An LCR measurement circuit

Abstract

The utility model discloses an LCR measurement circuit, which belongs to the technical field of measurement circuits, and comprises an operational amplifier; an operational amplifier feedback resistor group, which is connected between the negative input terminal and the output terminal of the operational amplifier, and comprises a third analog switch and a feedback resistor connected in series; a second analog switch, wherein the common connection point of the second analog switch, the third analog switch and the feedback resistor is a feedback point; and a component to be measured, wherein one end is connected to the second analog switch, and the other end is connected to a signal source through a standard resistor group. By adding a second analog switch, a new feedback point is formed at the connection point between the second analog switch and the operational amplifier feedback resistor group to replace the original feedback point at the operational amplifier, thereby eliminating the influence of the on-resistance of the third analog switch itself on the measurement result.

Description

Technical Field

[0001] The utility model relates to the technical field of measuring circuits, in particular to an LCR measuring circuit. Background Art

[0002] As the most basic components in the circuit field, the true value of resistance, capacitance and inductance plays an important role in the analysis, evaluation and realization of circuit performance. In order to accurately measure the value of resistance, capacitance and inductance in a wide range, relevant technicians have proposed many test methods, among which the most widely used is the self-balancing bridge method to measure LCR. The circuit of this method is designed with multiple different gears to measure resistance, capacitance and inductance of different sizes, and the gear switching is achieved by controlling the on and off of the relay / analog switch.

[0003] When using analog switches: Analog switches cause less signal interference, but their on-resistance can reach hundreds of ohms and is also affected by voltage and temperature. Current measurements show that when using analog switches to measure LCR, the overall performance of the board is better than when using relays for switching, but the existence and impact of the on-resistance RD of the analog switch is still a technical pain point.

[0004] The self-balancing bridge method is the most widely used method in the field of LCR measurement. Its principle is as follows Figure 1 As shown, RG is always equal to RF. According to the "virtual short, virtual open" characteristics of the operational amplifier, the voltage and inflow current at the reverse end of the amplifier can be regarded as 0. Therefore, the currents on RG, Z, and RF can be regarded as equal. By measuring the voltage across the Z of the component to be tested and the voltage across RF (i.e., the amplifier output voltage), the impedance of the Z to be tested can be known. The analysis is as follows:

[0005]

[0006] ,have to:

[0007]

[0008] ; When the device under test is a resistor:

[0009] Z=R

[0010] ; When the device under test is a capacitor:

[0011]

[0012] ; When the device under test is an inductor:

[0013] Z=2*pi*f*L

[0014] In general circuit design, analog switches are connected to circuits in the following ways: Figure 2As shown in the figure, since the analog switch is connected in series in the signal chain, its presence makes the actual feedback resistance of the amplifier:

[0015] RF'=RF+RD

[0016] , where RD is the on-resistance of the analog switch. Since the on-resistance of the analog switch changes with voltage and temperature, the actual feedback resistance RF' is not a fixed value in the calculation. It is difficult for the tester to know the true value of RF' in real time, which introduces errors to the LCR calculation.

[0017] Since the LCR board needs to measure the resistance, capacitance and inductance over a wide range, there are usually multiple gears when designing the circuit, that is, multiple sets of RG and RF values, generally 100R, 1K, 10K, 100K, etc., to measure resistance, capacitance and inductance of different sizes. When testing at a large gear, RF is above 10K, and a change of a few ohms in the internal resistance of the analog switch has little effect on the test results. However, when RF is 100R, a change of 10 ohms in the internal resistance of the analog switch will bring a 10% error to the test results, which is unacceptable to users. Therefore, in order to accurately measure the value of resistance, capacitance and inductance over a wide range, the problem of analog switch on-resistance must be solved. The fundamental reason why the on-resistance of the analog switch affects the test results is that it is connected in series to the main signal link, resulting in uncertainty in the true value of the amplifier feedback resistor. Utility Model Content

[0018] The purpose of this section is to summarize some aspects of the embodiments of the utility model and briefly introduce some preferred embodiments. Some simplifications or omissions may be made in this section and the specification abstract and utility model name of this application to avoid blurring the purpose of this section, specification abstract and utility model name, and such simplifications or omissions cannot be used to limit the scope of the utility model.

