Multiplexing output circuit
Through the combination of digital-to-analog conversion module, voltage amplification module and current conversion module, the problem of high cost of communication circuits is solved, and the output of 0-10V voltage and 4-20mA current is achieved, which is suitable for data communication of industrial automation and IoT devices.
Patent Information
- Application Number
- CN202422072034.3
- Authority / Receiving Office
- CN · China
- Patent Type
- Utility models(China)
- Current Assignee / Owner
- Filing Date
- 2024-08-26
- Publication Date
- 2025-08-08
- Estimated Expiration
- 2034-08-26
AI Technical Summary
In existing communication circuits, the 4-20mA current output and 0-10V voltage output mainly rely on expensive foreign chips, resulting in higher costs.
The digital-to-analog conversion module, voltage amplification module and current conversion module are adopted to convert the digital signal into an analog signal through the digital-to-analog conversion module. The voltage amplification module converts the analog voltage into a 0-10V voltage output, and the current conversion module converts the analog voltage into a 4-20mA current output to realize circuit multiplexing.
It reduces the cost of communication circuits and realizes the output of 0-10V voltage and 4-20mA current. It is suitable for sensor signal transmission and actuator control in industrial automation control systems, data communication of building automatic control systems, security monitoring systems and Internet of Things equipment.
Smart Images
Figure CN223207124U_ABST
Abstract
Description
Technical Field
[0001] The utility model relates to the technical field of communication circuits, in particular to a multiplexing output circuit. Background Art
[0002] 4-20mA current output includes 4mA-20mA, 0mA-20mA, and 0mA-24mA, and 0-10V voltage output includes 0V-5V, 0V-10V, ±5V, and ±10V. Currently, the communication methods for achieving 4-20mA current output and 0-10V voltage output mainly rely on foreign chips, which are relatively expensive and lead to high costs. Utility Model Content
[0003] The utility model provides a multiplexing output circuit, aiming to solve the problem of high cost of current communication circuits.
[0004] The utility model provides a multiplexed output circuit, which includes a digital-to-analog conversion module, a voltage amplification module and a current conversion module; the input end of the digital-to-analog conversion module is connected to a control module; the input end of the voltage amplification module is connected to the output end of the digital-to-analog conversion module, and the output end of the voltage amplification module is connected to an external device; the input end of the current conversion module is connected to the output end of the digital-to-analog conversion module, and the output end of the current conversion module is connected to the external device.
[0005] Furthermore, the digital-to-analog conversion module includes a first resistor, a second resistor, a third resistor, a first capacitor, a second capacitor and a third capacitor; one end of the first resistor is connected to the control module, the other end of the first resistor is respectively connected to one end of the second resistor and one end of the first capacitor, the other end of the second resistor is respectively connected to one end of the third resistor and one end of the second capacitor, the other end of the third resistor is respectively connected to the voltage amplification module, the current conversion module and one end of the third capacitor, and the other end of the first capacitor, the other end of the second capacitor and the other end of the third capacitor are all grounded.
[0006] Furthermore, the voltage amplification module includes a voltage follower circuit and a voltage conversion circuit; the input end of the voltage follower circuit is connected to the output end of the digital-to-analog conversion module, the output end of the voltage follower circuit is connected to the input end of the voltage conversion circuit, and the output end of the voltage conversion circuit is connected to the external device.
[0007] Furthermore, the voltage follower circuit includes a first operational amplifier, the non-inverting input terminal of the first operational amplifier is connected to the digital-to-analog conversion module, the inverting input terminal of the first operational amplifier is connected to the output terminal of the first operational amplifier, and the output terminal of the first operational amplifier is connected to the voltage conversion circuit.
[0008] Furthermore, the voltage conversion circuit includes a second operational amplifier and a first switching tube, the non-inverting input terminal of the second operational amplifier is connected to the output terminal of the first operational amplifier, the inverting input terminal of the second operational amplifier is connected to the first electrode of the first switching tube, the output terminal of the second operational amplifier is connected to the controlled terminal of the first switching tube, and the second electrode of the first switching tube is connected to the power supply terminal.
