CN217404394U - Protection and measurement dual-purpose high-precision large-dynamic-range alternating current sampling circuit - Google Patents
Protection and measurement dual-purpose high-precision large-dynamic-range alternating current sampling circuit Download PDFInfo
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Abstract
The application provides a protection and big dynamic range's of measurement dual-purpose high accuracy alternating current sampling circuit includes: an A/D converter; a plurality of current transformers; each current transformer occupies two sampling channels of the A/D converter, so that the A/D converter can select sampling values of different sampling channels according to different working states; a plurality of sampling resistors are electrically connected between each current transformer and the A/D converter; the number and resistance values of the sampling resistors are selected according to the transformation ratio characteristic of the selected A/D converter, the highest allowable input voltage and the input dynamic range required by the industry standard. The method and the device can realize high-precision sampling in a large dynamic range on a special mutual inductor for protection, and can realize AC sampling for high-precision metering. The mutual inductor and the sampling resistor are not required to be replaced in the whole process, the amplification factor of an operational amplifier of the preposed conditioning circuit is not required to be adjusted, and the problems that a protection device cannot accurately measure and a metering device cannot accurately measure in a large dynamic range are thoroughly solved.
Description
Technical Field
The application relates to the technical field of alternating current sampling circuits, in particular to a protection and measurement dual-purpose high-precision large-dynamic-range alternating current sampling circuit.
Background
In an electric power system, various relay protection devices, power distribution terminals, intelligent electric meters and the like are provided with Current transformers, the English name of which is a Current transformer and the symbol of which is TA, and the Current transformers are instruments which are composed of closed iron cores and primary and secondary windings and can convert primary large Current into secondary small Current according to the electromagnetic induction principle. The primary winding has fewer turns and is connected in a circuit needing to measure current, and the secondary winding has more turns and is connected in a measuring instrument or a protection loop.
The mutual inductors are divided into a protection special current mutual inductor and a metering (or measuring) special mutual inductor, the protection special current mutual inductor is not easy to saturate, the dynamic range is large, but the precision is low, and the measuring special mutual inductor is easy to saturate, and the dynamic range is small. Due to the special characteristic of the transformer and the saturation problem of the A/D sampling circuits such as the operational amplifier in the AC sampling circuit, the relay protection device can only be designed and installed with a special current transformer for protection, and other equipment devices needing accurate metering must be designed and installed with a special transformer for measurement.
Therefore, the current relay protection device cannot realize accurate measurement, and the metering device is easy to saturate and cannot realize large dynamic range measurement, so that the relay protection function cannot be realized.
SUMMERY OF THE UTILITY MODEL
In view of the above-mentioned shortcomings of the prior art, the technical problem to be solved by the present application is to provide a dual-purpose high-precision large-dynamic-range ac sampling circuit for protection and measurement, which is used to solve at least one problem in the prior art.
To achieve the above and other related objects, the present application provides an ac sampling circuit for dual purpose, high precision and large dynamic range protection and measurement, the ac sampling circuit comprising: an A/D converter; a plurality of current transformers; the secondary coil of each current transformer respectively occupies two sampling channels of the A/D converter, so that the A/D converter can select sampling values of different sampling channels according to different working states; a plurality of sampling resistors are electrically connected between the secondary coil of each current transformer and the A/D converter; the number and the resistance value of the sampling resistors are selected according to the transformation ratio characteristic of the selected A/D converter, the highest allowable input voltage and the input dynamic range required by the industry standard.
In an embodiment of the present application, the sampling resistor includes a first sampling resistor and a second sampling resistor; the sampling channels are respectively a first sampling channel and a second sampling channel.
In an embodiment of the present application, a sum of resistance values of the first sampling resistor and the second sampling resistor is not greater than a first reference resistance value obtained by dividing a highest allowable input voltage value of the selected a/D converter by a current value of a transformation ratio characteristic of the selected a/D converter.
In an embodiment of the present application, the resistance R2 of the second sampling resistor is not greater than the second reference resistance obtained by dividing the first reference resistance by the lowest multiple N of the input dynamic range required by the industry standard of the selected a/D converter; and selecting a standard resistor with the resistance value less than or equal to the second reference resistance value from standard resistors available in the market as a second sampling resistor R2.
