A noise suppression circuit and a test circuit including the noise suppression circuit

Abstract

The present invention provides a noise suppression circuit, which is connected in parallel with an impedance from a floating system to a common-mode reference ground, wherein the two ends of the impedance are the floating system reference ground and the common-mode reference ground, respectively. The noise suppression circuit comprises: an inverting active bandpass filter, wherein the impedance is connected between its two input ends, and its center frequency or passband frequency is set to the frequency point or frequency band of a known target noise, for inverting and amplifying the target noise; an impedance circuit is connected between one input end and the output end of the inverting active bandpass filter to attenuate the target noise. Since the center frequency or passband frequency of the inverting active bandpass filter is set at the frequency point or frequency band of the known target noise, its equivalent capacitance value is the largest at the frequency point or frequency band of the target noise, and the filtering effect on the target noise signal is the strongest. The present application also provides a test circuit including the noise suppression circuit.

Description

Technical Field

[0001] The present invention relates to the technical field of circuit testing, and in particular to a noise suppression circuit and a test circuit comprising the noise suppression circuit. Background Art

[0002] In the field of semiconductor automated testing, the testing of precision small signals is often accompanied by external noise interference, such as power frequency interference, which makes the test results less accurate and more discrete. Especially for the testing of high-resistance lines, such as small leakage tests, external noise interference is particularly serious. Therefore, in the above test applications, the suppression effect of test equipment such as floating systems on external noise often determines the test quality.

[0003] When measuring a DC floating system, since the floating system (such as a floating VI source meter) relies on a high-frequency transformer to achieve floating, the parasitic parameters of the high-frequency transformer cause the high-frequency transformer to conduct or generate AC common-mode noise of the power supply frequency into the floating system, causing the floating system measurement signal-to-noise ratio to decrease.

[0004] like Figure 5 As shown in the figure, the common mode noise source V1 is included. Common mode noise is also called asymmetric noise or line-to-ground noise. This noise exists at the input end of electrical equipment using AC power supply. Therefore, the common mode noise source V1 has obvious frequency characteristics. The common mode noise source V1 is connected to the impedance Z1 of the floating system. The floating system to the common mode reference ground (EG, Figure 5 The common mode noise source V1 and the impedance Z2 from the floating system to the common mode reference ground are connected to the common mode reference ground (EG), while the floating system is connected to the floating system reference ground (FG, Figure 5 According to the circuit principle, let the noise voltage of the common-mode reference ground V(EG) = 0, then the noise voltage of the floating system reference ground as the target noise V(FG) = V1×Z2 / (Z1+Z2), that is, the common-mode noise of the floating system reference ground is the voltage divided by the impedance Z1 of the floating system and the impedance Z2 from the floating system to the common-mode reference ground to the common-mode noise source V1.

[0005] Methods for reducing the common-mode noise V(FG) of floating systems include the following categories:

[0006] 1. Reduce common mode noise source V1

[0007] The common mode noise conducted or generated is suppressed by adjusting the high frequency transformer structure. However, this method is limited by the high frequency transformer manufacturing process, and it is difficult to accurately adjust the parasitic parameters of the high frequency transformer to achieve a better suppression effect.

[0008] 2. Reduce the impedance Z2

[0009] Alternatively, a larger capacitor can be connected between the floating system reference ground and the common mode reference ground. However, this method greatly sacrifices the AC floating characteristics of the floating system in the entire frequency domain in order to suppress the common mode noise of the point frequency.

[0010] Alternatively, an LC circuit is connected between the reference ground and the common-mode reference ground of the floating system to perform frequency selection filtering. However, considering the AC floating performance of the floating system, the inductance value L in the LC circuit needs to be designed to be above 0.1H. Such a high-frequency and large-inductance inductor has adverse effects on device selection, cost, and volume.

[0011] 3. Increase the impedance Z1 of the floating system

[0012] Impedance Z1 can be simply understood as a resistor connected in parallel with a capacitor. Increasing Z1 means increasing resistance and reducing capacitance. Increasing resistance is not difficult, but reducing capacitance to a small enough level is very difficult because parasitic capacitance in space always exists and cannot be avoided. However, common-mode noise is an AC component and is sensitive to capacitance, so it is very difficult to reduce impedance Z1 for common-mode noise.

