the low ripple of this isolated switching power supply. Ultimately, the ripple noise of this isolated power supply is around 40mVp-p, which is lower than most commercially available isolated DC / DC modules.

[0059] Furthermore, the 8VCC of the isolation power supply provides power to the CAN and RS485 interface circuits, the 15V supplies power to the analog transmitter module, the 9VA provides power to the sensor and its signal conditioning circuit, and the 3.3VCC provides power to the main control MCU and keyboard display circuits, achieving electrical isolation between them.

[0060] Furthermore, such as Figure 4 The strain gauge sensor signal conditioning circuit diagram shown provides a precision 5V power supply AVCC through Q1, U4, R11, R16, R18, R19, and related capacitors. U4 is a precision reference power supply RS432, Q1 is a current-amplifying transistor, Q2 and R8 limit current to prevent overcurrent, and U19 is a high-resolution 32-bit Σ-Δ ADC with an integrated PGA and a precision reference power supply with extremely low temperature drift. The sensor's differential signals AI+ and AI- are directly connected to the ADC, amplified by the internal PGA, and then converted from analog to digital by the internal ADC. The converted digital signal is read by the MCU via the SPI interface, coupled by a high-speed, high-performance gate isolator U14. Since this ADC is a typical mixed-signal device, its analog and digital power supplies are separate. To prevent crosstalk between the two power supplies, R40, R41, C60, and C61 perform π-type filtering to obtain the VCC_A and GA digital power supplies, which power the ADC's digital circuitry. To enable rapid sensor calibration, the MCU's internal code provides one-click zero-point calibration and full-scale calibration. The full-scale calibration program offers methods such as direct input of sensor sensitivity calibration and weight calibration.

[0061] Furthermore, such as Figure 5The circuit diagram shown depicts the CAN bus and RS485 bus interfaces. This part of the circuit is powered by the 8VCC of the system isolation power supply, which is regulated by U8 to obtain a stable VCC2 power supply. The CAN interface circuit uses a dual-channel capacitor-gate isolation chip NSI8021, and the RS485 interface uses a four-channel capacitor-gate isolation chip NIS8241. NSi8x products are UL1577 safety certified, support an insulation withstand voltage of 5kVrms, and have a data rate of up to 150Mbps. Compared to ordinary high-speed optocoupler isolation methods, this type of isolator features high integration and high reliability. U18 is a CAN transceiver, achieving dual protection through transient suppression D12 and D13 and a dedicated CAN bus ESD protection chip U16. R42-R45 and C65 provide energy absorption and noise suppression for bus reflection signals.

[0062] The U20 is a TTL to RS485 logic level conversion chip. The RS485 interface is used for data interaction with other smart devices. Pull-up and pull-down resistors R46 and R49 prevent bus logic malfunction due to RS485 bus failure. R47 and R48 provide energy absorption for bus reflected signals. Self-resetting fuses F2 and F3 and transient suppression diodes D14 and D15 prevent bus overcurrent and overvoltage, thereby improving the normal operation of the RS485 bus in harsh environments.

[0063] Furthermore, such as Figure 6 The circuit shown depicts a three-wire analog transmitter signal generating circuit for 4-20mA and a self-calibration module control circuit. An LDO chip U7 generates a regulated 5V voltage to power the NSA2860, which integrates a 16-bit DAC. A π-type filter then provides a 5V digital power supply (VCC3) and GND3 to power the gate isolator U12. U15 is a standard optocoupler; the MCU controls the normally open contact of J1 to achieve current transmitter output calibration and normal output circuit switching. The three-wire current transmitter signal is first generated by the MCU through the gate isolator U12 via an analog SPI interface, controlling the NSA2860 to generate the relevant voltage output. Then, it is generated by a mirror current source composed of U10, Q4, and Q3. When calibration is required, the MCU controls the normally open contact of J1 to close, and after I / V conversion via a precision sampling resistor R37, the signal is sent to the 24-bit analog-to-digital converter inside the NSA2860. Rapid PID adjustment enables fast calibration of the zero-point 4mA and full-scale 20mA.

[0064] For conventional analog output signal transmitters, the calibration of standard transmitter signals typically requires specialized testing instruments such as ammeters, and adjustments to certain components such as potentiometers. This can be inconvenient for customers who have already installed and are using the transmitter but need recalibration. This invention features online automatic calibration of the transmitter signal output. Simply send a relevant command to the MCU, and the 4-20mA output signal can be automatically calibrated via a PID controller, greatly simplifying the calibration process for ordinary customers. No other testing equipment is required throughout the process. After calibration, the MCU sends a control signal to open the contact of the miniature relay J1.