[0009] Furthermore, the current conversion module includes a voltage-to-current circuit and a mirror current source circuit; the input end of the voltage-to-current circuit is connected to the digital-to-analog conversion module, the output end of the voltage-to-current circuit is connected to the input end of the mirror current source circuit, and the output end of the mirror current source circuit is connected to the external device.
[0010] Furthermore, the voltage-to-current circuit includes a third operational amplifier and a second switching tube; the non-inverting input terminal of the third operational amplifier is connected to the digital-to-analog conversion module, the inverting input terminal of the third operational amplifier is connected to the first pole of the second switching tube, the output terminal of the third operational amplifier is connected to the controlled terminal of the second switching tube, and the second pole of the second switching tube is connected to the power supply terminal.
[0011] Furthermore, the mirror current source circuit includes a fourth operational amplifier and a third switching tube; the non-inverting input terminal of the fourth operational amplifier is connected to the voltage-to-current circuit, the inverting input terminal of the fourth operational amplifier is connected to the power supply terminal, the output terminal of the fourth operational amplifier is connected to the controlled terminal of the third switching tube, the first pole of the third switching tube is connected to the power supply terminal, and the second pole of the third switching tube is connected to the external device.
[0012] Furthermore, it also includes an output protection circuit, the input end of the output protection circuit is connected to the voltage amplification module and the current conversion module respectively, and the output end of the output protection circuit is connected to the external device.
[0013] Furthermore, the output protection circuit includes a fourth resistor, a fifth resistor, a sixth resistor, a fourth capacitor, a fifth capacitor and a voltage suppressor; one end of the fourth resistor is respectively connected to the voltage amplification module and the current conversion module, the other end of the fourth resistor is respectively connected to one end of the fifth resistor, one end of the fourth capacitor and one end of the voltage suppressor, the other end of the fifth resistor is respectively connected to one end of the fifth capacitor, one end of the sixth resistor and the external device, and the other end of the fourth capacitor, the other end of the fifth capacitor, the other end of the sixth resistor and the other end of the voltage suppressor are all grounded.
[0014] The multiplexed output circuit disclosed in the present utility model includes a digital-to-analog conversion module, a voltage amplification module and a current conversion module. The digital-to-analog conversion module can convert the digital signal sent by the control module into an analog signal, so as to provide an analog voltage for the voltage amplification module and the current conversion module. The voltage amplification module converts the received analog voltage into a voltage of 0-10V for output, and the current conversion module converts the analog voltage into a current of 4-20mA for output, thereby replacing the function of the chip and reducing costs. BRIEF DESCRIPTION OF THE DRAWINGS
[0015] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.
[0016] Figure 1 1 is a block diagram of a multiplexing output circuit provided in the first embodiment of the present utility model;
[0017] Figure 2 1 is a block diagram of a multiplexing output circuit provided in a second embodiment of the present utility model;
[0018] Figure 3 1 is a block diagram of a multiplexing output circuit provided in a third embodiment of the present utility model;
[0019] Figure 4 This is a circuit diagram of a multiplexed output circuit provided in one embodiment of the present utility model. DETAILED DESCRIPTION
[0020] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.
[0021] It will be understood that when used in this specification and the appended claims, the terms “include” and “comprising” indicate the presence of described features, integers, operations, elements and / or components, but do not preclude the presence or addition of one or more other features, integers, operations, elements, components and / or groups thereof.
[0022] It should also be understood that the terms used in this utility model specification are for the purpose of describing specific embodiments only and are not intended to limit the utility model. As used in this utility model specification and the appended claims, the singular forms "a," "an," and "the" are intended to include the plural forms unless the context clearly indicates otherwise. It should also be further understood that the term "and / or" used in this utility model specification and the appended claims refers to any and all possible combinations of one or more of the associated listed items, including and including these combinations.
[0023] Furthermore, directional terms used in this disclosure, such as "up," "down," "front," "back," "left," "right," "inside," "outside," and "side," refer only to the accompanying drawings and the orientation of the product in use. Therefore, these directional terms are intended to illustrate and facilitate understanding of this disclosure and are not intended to limit this disclosure. Furthermore, in the accompanying drawings, similar or identical structures are denoted by the same reference numerals.