[0013] In view of the fact that the above-mentioned solution of directly reducing the target noise has certain limitations, semiconductor automated testing requires a simpler and more direct solution. Summary of the invention

[0014] The main purpose of the present invention is to provide a noise suppression circuit to overcome the above-mentioned defects of the prior art. The noise suppression circuit is connected in parallel with an impedance from a floating system to a common mode reference ground, the two ends of the impedance are the floating system reference ground and the common mode reference ground respectively, and the noise suppression circuit comprises:

[0015] An inverting active bandpass filter, wherein the impedance is connected between the two input ends thereof, and the center frequency or passband frequency thereof is set to the frequency point or frequency band of the known target noise, and is used for inverting and amplifying the target noise;

[0016] An impedance circuit is connected between one input end and an output end of the inverting active bandpass filter to attenuate the target noise.

[0017] From the above, based on the Miller effect, the inverting active bandpass filter and the impedance circuit are equivalent to a capacitor whose capacitance varies with the frequency of the target noise. Since the center frequency or passband frequency of the inverting active bandpass filter is set at the known frequency point or frequency band of the target noise, its equivalent capacitance value is the largest at the frequency point or frequency band of the target noise, and the filtering effect on the target noise signal is the strongest.

[0018] Wherein, the inverting active bandpass filter comprises a bandpass part and a gain part,

[0019] The bandpass part includes: a first operational amplifier U1,

[0020] The floating system reference ground is connected to the inverting input terminal of the first operational amplifier U1;

[0021] The non-inverting input terminal of the first operational amplifier U1 is connected to the common mode reference ground;

[0022] The gain part includes: a second operational amplifier U2 and a third operational amplifier U3,

[0023] The inverting input terminal of the second operational amplifier U2 is connected to the output terminal of the first operational amplifier U1, a fourth resistor R4 is connected between the inverting input terminal of the second operational amplifier U2 and its output terminal, and the non-inverting input terminal of the second operational amplifier U2 is connected to the common mode reference ground;

[0024] The inverting input terminal of the third operational amplifier U3 is connected to the output terminal of the second operational amplifier U2, a sixth resistor R6 is connected between the inverting input terminal of the third operational amplifier U3 and its output terminal, and the non-inverting input terminal of the third operational amplifier U3 is connected to the common mode reference ground;

[0025] The positive and negative power supply terminals of the first, second and third operational amplifiers are connected to the positive and negative poles of the power supply respectively, and the positive and negative power supply terminals are also connected to the common mode reference ground respectively.

[0026] As described above, the inverting active bandpass filter implements inverting amplification processing for the target noise, thereby generating a suppression signal with a phase difference of 180° with the target noise signal.

[0027] Wherein, the inverting active bandpass filter comprises a bandpass part and a gain part,

[0028] The bandpass part includes: a first operational amplifier U1,

[0029] The common mode reference ground is connected to the inverting input terminal of the first operational amplifier U1;

[0030] The non-inverting input terminal of the first operational amplifier U1 is connected to the floating system reference ground;

[0031] The gain part includes: a second operational amplifier U2 and a third operational amplifier U3,

[0032] The inverting input terminal of the second operational amplifier U2 is connected to the output terminal of the first operational amplifier U1, a fourth resistor R4 is connected between the inverting input terminal of the second operational amplifier U2 and its output terminal, and the non-inverting input terminal of the second operational amplifier U2 is connected to the floating system reference ground;

[0033] The inverting input terminal of the third operational amplifier U3 is connected to the output terminal of the second operational amplifier U2, a sixth resistor R6 is connected between the inverting input terminal of the third operational amplifier U3 and its output terminal, and the non-inverting input terminal of the third operational amplifier U3 is connected to the floating system reference ground;

[0034] The positive and negative power supply terminals of the first, second and third operational amplifiers are connected to the positive and negative poles of the power supply respectively, and the positive and negative power supply terminals are also connected to the floating system reference ground respectively.

[0035] As described above, the inverting active bandpass filter realizes the inverting amplification processing of the target noise, thereby generating a suppression signal with a phase difference of 180° with the target noise signal. In addition, since the positive and negative poles of the power supply are the reference ground potential of the floating system, the noise suppression circuit can directly use the power supply of the floating system without the need for an additional power supply.

[0036] The bandpass part further includes: an RC series circuit connected in parallel with the first capacitor C1; a common connection end of the resistor and capacitor in the RC series circuit is connected to the output end of the first operational amplifier U1.

[0037] From the above, filtering of the input of the bandpass part is achieved through the RC series circuit.

[0038] The bandpass part further includes: a first forward diode D1 and a first reverse diode D2 respectively connected between the non-inverting input terminal and the inverting input terminal of the first operational amplifier U1.

[0039] As described above, the diodes D1 and D2 play a clamping role to protect the first operational amplifier from overvoltage damage.