In an embodiment of the present application, the resistance R1 of the first sampling resistor is: r1 ═ R2 (N-1) -R2; wherein N-1 represents a preset multiplying factor for amplifying the input voltage of the first sampling channel to be close to the lowest multiplying factor in the input dynamic range required by the industry standard of the selected A/D converter so as to improve the sampling precision.
In an embodiment of the present application, during sampling, the a/D converter samples two sampling channels corresponding to each current transformer simultaneously, so as to reduce an error caused by an alternating current input signal that changes at any time.
In an embodiment of the present application, in a normal operating state, the sampling value corresponding to the first sampling channel is selected, and the sampling value corresponding to the second sampling channel is discarded.
In an embodiment of the present application, when a line fails, if a resistance value obtained by the a/D converter is saturated, a sampling value corresponding to the first sampling channel is discarded, and a sampling value corresponding to the second sampling channel is multiplied by the lowest multiple N in an input dynamic range required by the selected a/D converter industry standard to serve as a final sampling value.
In an embodiment of the present application, when the sampled value obtained by the a/D converter is constantly the highest resistance value, it is determined that the first sampling channel is saturated.
In an embodiment of the present application, one end of the secondary coil of the current transformer is electrically connected to one end of the first sampling resistor, one end of the first capacitor, and the anode of the first sampling channel port of the a/D converter, respectively; the other end of the first sampling resistor is respectively and electrically connected with one end of the second sampling resistor, the other end of the first capacitor, one end of the second capacitor and the anode of the second sampling channel port of the A/D converter; and one end of a secondary coil of the current transformer is respectively and electrically connected with the other end of the second sampling resistor, the other end of the second capacitor, the cathode of the first sampling channel port of the A/D converter and the cathode of the second sampling channel port of the A/D converter.
As described above, the present application provides a dual-purpose high-precision large-dynamic-range ac sampling circuit for protection and measurement, which includes: an A/D converter; a plurality of current transformers; the secondary coil of each current transformer respectively occupies two sampling channels of the A/D converter, so that the A/D converter can select sampling values of different sampling channels according to different working states; a plurality of sampling resistors are electrically connected between the secondary coil of each current transformer and the A/D converter; the number and the resistance value of the sampling resistors are selected according to the transformation ratio characteristic of the selected A/D converter, the highest allowable input voltage and the input dynamic range required by the industry standard.
Has the following beneficial effects:
the method and the device can realize large dynamic range high-precision sampling on a special mutual inductor for protection, and realize that a circuit can realize alternating current sampling for large dynamic range protection and alternating current sampling for high-precision metering. The mutual inductor, the sampling resistor and the amplification factor of an operational amplifier of the prepositive conditioning circuit are not required to be replaced or switched in the whole process; the problems that the protection device cannot accurately measure and the metering device cannot accurately measure in a large dynamic range can be thoroughly solved.
Drawings
Fig. 1 is a schematic circuit diagram of an ac sampling circuit of a conventional relay protection device according to an embodiment of the present invention.
Fig. 2 is a circuit diagram of a dual-purpose high-precision large-dynamic-range ac sampling circuit for protection and measurement according to an embodiment of the present invention.
Detailed Description
The following description of the embodiments of the present application is provided by way of specific examples, and other advantages and effects of the present application will be readily apparent to those skilled in the art from the disclosure herein. The present application is capable of other and different embodiments and its several details are capable of modifications and/or changes in various respects, all without departing from the spirit of the present application. It is to be noted that the features in the following embodiments and examples may be combined with each other without conflict.
It should be noted that the drawings provided in the following embodiments are only schematic and illustrate the basic idea of the present application, and although the drawings only show the components related to the present application and are not drawn according to the number, shape and size of the components in actual implementation, the type, quantity and proportion of the components in actual implementation may be changed at will, and the layout of the components may be more complex.
Throughout the specification, when a part is referred to as being "connected" to another part, this includes not only a case of being "directly connected" but also a case of being "indirectly connected" with another element interposed therebetween. In addition, when a certain portion is said to "include" a certain constituent element, unless otherwise specified, it means that other constituent elements may be further included without excluding other constituent elements.
The terms first, second, third, etc. are used herein to describe various elements, components, regions, layers and/or sections, but are not limited thereto. These terms are only used to distinguish one element, component, region, layer or section from another element, component, region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the scope of the present application.
