ADC signal input circuit

Abstract

The invention provides an ADC signal input circuit, and relates to the technical field of analog signal acquisition and processing, the ADC signal input circuit comprises an operational amplifier, an input branch, a feedback branch, a sampling branch and a compensation branch, and the operational amplifier is provided with a positive input end, a negative input end and an output end; the input branch is connected with the positive input end of the operational amplifier; the feedback branch is connected with the output end and the negative input end of the operational amplifier; the sampling branch comprises a first resistor and a first capacitor which are connected in series, one end of the first resistor is connected with the output end of the operational amplifier, and the other end serves as a sampling point to be connected with the analog input end of the ADC. According to the circuit, through the design of the compensation branch, the equivalent input impedance of the sampling branch is reduced by utilizing the feedback effect of the operational amplifier, the problem of measurement errors caused by unstable voltage drop generated by ADC input current on a sampling resistor is solved, and meanwhile, the reliability of the circuit is ensured.

Description

Technical Field

[0001] The present invention relates to the technical field of analog signal acquisition and processing, and particularly relates to an ADC signal input circuit. Background Art

[0002] In the design of the application circuit of an analog-to-digital converter (ADC), the input pin impedance of the ADC is not ideally infinite, but there is an input current. Taking a common ADC chip as an example, both its AN+ and AN- pins have input currents, and this input current fluctuates with the working conditions. In precision measurement applications, to reduce the influence of the output impedance of the signal source, an operational amplifier is usually used to buffer the input signal.

[0003] The existing ADC input buffer circuits usually use an operational amplifier as a voltage follower, and a resistor is connected in series at the output end of the operational amplifier and cooperates with a capacitor to form a first-order low-pass filter circuit to eliminate high-frequency interference. However, due to the input current existing at the ADC input pin, this current generates a voltage drop across the series resistor, resulting in the actual sampling voltage of the ADC being lower than the signal source voltage. More seriously, due to the fluctuation of the ADC input current, the voltage drop value also changes accordingly, and this unstable voltage drop further introduces a measurement error at the microvolt level.

[0004] The above measurement error may not be obvious in general applications, but it will significantly affect the measurement accuracy of the system in high-precision measurement scenarios. In addition, if the series resistor is directly reduced or removed to reduce the voltage drop, the anti-interference ability of the circuit will be weakened, and the ADC chip may be damaged in abnormal situations. Summary of the Invention

[0005] The purpose of the present invention is to provide an ADC signal input circuit. By adding a feedback loop from the sampling point to the negative input terminal of the operational amplifier in the ADC input buffer, the problem of the microvolt-level measurement error caused by the unstable voltage drop generated by the ADC input current across the traditional series resistor is ingeniously solved.

[0006] To achieve the above purpose, the present invention is realized through the following technical solutions:

[0007] An ADC signal input circuit, comprising: an operational amplifier having a positive input terminal, a negative input terminal, and an output terminal;

[0008] An input branch connected to the positive input terminal of the operational amplifier;

[0009] A feedback branch connected to the output terminal and the negative input terminal of the operational amplifier;

[0010] The sampling branch includes a first resistor and a first capacitor connected in series. One end of the first resistor is connected to the output end of the operational amplifier, and the other end serves as a sampling point and is connected to the analog input end of the ADC.

[0011] The compensation branch includes a second resistor. One end of the second resistor is connected to the sampling point, and the other end is connected to the negative input end of the operational amplifier.

[0012] Further: The input branch includes a voltage dividing circuit.

[0013] Further: The voltage dividing circuit includes a third resistor and a fourth resistor connected in series. One end of the third resistor is used to connect to the signal source, and one end of the fourth resistor is grounded.

[0014] Further: The feedback branch includes a fifth resistor.

[0015] Further: The resistance value of the second resistor in the compensation branch is greater than the resistance value of the first resistor in the sampling branch.

[0016] Further: The resistance value of the second resistor is 100 - 10 MΩ, and the resistance value of the first resistor is 10 - 10 KΩ.

[0017] Further: It further includes a power supply circuit, and the power supply circuit is used to provide positive and negative power supplies for the operational amplifier.

[0018] Further: The power supply circuit includes a positive power supply and a negative power supply. The voltage of the positive power supply is +2.7V to +30V, and the voltage of the negative power supply is -30V to 0V.