[0040] Wherein, a second forward diode D3 is connected between the output end of the third operational amplifier U3 and the positive electrode of the power supply;

[0041] A second reverse diode D4 is connected between the output end of the third operational amplifier U3 and the negative electrode of the power supply.

[0042] As described above, the second forward diode D3 and the second reverse diode D4 play a clamping role to protect the second and third operational amplifiers from being damaged by overvoltage.

[0043] Wherein, the impedance circuit is a series RC circuit.

[0044] Wherein, the impedance circuit is a resistor.

[0045] Correspondingly, the present application also provides a test circuit, including a common-mode noise source V1, an impedance Z1, a floating system and an impedance Z2 connected in series at both ends of a common-mode reference ground, characterized in that it also includes the aforementioned noise suppression circuit, which is connected in parallel with the impedance Z2.

[0046] From the above, based on the Miller effect, the inverting active bandpass filter and the impedance circuit are equivalent to a capacitor whose capacitance varies with the frequency of the target noise. Since the center frequency or passband frequency of the inverting active bandpass filter is set at the known frequency point or frequency band of the target noise, its equivalent capacitance value is the largest at the frequency point or frequency band of the target noise, which effectively reduces the target noise signal interference caused by the use of a floating system for measurement, and achieves the optimal filtering effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 is the circuit principle block diagram of the test circuit;

[0048] Figure 2 It is the circuit principle block diagram of the noise suppression circuit;

[0049] FIG3(A) is a circuit diagram of a first embodiment of an inverting active bandpass filter 110;

[0050] FIG3(B) is a circuit diagram of a second embodiment of an inverting active bandpass filter 110;

[0051] Figure 4 is a circuit diagram of an impedance circuit 120;

[0052] Figure 5 Figure 2 is the noise model diagram of the floating system. DETAILED DESCRIPTION

[0053] See below Figure 1 to Figure 4 The noise suppression circuit of the present invention and a test circuit including the noise suppression circuit are described in detail.

[0054] Figure 1 The circuit principle block diagram of the test circuit is as follows: Figure 1 As shown, the core idea of ​​the present application is to configure a noise suppression circuit ZP, which is connected to Figure 5 The impedance Z2 from the floating system to the common mode reference ground shown in FIG. 1 is connected in parallel, thereby reducing the impedance Z2 from the floating system to the common mode reference ground to achieve noise suppression.

[0055] Considering the DC floating characteristics of the floating system, the noise suppression circuit ZP cannot be purely resistive. If the noise suppression circuit ZP is purely capacitive, the AC floating characteristics of the floating system will be reduced. Since the common mode noise source V1 has obvious frequency characteristics in actual use, the noise suppression circuit ZP can be designed to show low impedance at a specific frequency point. Figure 2 The figure shows a principle block diagram of an equivalent circuit of a noise suppression circuit ZP, which includes an inverting active bandpass filter 110 , whose input and output ends are respectively connected to two ends of an impedance circuit 120 .

[0056] As shown in Fig. 3(A) and Fig. 3(B), the inverting active bandpass filter 110 includes a bandpass part and a gain part. The core circuits of the bandpass part and the gain part are both operational amplifiers, wherein the core circuit of the bandpass part is the first operational amplifier U1, and the core circuit of the gain part is the second and third operational amplifiers U2 and U3.

[0057] In Fig. 3(A), Vin represents the potential of the floating system reference ground (FG), which is used as the input terminal of the bandpass part, and is connected to the inverting input terminal of the first operational amplifier U1 after being connected in series with the first resistor R1 and the first capacitor C1. In a preferred embodiment, an RC series circuit composed of a second capacitor C2 and a second resistor R2 is connected in parallel with the capacitor C1, and the common connection terminal of the second capacitor C2 and the second resistor R2 is connected to the output terminal of the first operational amplifier U1.

[0058] The non-inverting input terminal of the first operational amplifier U1 is connected to the common mode reference ground (EG). In a preferred embodiment, a first forward diode D1 and a first reverse diode D2 are connected between the non-inverting input terminal and the inverting input terminal of the first operational amplifier U1, respectively. The diodes D1 and D2 act as clamps to protect the first operational amplifier U1 from overvoltage damage.

[0059] The positive and negative power supply terminals of the first operational amplifier U1 are connected to +15V and -15V power supplies respectively. The positive and negative power supply terminals of the first operational amplifier U1 are also connected to a capacitor respectively and then connected to the common mode reference ground (EG).