Also, as used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context indicates otherwise. It will be further understood that the terms "comprises," "comprising," and/or "comprising," when used in this specification, specify the presence of stated features, operations, elements, components, items, species, and/or groups, but do not preclude the presence, or addition of one or more other features, operations, elements, components, items, species, and/or groups thereof. The terms "or" and/or "as used herein are to be construed as inclusive or meaning any one or any combination. Thus, "A, B or C" or "A, B and/or C" means "any of the following: a; b; c; a and B; a and C; b and C; A. b and C ". An exception to this definition will occur only when a combination of elements, functions or operations are inherently mutually exclusive in some way.
The existing relay protection device can not realize accurate measurement, and the metering device is easy to saturate and can not realize large dynamic range measurement, so that the relay protection function can not be realized. In order to solve the problems, the protection and measurement dual-purpose high-precision large-dynamic-range alternating current sampling circuit can achieve high-dynamic-range high-precision sampling on a protection special mutual inductor, and meanwhile, the circuit can achieve alternating current sampling for large-dynamic-range protection and alternating current sampling for high-precision metering. The mutual inductor does not need to be replaced in the whole process, the sampling resistor does not need to be switched in a good replacing mode, and the amplification factor of an operational amplifier of the preposed conditioning circuit does not need to be adjusted. The problems that the protection device cannot accurately measure and the metering device cannot accurately measure in a large dynamic range are thoroughly solved.
Fig. 1 shows a schematic circuit diagram of an ac sampling circuit of a conventional relay protection device. Based on fig. 1, the problems of the conventional current transformer will be further explained.
As shown in fig. 1, each of the four current transformers T has an ohmic sampling resistor R, converts the current into a voltage, and inputs the voltage to four a/D conversion channels AIN1-AIN4 corresponding to the a/D converter. To prevent saturation of the a/D converter, the four sampling resistors R are all of relatively small value, e.g., 120 ohms. Because the CPU of the relay protection device is usually a fixed 3.3V working power supply, in order to match the level of the A/D converter with the CPU, the working power supply of the A/D converter can only be 3.3V, otherwise, for example, selecting 5V can possibly burn the I/O port of the CPU, and the input voltage of the A/D converter can only reach 3V at most.
If the input dynamic range required by the corresponding industry standard of the a/D converter is calculated according to the 10-fold dynamic range, for example, when a power system fails, a rated operating current which exceeds about 10 times or even more flows through the current transformer T, if the current transformer T of 5A/5mA is selected, the rated operating current can only reach 5-fold dynamic range, the a/D converter which exceeds 5-fold dynamic range is saturated and cannot be measured, in order to measure the 10-fold current, the sampling resistance change is usually small, so that the input signal of the a/D converter under the rated operating current is small, and therefore the conversion error of the a/D converter is large, and accurate measurement cannot be performed.
Based on the working characteristics and the defects of the alternating current sampling A/D conversion circuit of the traditional relay protection device, the current A/D sampling circuit is improved based on the circuit structure shown in figure 1 so as to solve the problems.
Fig. 2 is a schematic circuit diagram of a dual-purpose high-precision large-dynamic-range ac sampling circuit for protection and measurement according to an embodiment of the present invention. As shown, the ac sampling circuit includes: an A/D converter U; a plurality of current transformers T; the secondary coil of each current transformer T respectively occupies two sampling channels AIN (including AIN + and AIN-) of the A/D converter U; and a plurality of sampling resistors R are electrically connected between the secondary coil of each current transformer T and the A/D converter U.
In short, the input signal of each current transformer T occupies two sampling channels AIN, and the current of the input signal of each current transformer T and the secondary coil of the voltage transformer are designed with two sampling resistors R connected in series.
It should be noted that, in the present application, a plurality of sampling resistors R are electrically connected between the secondary coil of each current transformer T and the a/D converter U, specifically, the number and the resistance value of the sampling resistors R are selected according to the transformation ratio characteristic of the selected a/D converter U, the maximum allowable input voltage, and the input dynamic range required by the industry standard, and in fig. 2, the number of the sampling resistors electrically connected between the secondary coil of each current transformer T and the a/D converter U is set to be 2 for easy understanding, that is, the sampling resistors R include the first sampling resistor R1 and the second sampling resistor R2. It should be understood that when the ratio-change characteristics of the selected a/D converter U, the maximum allowable input voltage, and the input dynamic range required by the industry standard vary, the number, connection mode, and resistance of the sampling resistors are easily adjusted by those skilled in the art based on the circuit connection of the two sampling resistors R as shown in fig. 2.