[0024] See also Figures 1 to 4 , Figure 1 1 is a block diagram of a multiplexing output circuit 100 provided in a first embodiment of the present invention; Figure 2 1 is a block diagram of a multiplexing output circuit 100 provided in a second embodiment of the present invention; Figure 3 1 is a block diagram of a multiplexing output circuit 100 provided in a third embodiment of the present invention; Figure 4 1 is a circuit diagram of a multiplexing output circuit 100 provided in one embodiment of the present invention.
[0025] like Figure 1As shown, the multiplexed output circuit 100 includes a digital-to-analog conversion module 10, a voltage amplification module 20 and a current conversion module 30; the input end of the digital-to-analog conversion module 10 is connected to the control module 200; the input end of the voltage amplification module 20 is connected to the output end of the digital-to-analog conversion module 10, and the output end of the voltage amplification module 20 is connected to the external device 300; the input end of the current conversion module 30 is connected to the output end of the digital-to-analog conversion module 10, and the output end of the current conversion module 30 is connected to the external device 300.
[0026] Specifically, 4-20mA current output and 0-10V voltage output are commonly used communication methods in industrial settings, and can be applied to sensor signal transmission and actuator control in industrial automation control systems. For example, various industrial instruments and meters, such as flow meters, pressure gauges, and temperature controllers, typically require 4-20mA or 0-10V signals for data transmission and remote control. Building automation systems and security monitoring systems also rely on 4-20mA and 0-10V signals for inter-device communication and control. Data communication between IoT devices can also use 4-20mA and 0-10V signals.
[0027] The multiplexed output circuit 100 may include a digital-to-analog conversion module 10, a voltage amplification module 20, and a current conversion module 30. The digital-to-analog conversion module 10 is connected to the control module 200 and is configured to receive a control signal from the control module 200. The control signal is generally a digital signal. The digital-to-analog conversion module 10 then converts the digital signal into an analog signal and outputs it. For example, the control module 200 may output a PWM signal to the digital-to-analog conversion module 10. The digital-to-analog conversion module 10 converts the PWM signal into an analog signal, thereby generating a DC voltage. The DC voltage range can be controlled by controlling the duty cycle of the PWM signal; for example, the DC voltage can vary between 0 and 3.3 V.
[0028] The voltage amplification module 20 is connected to the output end of the digital-to-analog conversion module 10 and is used to amplify the DC voltage output by the digital-to-analog conversion module 10 and then output it to the external device 300. For example, the 0-3.3V voltage output by the digital-to-analog conversion module 10 can be converted into a 0-10V voltage and the converted voltage can be output to the external device 300, thereby achieving a 0-10V voltage output.
[0029] The current conversion module 30 is connected to the output terminal of the digital-to-analog conversion module 10 and is used to convert the DC voltage output by the digital-to-analog conversion module 10 into a current and output it to an external device. For example, the 0-3.3V voltage output by the digital-to-analog conversion module 10 can be converted into a current of 4mA to 20mA and the converted current can be output to the external device 300, thereby achieving a current output of 4mA to 20mA.
[0030] In actual use, the digital-to-analog conversion module 10 converts the PWM signal into a DC voltage and outputs it to the current conversion module 30 and the voltage amplification module 20. The current conversion module 30 converts the voltage of 0-3.3V into a current of 4mA~20mA, and the voltage amplification module 20 converts the voltage of 0~3.3V into a voltage of 0-10V. According to the requirements of the external device 300, it can be determined whether the final output is a current of 4mA~20mA or a voltage of 0-10V.
[0031] As a further embodiment, the digital-to-analog conversion module 10 includes a first resistor R1, a second resistor R2, a third resistor R3, a first capacitor C1, a second capacitor C2 and a third capacitor C3; one end of the first resistor R1 is connected to the control module 200, the other end of the first resistor R1 is respectively connected to one end of the second resistor R2 and one end of the first capacitor C1, the other end of the second resistor R2 is respectively connected to one end of the third resistor R3 and one end of the second capacitor C2, the other end of the third resistor R3 is respectively connected to the voltage amplification module 20, the current conversion module 30 and one end of the third capacitor C3, the other end of the first capacitor C1, the other end of the second capacitor C2 and the other end of the third capacitor C3 are all grounded.