[0019] In view of the above problems and / or the problems existing in the prior art, the present utility model is proposed.

[0020] Therefore, the problem to be solved by the present invention is how to solve the problem of uncertainty in the true value of the feedback resistor of the existing amplifier.

[0021] In order to solve the above technical problems, the utility model provides the following technical solutions: an LCR measurement circuit, comprising an operational amplifier; an operational amplifier feedback resistor group, which is connected between the negative input terminal and the output terminal of the operational amplifier, and includes a third analog switch and a feedback resistor connected in series; a second analog switch, and the common connection point of the second analog switch, the third analog switch and the feedback resistor is a feedback point; a component to be measured, one end of which is connected to the second analog switch, and the other end of which is connected to a signal source through a standard resistor group.

[0022] As a preferred solution of the LCR measurement circuit of the utility model, wherein: the standard resistor group includes standard resistors, and the number of the standard resistors is n, where n is an integer ≥1.

[0023] As a preferred solution of the LCR measurement circuit of the utility model, the standard resistor group also includes a first analog switch, the first analog switch is provided with a first switch contact, the first switch contact is provided with n, n is an integer ≥1, and the first switch contact corresponds one-to-one with the standard resistor.

[0024] As a preferred solution of the LCR measurement circuit of the utility model, the component to be measured can be selected from one of resistance, capacitance and inductance.

[0025] As a preferred solution of the LCR measurement circuit of the utility model, wherein: the second analog switch is provided with a second switch contact, and the number of the second switch contacts is n, where n is an integer ≥1.

[0026] As a preferred solution of the LCR measurement circuit of the utility model, wherein: the feedback resistors are provided with n, n is an integer ≥1, the number of the feedback resistors is the same as the number of the standard resistors, one end of the feedback resistor is connected to the second switch contact, and the other end of the feedback resistor is connected to the output end of the operational amplifier.

[0027] As a preferred solution of the LCR measurement circuit of the utility model, wherein: the third analog switch is provided with a third switch contact, the third switch contact is provided with n, n is an integer ≥1, and the third switch contact corresponds to the feedback resistor one by one.

[0028] As a preferred solution of the LCR measurement circuit of the utility model, wherein: the operational amplifier further includes a positive input terminal, and the positive input terminal of the operational amplifier is grounded.

[0029] The beneficial effect of the utility model is that by adding a second analog switch, a new feedback point is formed at the connection between the second analog switch and the operational amplifier feedback resistor group to replace the original feedback point at the operational amplifier, thereby eliminating the influence of the on-resistance of the third analog switch itself on the measurement result. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the technical solutions of the embodiments of the present utility model, the following briefly introduces the drawings required for the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present utility model. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative labor. Among them:

[0031] Figure 1 FIG. 4 is a diagram of an LCR measurement link in the background technology.

[0032] Figure 2 1 is a circuit diagram of an existing LCR measurement circuit in the background technology.

[0033] Figure 3 The utility model is a circuit diagram of the LCR measurement circuit. DETAILED DESCRIPTION

[0034] In order to make the above-mentioned purposes, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are described in detail below in conjunction with the accompanying drawings.

[0035] In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention may also be implemented in other ways different from those described herein, and those skilled in the art may make similar generalizations without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0036] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure or characteristic that may be included in at least one implementation of the present invention. The term "in one embodiment" that appears in different places in this specification does not necessarily refer to the same embodiment, nor is it a separate or selective embodiment that is mutually exclusive with other embodiments.

[0037] Example 1

[0038] Reference Figure 3 , is the first embodiment of the utility model, which provides an LCR measurement circuit.

[0039] Specifically, an operational amplifier 500; an operational amplifier feedback resistor group 400, which is connected between the negative input terminal and the output terminal of the operational amplifier 500, and includes a third analog switch 401 and a feedback resistor RF connected in series; a second analog switch 300, and the common connection point of the second analog switch 300, the third analog switch 401 and the feedback resistor RF is a feedback point 402; a component to be measured Z, one end of which is connected to the second analog switch 300, and the other end of which is connected to the signal source 100 through the standard resistor group 200.