The scheme of how to select the number of sampling resistors R and the resistance value according to the transformation ratio characteristic of the selected a/D converter U, the maximum allowable input voltage, and the input dynamic range required by the industry standard in the present application is described in detail below.
In the embodiment of the present application, the ac sampling circuit may specifically refer to fig. 1 as follows:
one end of a secondary coil of the current transformer T is electrically connected with one end of a first sampling resistor R1, one end of a first capacitor C1 and the anode of a first sampling channel AIN1 port of the A/D converter U respectively; the other end of the first sampling resistor R1 is respectively and electrically connected with one end of a second sampling resistor R2, the other end of a first capacitor C1, one end of a second capacitor C2 and the anode of a second sampling channel AIN2 port of the A/D converter U; one end of a secondary coil of the current transformer T is respectively and electrically connected with the other end of the second sampling resistor R2, the other end of the second capacitor C2, the negative electrode of a first sampling channel AIN1 port of the A/D converter U and the negative electrode of a second sampling channel AIN2 port of the A/D converter U.
Preferably, the capacitance values of the first capacitor C1 and the second capacitor C2 are 0.1 μ F.
In the present application, the secondary coil of each current transformer T occupies two sampling channels AIN of the a/D converter U, respectively, as AIN1+ and AIN 1-in fig. 2 are one sampling channel, and so on. Enabling the A/D converter U to select sampling values of different sampling channels AIN according to different working states; a plurality of sampling resistors R are electrically connected between the secondary coil of each current transformer T and the A/D converter U; the number and the resistance value of the sampling resistors R are selected according to the transformation ratio characteristic of the selected A/D converter U, the highest allowable input voltage and the input dynamic range required by the industry standard.
The input dynamic range required by the industry standard of the a/D converter U means that when a power system fails, a rated operating current which exceeds about N times or more flows through the current transformer T.
Preferably, the sampling resistor R includes a first sampling resistor RR1 and a second sampling resistor R, although the present application is not limited to the number of the sampling resistors R being 2, and the number of the sampling resistors R can be adjusted according to the ratio-change characteristic of the selected a/D converter U, the maximum allowable input voltage, and the input dynamic range required by the industry standard.
Specifically, when the number of the sampling resistors R is 2, the sum of the resistance values of the first sampling resistor R1 and the second sampling resistor R2 is not greater than a first reference resistance value R obtained by dividing the voltage value of the highest allowable input of the selected a/D converter U by the current value of the conversion ratio characteristic of the selected a/D converter U C1 。
For example, assuming that the transformation ratio of the current transformer T is 5A/2mA and the maximum allowable input voltage of the a/D converter U is 3V, the 3V/2mA is 1500 ohms, so that the sum of the resistance values of the first sampling resistor R1 and the second sampling resistor R2 is firstly ensured to be less than 1500.
In an embodiment of the present application, the resistance of the second sampling resistor R2 is not greater than the first reference resistance R C1 Dividing by a second reference resistance value R obtained by the lowest multiple N in the input dynamic range required by the selected A/D converter U industry standard C2 (ii) a Selecting the standard resistor with the resistance value less than or equal to the second reference resistance value R in the market selectable standard resistor C2 Standard of (2)The resistance serves as a second sampling resistor R2.
For another example, since the dynamic range of the input of the a/D converter U is required to be greater than 8 times according to the relevant industry standard, and then 3V/(2mA × 8) is 187.5 ohms, the theoretical value of the second sampling resistor R2 cannot exceed 187.5 ohms at the maximum, since 187.5 is a non-standard resistor, which cannot be purchased and is not easy to purchase, the grade is reduced, and the standard 120 ohms is selected, so that the standard 120 ohms is selected as the second sampling resistor R2.
In an embodiment of the present application, the resistance value of the first sampling resistor R1 is: r1 ═ R2 (N-1) -R2; wherein N-1 represents a preset multiplying factor for amplifying the input voltage of the first sampling channel AIN1 to be close to the lowest multiplying factor N in the input dynamic range required by the industry standard of the selected A/D converter U so as to improve the sampling precision.