[0032] The digital-to-analog conversion module 10 includes a three-stage filtering circuit. The first stage filtering circuit consists of a first resistor R1 and a first capacitor C1; the second stage filtering circuit consists of a second resistor R2 and a second capacitor C2; and the third stage filtering circuit consists of a third resistor R3 and a third capacitor C3. The PWM signal output by the control module 200 is filtered by the three-stage filtering circuit to produce a stable DC power supply, which is used to provide a DC voltage to the voltage amplification module 20 and the current conversion module 30. The output voltage of the resulting DC power supply typically ranges from 0 to 3.3V, and the output voltage can be adjusted by varying the duty cycle of the PWM signal.
[0033] As a further embodiment, the voltage amplification module 20 includes a voltage follower circuit 21 and a voltage conversion circuit 22; the input end of the voltage follower circuit 21 is connected to the output end of the digital-to-analog conversion module 10, the output end of the voltage follower circuit 21 is connected to the input end of the voltage conversion circuit 22, and the output end of the voltage conversion circuit 22 is connected to the external device 300.
[0034] The input of the voltage follower circuit 21 is connected to the digital-to-analog conversion module 10, and its output is connected to the voltage conversion circuit 22. This circuit is used to stabilize the voltage output by the digital-to-analog conversion module 10 to improve output accuracy. The voltage conversion circuit 22 converts the voltage output of the voltage follower circuit 21 into a 0-10V voltage for output, meeting the requirements of the external device 300.
[0035] As a further embodiment, the voltage follower circuit 21 includes a first operational amplifier U1, the non-inverting input terminal of the first operational amplifier U1 is connected to the digital-to-analog conversion module 10, the inverting input terminal of the first operational amplifier U1 is connected to the output terminal of the first operational amplifier U1, and the output terminal of the first operational amplifier U1 is connected to the voltage conversion circuit 22.
[0036] Among them, the first operational amplifier U1 acts as a voltage follower, the non-inverting input terminal of the first operational amplifier U1 is connected to the digital-to-analog conversion module 10, and the output terminal of the first operational amplifier U1 is respectively connected to the inverting input terminal of the first operational amplifier U1 and the voltage conversion circuit 22, which is used to provide a low output impedance and high input impedance buffer, so that the influence of the subsequent circuit on the previous circuit is minimized, thereby ensuring the stability of the output voltage.
[0037] As a further embodiment, the voltage conversion circuit 22 includes a second operational amplifier U2 and a first switch tube Q1, the non-inverting input terminal of the second operational amplifier U2 is connected to the output terminal of the first operational amplifier U1, the inverting input terminal of the second operational amplifier U2 is connected to the first electrode of the first switch tube Q1, the output terminal of the second operational amplifier U2 is connected to the controlled terminal of the first switch tube Q1, and the second electrode of the first switch tube Q1 is connected to the power supply terminal.
[0038] The non-inverting input of the second operational amplifier U2 is connected to the output of the first operational amplifier U1 for receiving a DC voltage. The inverting input of the second operational amplifier U2 is connected to the first terminal of the first switching transistor Q1 for providing a reference voltage, and the second terminal of the first switching transistor Q1 is connected to the power supply terminal. Furthermore, the amplification factor of the second operational amplifier U2 can be adjusted by adjusting external resistors, for example, using the eighth resistor R8 and the ninth resistor R9. The second operational amplifier U2 amplifies the received DC voltage to a voltage between 0 and 10V and outputs it.
[0039] As a further embodiment, the current conversion module 30 includes a voltage-to-current circuit 31 and a mirror current source circuit 32; the input end of the voltage-to-current circuit 31 is connected to the digital-to-analog conversion module 10, the output end of the voltage-to-current circuit 31 is connected to the input end of the mirror current source circuit 32, and the output end of the mirror current source circuit 32 is connected to the external device 300. Furthermore, the voltage-to-current circuit 31 includes a third operational amplifier U3 and a second switch tube Q2; the non-inverting input end of the third operational amplifier U3 is connected to the digital-to-analog conversion module 10, the inverting input end of the third operational amplifier U3 is connected to the first terminal of the second switch tube Q2, the output end of the third operational amplifier U3 is connected to the controlled terminal of the second switch tube Q2, and the second terminal of the second switch tube Q2 is connected to the power supply end. Furthermore, the mirror current source circuit 32 includes a fourth operational amplifier U4 and a third switch tube Q3; the non-inverting input terminal of the fourth operational amplifier U4 is connected to the voltage-to-current circuit 31, the inverting input terminal of the fourth operational amplifier U4 is connected to the power supply terminal, the output terminal of the fourth operational amplifier U4 is connected to the controlled terminal of the third switch tube Q3, the first electrode of the third switch tube Q3 is connected to the power supply terminal, and the second electrode of the third switch tube Q3 is connected to the external device 300.