[0040] When in use, by adding a second analog switch 300, a new feedback point 402 at the common connection of the second analog switch 300, the third analog switch 401 and the feedback resistor RF replaces the old feedback point originally at the operational amplifier 500, thereby eliminating the influence of the on-resistance RD of the third analog switch 401 itself on the measurement result.

[0041] Example 2

[0042] See also Figure 3 , which is the second embodiment of the utility model, and this embodiment is based on the previous embodiment.

[0043] Specifically, the standard resistor RG group 200 includes n standard resistors RG, where n is an integer ≥ 1. For example, there are four standard resistors RG, which are connected in parallel. The resistance values ​​of the four standard resistors RG are 100Ω, 1KΩ, 10KΩ and 100KΩ respectively.

[0044] Preferably, the standard resistor RG group 200 also includes a first analog switch 201, and the first analog switch 201 is provided with a first switch contact 201a, and the first switch contact 201a is provided with n, and n is an integer ≥1. For example, there are four first switch contacts 201a, and the first switch contacts 201a are arranged in a one-to-one correspondence with the standard resistors RG, and one end of the four first switch contacts 201a is connected to the signal source 100, and the other end of the four first switch contacts 201a is respectively connected to the four standard resistors RG. The component Z to be measured includes a resistor, a capacitor and an inductor. The second analog switch 300 is provided with a second switch contact 301, and the second switch contact 301 is provided with n, and n is an integer ≥1. For example, there are four second switch contacts 301, and one end of the four second switch contacts 301 is connected to the component Z to be measured.

[0045] Preferably, the operational amplifier feedback resistor RF group includes a feedback resistor RF, one end of the feedback resistor RF is connected to the other end of the second switch contact 301, and the other end of the feedback resistor RF is connected to the output end of the operational amplifier 500. There are n feedback resistors RF, n is an integer ≥ 1, and the number of feedback resistors RF is the same as the number of standard resistors RG. For example, there are four feedback resistors RF, the four feedback resistors RF are connected in parallel, the resistance values ​​of the four feedback resistors RF are 100Ω, 1KΩ, 10KΩ and 100KΩ respectively, and the four feedback resistors RF are connected to the four second switch contacts 301 respectively.

[0046] Preferably, the operational amplifier feedback resistor RF group further includes a third analog switch 401, the third analog switch 401 is provided with a third switch contact 401a, the second switch contact 301 is provided with n, n is an integer ≥ 1, such as the third switch contact 401a is provided with four, the third switch contact 401a corresponds to the feedback resistor RF one by one. The operational amplifier 500 includes a positive input terminal, and the positive input terminal of the operational amplifier 500 is grounded. It is worth noting that the tests of resistance, capacitance and inductance are based on

[0047] The principle is as follows, wherein the component under test Z is the impedance value of the resistance, capacitance and inductance to be measured, and U and I are the corresponding voltage and current respectively; a self-balancing bridge method based on an operational amplifier 500 is used, and the method calculates the values ​​of resistance, capacitance and inductance by measuring the voltage VZ on both sides of the component under test Z and the voltage VOUT on both sides of the amplifier feedback resistor RF, wherein VOUT is used to calculate the current I on the component under test Z;

[0048] According to the characteristic of infinite input impedance of the operational amplifier 500, the input of the third analog switch 401 for controlling the feedback resistor RF is connected to the reverse input terminal, and the output of the third analog switch 401 is connected to the feedback resistor RF and the element to be measured Z, so that its on-resistance RD is not in the feedback loop of the amplifier;

[0049] During the test, select different gears according to the size of the LCR to be tested, and ensure that the standard resistance RG = feedback resistance RF;

[0050] Calculate the value of the LCR to be measured based on the feedback resistor RF and the measured VZ and VOUT.

[0051]

[0052] According to the “virtual short, virtual open”, the voltage at the output end of the third analog switch 401 and the current flowing in can be regarded as 0. Therefore, the current on the standard resistor RG, the feedback resistor RF and the element under test Z are still equal. Formula

[0053]

[0054] In the example, VOUT corresponds to the voltage across the feedback resistor RF, i.e., the output of the operational amplifier 500 , and VZ corresponds to the voltage across the device under test Z, which is no longer affected by the on-resistance RD of the third analog switch 401 .