For another example, after the second sampling resistor R2 is selected to be 120 ohms, in order to improve the sampling accuracy, the input voltage of the first sampling channel AIN1 needs to be amplified to be as close to the lowest multiple of 8 times in the dynamic range as possible under the rated working condition, so the magnification N-1 is selected to be 7, 120 × 7 is 840 ohms, 840 — 120 is 720 ohms, and therefore the first sampling resistor R2 is 720 ohms.
It should be noted that the above example is performed when it is determined that the transformation ratio of the current transformer T is 5A/2mA and the maximum allowable input voltage of the a/D converter U is 3V. If in a plurality of real-time examples, mutual current sensors T of different manufacturers are replaced, the transformation ratio characteristic is changed, for example, the mutual current sensors T of 5A/5mA are changed, the parameters are recalculated and changed accordingly, for example, the input voltage range of some A/D converters U is +5V and is not 3.0V, or the input dynamic range of the A/D converters U is required to be more than 20 times, the parameters are recalculated and selected according to 20 times. Therefore, the resistance value of the sampling resistor R requires re-calculation modification of the ratio-change characteristics of the specific a/D converter U, the maximum allowable input voltage, and the input dynamic range or saturation limit required by industry standards.
In an embodiment of the present application, during sampling, the a/D converter U samples two sampling channels AIN corresponding to each current transformer T at the same time, so as to reduce an error caused by an alternating current input signal that changes at any time.
For example, taking the input signal IA corresponding to the current transformer T as an example, during sampling, the sampling channels AIN1 and AIN2 sample simultaneously, and it is particularly emphasized that "simultaneously", that is, sampling of AIN1 and AIN2 must be simultaneously sampled, since the input signal is generally an ac signal of 50Hz, which is a signal that changes at any time, if the input signal is not simultaneously sampled, a large error will be brought.
In an embodiment of the present application, when the sampling device is in a normal rated current operating state, the sampling value corresponding to the first sampling channel AIN1 is selected, and the sampling value corresponding to the second sampling channel AIN2 is discarded; when a line has a fault, if the resistance value acquired by the A/D converter U is saturated, discarding the sampling value corresponding to the first sampling channel AIN1, and multiplying the sampling value corresponding to the second sampling channel AIN2 by the lowest multiple N in the input dynamic range required by the industry standard of the selected A/D converter U to be used as a final sampling value. When the sampling value obtained by the a/D converter U is constantly the highest resistance value, the first sampling channel AIN1 is determined to be saturated.
For example, under the normal rated current operation state, assuming that R1+ R2 is 840 ohms, the first sampling channel AIN1 is not saturated and the input signal amplitude is relatively large, the sampling precision is high, the CPU always uses the sampling value of the first sampling channel AIN1 for various subsequent applications, and the sampling value of the second sampling channel AIN2 is discarded.
When a line fails, IA becomes large, so that the first sampling channel AIN1 is saturated, but the second sampling channel AIN2 is not saturated at the time, because the sampling value of the 16-bit A/D converter is constant to be the maximum value 65535 after saturation, and according to the characteristic, the CPU can judge that the sampling value is saturated by acquiring the 65535, abandons the sampling value, and uses the sampling value of the second sampling channel AIN2 to multiply by 7 as the sampling value of the input current IA to participate in subsequent application.
It should be noted that since the sampling resistance multiplying factor for converting the current into the voltage must be smaller than the input dynamic range required by the industry standard of the a/D converter U, such as 8 times, 7 can be selected in this application. Of course, it can be ensured that the input dynamic range of the a/D converter U is larger than 8 times, and it is also possible to take 7.5, 7.9, and so on. Since the 840 ohm resistor is also a standard resistor and is also easily available, 7 is selected in the present application. Otherwise, a slightly larger point, for example 7.5 or a slightly smaller point, for example 6.8, is also possible, as long as a maximum standard resistance of not more than 8 times is selected.
By analogy, the current input channels such as IB, IC and ID which correspond to the other current transformers T2-T4 respectively are also used for sampling according to the scheme, so that the aims of anti-saturation, high-precision and high-speed sampling of all input signals are fulfilled.