[0040] The voltage-to-current circuit 31 is connected to the digital-to-analog conversion module 10 and is used to convert the DC voltage output by the digital-to-analog conversion module 10 into a current of 4 mA to 20 mA. The mirror current source circuit 32 is connected to the output of the voltage-to-current circuit 31 and is used to enhance and stabilize the current output by the voltage-to-current circuit 31, ensuring the stability and accuracy of the final output current of 4 mA to 20 mA. The voltage-to-current circuit 31 can specifically include a third operational amplifier U3 and a second switch tube Q2. The non-inverting input terminal of the third operational amplifier U3 is connected to the digital-to-analog conversion module 10, the inverting input terminal is connected to the first electrode of the second switch tube Q2, and the output terminal is connected to the controlled terminal of the second switch tube Q2. The mirror current source circuit 32 can specifically include a fourth operational amplifier U4 and a third switch tube Q3. The non-inverting input terminal of the fourth operational amplifier U4 is connected to the power supply terminal, the output terminal is connected to the controlled terminal of the second switch tube Q2, and the inverting input terminal is connected to the power supply terminal.
[0041] As a further embodiment, the present invention further includes an output protection circuit 40, wherein the input end of the output protection circuit 40 is respectively connected to the voltage amplification module 20 and the current conversion module 30, and the output end of the output protection circuit 40 is connected to the external device 300. Furthermore, the output protection circuit 40 includes a fourth resistor R4, a fifth resistor R5, a sixth resistor R6, a fourth capacitor C4, a fifth capacitor C5, and a voltage suppressor TVS; one end of the fourth resistor R4 is respectively connected to the voltage amplification module 20 and the current conversion module 30, the other end of the fourth resistor R4 is respectively connected to one end of the fifth resistor R5, one end of the fourth capacitor C4, and one end of the voltage suppressor TVS, the other end of the fifth resistor R5 is respectively connected to one end of the fifth capacitor C5, one end of the sixth resistor R6, and the external device 300, and the other end of the fourth capacitor C4, the other end of the fifth capacitor C5, the other end of the sixth resistor R6, and the other end of the voltage suppressor TVS are all grounded.
[0042] The output protection circuit 40 is used to provide circuit filtering protection, overvoltage protection, and false touch protection for the multiplexed output circuit 100. For example, the fourth resistor R4 is an overcurrent protection resistor used to provide overcurrent protection. When the loop current is too large, the fourth resistor R4 burns out to prevent the preceding circuit from burning out. The voltage suppressor TVS can be a TVS tube, which is used to provide overvoltage protection. The fifth resistor R5, the fourth capacitor C4, and the fifth capacitor C5 form a filter circuit for providing filtering to ensure output stability. The sixth resistor R6 is used to adjust the output accuracy.
[0043] The multiplexed output circuit disclosed in the present utility model can convert digital signals into analog signals through a digital-to-analog conversion module to provide a DC voltage. The voltage amplification module can convert the DC voltage into a voltage output of 0-10V. The current conversion module can convert the DC voltage into a current output of 4mA-20mA, thereby realizing circuit multiplexing at a low cost.
[0044] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in the present invention, and such modifications or substitutions are intended to be within the scope of protection of the present invention. Therefore, the scope of protection of the present invention shall be subject to the scope of protection of the claims.
Claims
1. A multiplexing output circuit, characterized in that: include: A digital-to-analog conversion module, wherein an input end of the digital-to-analog conversion module is connected to the control module; a voltage amplifying module, wherein the input end of the voltage amplifying module is connected to the output end of the digital-to-analog conversion module, and the output end of the voltage amplifying module is connected to an external device; A current conversion module, wherein the input end of the current conversion module is connected to the output end of the digital-to-analog conversion module, and the output end of the current conversion module is connected to the external device.