[0019] Further: It further includes a stabilization circuit, and the stabilization circuit includes an RC filter circuit connected to the output end of the operational amplifier.

[0020] Further: The capacitance value of the first capacitor is 100 pF - 100 μF.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] First, it significantly improves the sampling accuracy of the ADC: Through the design of the compensation branch, the equivalent resistance value of the first resistor in the sampling branch is reduced to 1 / (1 + AF) of the original value by using the feedback effect of the operational amplifier. Thus, even if there is an input current at the analog input end of the ADC and the current is unstable, due to the extremely small equivalent resistance value, the voltage drop generated on the first resistor becomes extremely small and stable, ensuring that the actual sampling voltage of the ADC can approach the true voltage value of the signal source to the greatest extent.

[0023] II. Effectively solve the measurement error problem: By adopting the design scheme of the compensation branch, the microvolt-level measurement error problem caused by the unstable voltage drop generated by the ADC input current on the series resistor in the traditional circuit is successfully solved.

[0024] III. Ensure the reliability of the circuit: In this scheme, a first resistor is set in the sampling branch. Although the voltage drop effect caused by this resistor is eliminated through the feedback action of the compensation branch, the actual value of the resistor still remains at the same order of magnitude. This design does not affect the measurement accuracy during normal operation, while in the event of an abnormality in the circuit, the first resistor can effectively limit the current amplitude and prevent the ADC chip from being damaged by excessive current, thereby improving the reliability of the circuit.

[0025] IV. This scheme improves the sampling accuracy while ensuring the reliability of the circuit, and is particularly suitable for applications where high measurement accuracy is required. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a schematic diagram of the circuit structure of an ADC signal input circuit of the present invention in an embodiment;

[0027] In the figure:

[0028] 1. First resistor; 2. Second resistor; 3. Third resistor; 4. Fourth resistor; 5. Fifth resistor; 6. First capacitor. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0029] The technical solutions of the present invention will be clearly and completely described below in conjunction with the accompanying drawings. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0030] In the description of the present invention, it should be noted that the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc. indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be construed as a limitation of the present invention. In addition, the terms "first", "second", "third" are only used for descriptive purposes and cannot be construed as indicating or implying relative importance.

[0031] Such as Figure 1As shown: The ADC signal input circuit includes an operational amplifier, an input branch, a feedback branch, a sampling branch, and a compensation branch. The operational amplifier is powered by a dual power supply and has a positive input terminal, a negative input terminal, and an output terminal. Among them, the operational amplifier is powered by a power supply circuit, and the power supply circuit includes a positive power supply and a negative power supply. The voltage of the positive power supply is +2.7V to +30V, and the voltage of the negative power supply is -30V to 0V. In this embodiment, preferably, the positive power supply voltage is +11V, and the negative power supply voltage is -11V.

[0032] The input branch is connected to the positive input terminal of the operational amplifier and includes a voltage division circuit. The voltage division circuit is composed of two resistors connected in series. Among them, the resistance value of the third resistor 3 is 10Ω - 1MΩ, preferably 10KΩ in this embodiment, and one end is used to connect to the signal source; the fourth resistor 4 is a current sampling resistor, with a resistance value of 1KΩ - 5KΩ, and one end is grounded. The voltage division circuit is used to adjust the input signal to an appropriate voltage range.

[0033] The feedback branch includes a fifth resistor 5, which is connected between the output terminal and the negative input terminal of the operational amplifier to form a basic voltage follower structure. The sampling branch includes a first resistor 1 and a first capacitor 6 connected in series. Among them, the resistance value of the first resistor 1 is 10Ω - 10KΩ, preferably 1KΩ in this embodiment, and one end is connected to the output terminal of the operational amplifier; the capacitance value of the first capacitor 6 is 100pF - 100μF, preferably 104pF in this embodiment. One end of it is connected to the other end of the first resistor 1 to form a sampling point, and this sampling point is connected to the analog input terminal AIN+ of the ADC. The first resistor 1 and the first capacitor 6 in the sampling branch form a first-order low-pass filter circuit to filter out high-frequency interference signals.

[0034] An important feature of this application is the addition of a compensation branch. The compensation branch includes a second resistor 2, with a resistance value of 100Ω - 1MΩ, preferably 10KΩ in this embodiment. One end of the second resistor 2 is connected to the sampling point, and the other end is connected to the negative input terminal of the operational amplifier to form a new feedback path. The function of this compensation branch is to reduce the equivalent input impedance of the sampling branch through the feedback action of the operational amplifier, thereby reducing the voltage drop generated by the ADC input current on the first resistor 1.