To sum up, protection and big dynamic range's of measurement dual purpose high accuracy alternating current sampling circuit, can realize big dynamic range high accuracy sampling on protection special mutual-inductor, realized that a circuit both can be big dynamic range protection with exchanging sampling, can high accuracy measurement again with exchanging sampling. The mutual inductor, the sampling resistor and the amplification factor of an operational amplifier of the preposed conditioning circuit are not required to be replaced or switched in the whole process; the problems that the protection device cannot accurately measure and the metering device cannot accurately measure in a large dynamic range can be thoroughly solved.
The application effectively overcomes various defects in the prior art and has high industrial utilization value.
The above embodiments are merely illustrative of the principles and utilities of the present application and are not intended to limit the application. Any person skilled in the art can modify or change the above-described embodiments without departing from the spirit and scope of the present application. Accordingly, it is intended that all equivalent modifications or changes which can be made by those skilled in the art without departing from the spirit and technical concepts disclosed in the present application shall be covered by the claims of the present application.
Claims (10)
1. A dual-purpose high-precision large-dynamic-range AC sampling circuit for protection and measurement, the AC sampling circuit comprising:
an A/D converter;
a plurality of current transformers;
the secondary coil of each current transformer respectively occupies two sampling channels of the A/D converter, so that the A/D converter can select sampling values of different sampling channels according to different working states;
a plurality of sampling resistors are electrically connected between the secondary coil of each current transformer and the A/D converter; the number and the resistance value of the sampling resistors are selected according to the transformation ratio characteristic of the selected A/D converter, the highest allowable input voltage and the input dynamic range required by the industry standard.
2. The ac sampling circuit of claim 1, wherein the sampling resistor comprises a first sampling resistor and a second sampling resistor; the sampling channels are respectively a first sampling channel and a second sampling channel.
3. The ac sampling circuit according to claim 2, wherein the sum of the resistance values of the first sampling resistor and the second sampling resistor is not greater than a first reference resistance value obtained by dividing a voltage value of a maximum allowable input of the selected a/D converter by a current value of a transformation ratio characteristic of the selected a/D converter.
4. The ac sampling circuit of claim 3, wherein the resistance value R2 of the second sampling resistor is not greater than the second reference resistance value obtained by dividing the first reference resistance value by the lowest multiple N of the input dynamic range required by the industry standard of the selected a/D converter; and selecting a standard resistor with the resistance value less than or equal to the second reference resistance value from standard resistors available in the market as a second sampling resistor R2.
5. The AC sampling circuit of claim 4, wherein the resistance value R1 of the first sampling resistor is:
R1＝R2*(N-1)-R2；
wherein N-1 represents a preset multiplying factor for amplifying the input voltage of the first sampling channel to be close to the lowest multiplying factor in the input dynamic range required by the industry standard of the selected A/D converter so as to improve the sampling precision.
6. The ac sampling circuit of claim 1, wherein during sampling operation, the a/D converter samples two sampling channels corresponding to each current transformer simultaneously to reduce errors caused by the ac input signal varying with time.
7. The ac sampling circuit of claim 2, wherein in a normal operating state, the sampling value corresponding to the first sampling channel is selected, and the sampling value corresponding to the second sampling channel is discarded.
8. The ac sampling circuit of claim 2, wherein when a line fails, if the resistance value obtained by the a/D converter is saturated, the sampling value corresponding to the first sampling channel is discarded, and the sampling value corresponding to the second sampling channel is multiplied by the lowest multiple N in the input dynamic range required by the industry standard of the selected a/D converter to be used as the final sampling value.
9. The ac sampling circuit of claim 8, wherein the first sampling channel is determined to be saturated when the sampled value obtained by the a/D converter is constantly at the highest resistance value.
10. The ac sampling circuit according to claim 2, wherein one end of the secondary winding of the current transformer is electrically connected to one end of the first sampling resistor, one end of the first capacitor, and the anode of the first sampling channel port of the a/D converter, respectively;
the other end of the first sampling resistor is respectively and electrically connected with one end of the second sampling resistor, the other end of the first capacitor, one end of the second capacitor and the anode of the second sampling channel port of the A/D converter;
and one end of a secondary coil of the current transformer is respectively and electrically connected with the other end of the second sampling resistor, the other end of the second capacitor, the cathode of the first sampling channel port of the A/D converter and the cathode of the second sampling channel port of the A/D converter.
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