[0065] Furthermore, the isolation between the MCU and NSA2860 abandons the commonly used optocoupler isolation method and adopts the highly integrated and highly reliable four-channel digital isolation chip NSi8241 based on capacitive gate isolation. The NSi824x products have passed UL1577 safety certification, support insulation withstand voltage of 5kVrms, and have a data rate of up to 150Mbps.

[0066] like Figure 7 The diagram shows the MCU, touch keyboard, and OLED display interface circuit. The MCU in the diagram uses the NUC131 series with an ARM-Cortex core, which has abundant internal resources and can operate at a frequency of 50MHz. Its internal FLASH supports in-system programming and can allocate some space for storing system parameters. The display can be a self-emissive OLED display. The keyboard is a touch keyboard. U5 and U6 are touch keyboard driver chips, P2 is the SWD debugging interface, R18 and C54 provide power-on reset operation, and P1 is the OLED display interface.

[0067] Although the present invention has been specifically described above with reference to preferred embodiments, it should be understood that the present invention is not limited to the embodiments described above. Rather, various modifications and variations can be made by those skilled in the art without departing from the essence of the present invention, and such modifications and variations should fall within the scope defined by the appended claims and their equivalents.

Claims

1. A fully isolated data acquisition circuit, characterized by, The data acquisition circuit comprises: An isolated power supply module for converting 8-36V DC input voltage into four mutually isolated low-ripple output power supplies, respectively for a sensor signal conditioning circuit, a microcontroller module, a communication interface module and an analog transmission module; A signal conditioning module comprising a differential signal input interface, a programmable gain amplifier and a 32-bit sigma-delta analog-to-digital converter (ADC), the ADC being connected to the SPI bus of the microcontroller module through a capacitive grid isolator; A microcontroller module integrated with an ARM Cortex M0 core for controlling signal acquisition, storing calibration parameters and analyzing communication protocols; An analog transmission module comprising a 16-bit DAC chip, a mirror current source and a PID adjustment circuit for generating a 4-20mA isolated output current signal; A communication interface module comprising an RS485 bus interface circuit and a CAN bus interface circuit, and interacting with the microcontroller module through a high-speed capacitive grid isolator.

2. The fully isolated data acquisition circuit of claim 1, wherein, The isolated power supply module comprises: A power supply control chip U1 supporting a Flyback topology structure and secondary side feedback, the input end of which is connected to the primary winding of a high-frequency transformer T1, and the output end thereof is rectified and filtered through Schottky diodes D1-D5; The secondary winding of the high-frequency transformer T1 outputs four isolated voltages, respectively providing 9V for the sensor signal conditioning circuit, 3.3V for the microcontroller module, 8V for the communication interface module and 15V for the analog transmission module; A pi-type filter circuit is arranged at each output end, which is composed of a resistor R20, capacitors C36-C40 and an inductor L1, for suppressing the output ripple to below 40mVp-p.

3. The fully isolated data acquisition circuit of claim 1, wherein, The signal conditioning module comprises: A precision reference voltage source U4 generating a 5V analog power supply AVCC after current expansion through a transistor Q1, and providing a common-mode voltage bias for the differential input signal; An overcurrent protection circuit comprising a transistor Q2, a resistor R8 and a capacitor C12, which cuts off the signal path when the input current exceeds 10mA.

4. The fully isolated data acquisition circuit of claim 1, wherein, In the analog transmission module: The 16-bit DAC chip is connected to the SPI bus of the microcontroller module through a capacitive grid isolator U12, and the output end thereof is connected to the non-inverting input end of an operational amplifier U10; The mirror current source comprises an operational amplifier U10, transistors Q3-Q4 and a sampling resistor R37, and the voltage across the sampling resistor R37 is fed back to the microcontroller module through a 24-bit ADC.

5. The fully isolated data acquisition circuit of claim 1, wherein, In the communication interface module: The RS485 bus interface comprises an isolator NIS8241 and a transceiver U20, and a resistor R47-R48 is connected in parallel at the bus end to absorb reflected signals; The CAN bus interface comprises an isolator NSI8021 and a transceiver U18, and an ESD protection chip U16 is used to suppress transient surges.

6. The fully isolated data acquisition circuit of claim 1, wherein, The capacitive grid isolator comprises a four-channel digital isolator chip NSi8241 supporting 5kVrms insulation withstand voltage and 150Mbps data transmission rate, and is used to realize signal isolation of the SPI bus and the communication interface module.

7. The fully isolated data acquisition circuit of claim 1, wherein, The differential signal input interface adopts a shielded twisted pair connector, and TVS diodes D6-D7 are connected in parallel at the input end to suppress electrostatic pulse interference.

8. The fully isolated data acquisition circuit of claim 1, wherein, A low-pass filter network composed of resistor R35 and capacitor C28 is arranged between the mirror current source and the 16-bit DAC chip to suppress the influence of high-frequency noise on current output.

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