[0055] In order to facilitate understanding of the technical solution of the utility model, a brief description of its working process is given below:

[0056] The impedance of the operational amplifier 500 is infinite, so the signal will not go in the direction of the operational amplifier 500 when it reaches the connection between the feedback resistor RF and the second switch contact 301, but can only go along the feedback resistor RF. According to the virtual disconnection characteristic of the operational amplifier 500, the current of the third analog switch 401 below is 0, resulting in the output voltage of the third analog switch 401 being 0, so the connection between the feedback resistor RF and the second switch contact 301 becomes a new feedback point 402; the feedback loop is between the output end of the operational amplifier 500 and the new feedback point 402; the four new feedback points 402 correspond to four-way selection;

[0057] The virtual short and virtual open characteristics of the operational amplifier are characteristics that are well known to technicians, but a large input impedance (generally above MΩ) is also an important characteristic of the operational amplifier. Since the on-resistance RD of the third analog switch 401 affects the test, the impedance problem of the operational amplifier is taken into consideration, and the on-resistance RD of the third analog switch 401 and the input impedance of the operational amplifier 500 are put together, and its output is used as the feedback point 402. On the other hand, the LCR test is a multi-directional test, which requires switching the value of the feedback resistor RF. Therefore, it is designed in the form of multiple feedback points 402, and each feedback point 402 corresponds to a different feedback resistor RF. During the test, the same channel of the first analog switch 201, the second analog switch 300 and the third analog switch 401 is turned on.

[0058] It should be noted that the above embodiments are only used to illustrate the technical solution of the utility model rather than to limit it. Although the utility model is described in detail with reference to the preferred embodiments, ordinary technicians in the field should understand that the technical solution of the utility model can be modified or replaced by equivalents without departing from the spirit and scope of the technical solution of the utility model, which should be included in the scope of the claims of the utility model.

Claims

1. An LCR measurement circuit, characterized in that: include, Operational amplifier (500); An operational amplifier feedback resistor group (400), connected between the negative input terminal and the output terminal of the operational amplifier (500), comprising a third analog switch (401) and a feedback resistor (RF) connected in series; A second analog switch (300), wherein a common connection point of the second analog switch (300), the third analog switch (401) and the feedback resistor (RF) is a feedback point (402); The element to be tested (Z) has one end connected to the second analog switch (300) and the other end connected to the signal source (100) via a standard resistor group (200).

2. The LCR measurement circuit according to claim 1, characterized in that: The standard resistor group (200) comprises a standard resistor (RG), wherein n standard resistors (RG) are provided, and n is an integer ≥1.

3. The LCR measurement circuit according to claim 2, characterized in that: The standard resistor group (200) further comprises a first analog switch (201), the first analog switch (201) being provided with a first switch contact (201a), the first switch contact (201a) being provided with n numbers, n being an integer ≥1, and the first switch contacts (201a) corresponding one to one with the standard resistor (RG).

4. The LCR measurement circuit according to claim 1, 2 or 3, characterized in that: The element to be measured (Z) may be selected from one of a resistor, a capacitor and an inductor.

5. The LCR measurement circuit according to claim 4, characterized in that: The second analog switch (300) is provided with a second switch contact (301), and the number of the second switch contacts (301) is n, where n is an integer ≥1.

6. The LCR measurement circuit according to claim 5, characterized in that: The feedback resistor (RF) is provided with n, where n is an integer ≥ 1, and the number of the feedback resistor (RF) is the same as the number of the standard resistor (RG). One end of the feedback resistor (RF) is connected to the second switch contact (301), and the other end of the feedback resistor (RF) is connected to the output end of the operational amplifier (500).

7. The LCR measurement circuit as claimed in claim 6, characterized in that: The third analog switch (401) is provided with a third switch contact (401a), and the number of the third switch contacts (401a) is n, where n is an integer ≥1, and the third switch contacts (401a) correspond one-to-one to the feedback resistor (RF).

8. The LCR measurement circuit according to claim 7, characterized in that: The operational amplifier (500) further comprises a positive input terminal, and the positive input terminal of the operational amplifier is grounded.

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