[0035] In practical applications, there is an input current on the input pin of the ADC, and this current fluctuates with the operating conditions. In traditional circuits, this input current generates a voltage drop across the first resistor 1 in the sampling branch, resulting in the actual sampling voltage of the ADC being lower than the signal source voltage. Moreover, due to the instability of the input current, this voltage drop is also unstable, thereby introducing a measurement error in the microvolt range. However, through the design of the compensation branch in this application, by utilizing the feedback effect of the operational amplifier, the equivalent resistance value of the first resistor 1 is reduced to 1 / (1 + AF) of the original value, where AF is the open-loop gain. In this way, even if there is an input current, since the equivalent resistance value is extremely small, the voltage drop generated across the first resistor 1 becomes extremely small and stable, ensuring that the voltage actually sampled by the ADC can approach the true voltage of the signal source to the greatest extent.

[0036] Meanwhile, although the compensation branch reduces the equivalent input impedance of the sampling branch, the actual resistance value of the first resistor 1 still remains at 1 kΩ. When an abnormality occurs in the circuit, this resistor can play a role in current limiting and protection to avoid damaging the ADC chip. In addition, this circuit also includes a stabilization circuit, which is connected to the output terminal of the operational amplifier using an RC filter circuit to improve the stability of the circuit.

[0037] The present invention provides an ADC signal input circuit. By adding a compensation branch between the sampling branch and the negative input terminal of the operational amplifier, and utilizing the feedback effect of the operational amplifier to reduce the equivalent input impedance of the sampling branch, it effectively solves the problem of measurement error caused by the unstable voltage drop generated by the ADC input current across the sampling resistor. This solution improves the sampling accuracy while ensuring the reliability of the circuit.

[0038] The above embodiments are only for illustrating the technical concept and features of the present invention, and their purpose is to enable those familiar with this technology to understand the content of the present invention and implement it accordingly, and should not be used to limit the protection scope of the present invention. Any equivalent transformation or modification made according to the spirit and essence of the present invention should be covered within the protection scope of the present invention.

Claims

1. An ADC signal input circuit, characterized in that, Comprising: An operational amplifier having a positive input terminal, a negative input terminal, and an output terminal; An input branch connected to the positive input terminal of the operational amplifier; A feedback branch connected to the output terminal and the negative input terminal of the operational amplifier; A sampling branch including a first resistor and a first capacitor connected in series, one end of the first resistor being connected to the output terminal of the operational amplifier and the other end being used as a sampling point and connected to the analog input terminal of an ADC; A compensation branch including a second resistor, one end of the second resistor being connected to the sampling point and the other end being connected to the negative input terminal of the operational amplifier.

2. The ADC signal input circuit according to claim 1, wherein The input branch includes a voltage dividing circuit.

3. An ADC signal input circuit according to claim 2, characterized in that, The voltage dividing circuit includes a third resistor and a fourth resistor connected in series, one end of the third resistor being used to connect to a signal source and one end of the fourth resistor being grounded.

4. An ADC signal input circuit according to claim 1, characterized in that, The feedback branch includes a fifth resistor.

5. An ADC signal input circuit according to claim 1, characterized in that, The resistance value of the second resistor in the compensation branch is greater than the resistance value of the first resistor in the sampling branch.

6. An ADC signal input circuit according to claim 5, characterized in that, The resistance value of the second resistor is 100 Ω - 10 MΩ, and the resistance value of the first resistor is 10 Ω - 10 KΩ.

7. An ADC signal input circuit according to claim 1, wherein It further includes a power supply circuit for providing positive and negative power supplies for the operational amplifier.

8. An ADC signal input circuit according to claim 7, characterized in that, The power supply circuit includes a positive power supply and a negative power supply, the voltage of the positive power supply being +2.7V to +30V, and the voltage of the negative power supply being -30V to 0V.

9. An ADC signal input circuit according to claim 1, characterized in that, It further includes a stabilization circuit, and the stabilization circuit includes an RC filter circuit connected to the output terminal of the operational amplifier.

10. An ADC signal input circuit according to claim 1, characterized in that, The capacitance value of the first capacitor is 100 pF - 100 μF.

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