2. The multiplexing output circuit according to claim 1, wherein: The digital-to-analog conversion module includes a first resistor, a second resistor, a third resistor, a first capacitor, a second capacitor, and a third capacitor; One end of the first resistor is connected to the control module, the other end of the first resistor is respectively connected to one end of the second resistor and one end of the first capacitor, the other end of the second resistor is respectively connected to one end of the third resistor and one end of the second capacitor, the other end of the third resistor is respectively connected to the voltage amplification module, the current conversion module and one end of the third capacitor, and the other end of the first capacitor, the other end of the second capacitor and the other end of the third capacitor are all grounded.
3. The multiplexing output circuit according to claim 1, wherein: The voltage amplification module includes a voltage follower circuit and a voltage conversion circuit; The input end of the voltage follower circuit is connected to the output end of the digital-to-analog conversion module, the output end of the voltage follower circuit is connected to the input end of the voltage conversion circuit, and the output end of the voltage conversion circuit is connected to the external device.
4. The multiplexing output circuit according to claim 3, wherein: The voltage follower circuit includes a first operational amplifier, the non-inverting input terminal of the first operational amplifier is connected to the digital-to-analog conversion module, the inverting input terminal of the first operational amplifier is connected to the output terminal of the first operational amplifier, and the output terminal of the first operational amplifier is connected to the voltage conversion circuit.
5. The multiplexing output circuit according to claim 4, wherein: The voltage conversion circuit includes a second operational amplifier and a first switching tube, the non-inverting input terminal of the second operational amplifier is connected to the output terminal of the first operational amplifier, the inverting input terminal of the second operational amplifier is connected to the first electrode of the first switching tube, the output terminal of the second operational amplifier is connected to the controlled terminal of the first switching tube, and the second electrode of the first switching tube is connected to the power supply terminal.
6. The multiplexing output circuit according to claim 1, wherein: The current conversion module includes a voltage-to-current circuit and a mirror current source circuit; The input end of the voltage-to-current circuit is connected to the digital-to-analog conversion module, the output end of the voltage-to-current circuit is connected to the input end of the mirror current source circuit, and the output end of the mirror current source circuit is connected to the external device.
7. The multiplexing output circuit according to claim 6, wherein: The voltage-to-current circuit includes a third operational amplifier and a second switch tube; The non-inverting input terminal of the third operational amplifier is connected to the digital-to-analog conversion module, the inverting input terminal of the third operational amplifier is connected to the first electrode of the second switching tube, the output terminal of the third operational amplifier is connected to the controlled terminal of the second switching tube, and the second electrode of the second switching tube is connected to the power supply terminal.
8. The multiplexing output circuit according to claim 6, wherein: The mirror current source circuit includes a fourth operational amplifier and a third switch tube; The non-inverting input terminal of the fourth operational amplifier is connected to the voltage-to-current circuit, the inverting input terminal of the fourth operational amplifier is connected to the power supply terminal, the output terminal of the fourth operational amplifier is connected to the controlled terminal of the third switching tube, the first terminal of the third switching tube is connected to the power supply terminal, and the second terminal of the third switching tube is connected to the external device.
9. The multiplexing output circuit according to claim 1, wherein: It also includes an output protection circuit, the input end of the output protection circuit is connected to the voltage amplification module and the current conversion module respectively, and the output end of the output protection circuit is connected to the external device.
10. The multiplexing output circuit according to claim 9, wherein: The output protection circuit includes a fourth resistor, a fifth resistor, a sixth resistor, a fourth capacitor, a fifth capacitor and a voltage suppressor; One end of the fourth resistor is respectively connected to the voltage amplification module and the current conversion module, the other end of the fourth resistor is respectively connected to one end of the fifth resistor, one end of the fourth capacitor and one end of the voltage suppressor, the other end of the fifth resistor is respectively connected to one end of the fifth capacitor, one end of the sixth resistor and the external device, and the other end of the fourth capacitor, the other end of the fifth capacitor, the other end of the sixth resistor and the other end of the voltage suppressor are all grounded.