[0060] The passband frequency or center frequency f of the passband section is:

[0061]

[0062] The quality factor Q of the bandpass section is:

[0063]

[0064] Wherein R1 and R2 represent the resistance of the first resistor R1 and the second resistor R2 respectively, and C represents the capacitance of any capacitor C1 or C2 with the same capacitance. The frequency point or frequency band of the inverting active bandpass filter can be adjusted by adjusting the quality factor Q.

[0065] In the gain part, the inverting input terminal of the second operational amplifier U2 is connected to the output terminal of the aforementioned first operational amplifier U1 through the third resistor R3, a fourth resistor R4 is connected in series between the inverting input terminal and the output terminal, and the non-inverting input terminal of the second operational amplifier U2 is connected to the common mode reference ground (EG).

[0066] The inverting input terminal of the third operational amplifier U3 is connected to the output terminal of the second operational amplifier U2 through the fifth resistor R3, a sixth resistor R6 is connected between the inverting input terminal and the output terminal, and the non-inverting input terminal of the third operational amplifier U3 is connected to the common mode reference ground (EG).

[0067] The positive and negative power supply terminals of the second and third operational amplifiers are connected to +15V and -15V power supplies respectively. The positive and negative power supply terminals of the second and third operational amplifiers are also connected to a capacitor and then connected to the common mode reference ground (EG).

[0068] In a preferred embodiment, the output end of the third operational amplifier U3 is further connected to a second forward diode D3 and then to a +15V power supply; the output end of the third operational amplifier U3 is further connected to a second reverse diode D4 and then to a -15V power supply.

[0069] The second forward diode D3 and the second reverse diode D4 play a clamping role to protect the second and third operational amplifiers from being damaged by overvoltage.

[0070] The output terminal of the third operational amplifier U3 serves as the output terminal of the inverting active bandpass filter 110, and its output current represents the current between the common mode reference ground (EG) and the floating system reference ground (FG).

[0071] The product of the gain of the bandpass part at the center frequency and the gain of the gain part at the center frequency is expressed as:

[0072]

[0073] The gain of the gain part at the center frequency corresponds to the gain of the second operational amplifier U2 and the gain of the third operational amplifier U3 in Figure 3, where R3 to R6 represent the resistance values ​​of the third, fourth, fifth and sixth resistors respectively. The gain at the center frequency is set at the common mode noise frequency conducted or generated by the high frequency transformer.

[0074] In the above circuit, the withstand voltage values ​​of the first and second capacitors C1-C2 meet the isolation withstand voltage index of the floating system. Preferably, the first, second and third operational amplifiers are AD843 devices.

[0075] Figure 4 The circuit diagram of the impedance circuit 120 is shown, which includes a third capacitor C3 and a seventh resistor R7 connected in series to form a series RC circuit. The third capacitor C3 plays a role of blocking DC during floating measurement, and the seventh resistor R7 plays a role of limiting the current of the third capacitor C3.

[0076] Further, when a non-floating system is used for measurement, the impedance circuit 120 only includes the seventh resistor R7.

[0077] Based on the Miller effect, the present application makes the noise suppression circuit composed of the inverting active bandpass filter 110 and the impedance circuit 120 equivalent to a capacitor whose capacitance varies with the frequency of the target noise. The principle is that the capacitor C3 connected in parallel between the input and output ends of the inverting active bandpass filter 110, due to the inverting amplification effect of the inverting active bandpass filter 110, has an equivalent capacitance value acting on the input end of the inverting active bandpass filter 110: C×(1+G(f)), where C is the capacitance of the capacitor C3 in the impedance circuit, G(f) is the response of the gain of the bandpass part and the gain product of the gain part with the target noise frequency, and when f is the center frequency, the corresponding G(f) value is the maximum value, which is the gain of the aforementioned bandpass part at the center frequency and the gain product A of the gain part at the center frequency.

[0078] The center frequency or passband frequency of the inverting active bandpass filter 110 is set at the frequency point or frequency band of the known target noise. The capacitor whose capacitance varies with the frequency of the target noise, formed by the inverting active bandpass filter 110 and the impedance circuit 120, has the largest equivalent capacitance value at the frequency point or frequency band of the target noise, and has the strongest filtering effect on the target noise signal.

[0079] The voltage of the common-mode noise of a specific frequency is detected by the inverting active bandpass filter 110, and the voltage of the common-mode noise of a specific frequency is converted into a common-mode feedback current through an impedance circuit, so as to realize a low-impedance common-mode noise current loop between the floating system reference ground and the common-mode reference ground at a specific frequency, thereby achieving the purpose of suppressing the common-mode noise voltage. The excellent effect of the above circuit is that it does not affect the DC floating, has less influence on the AC floating characteristics compared with the traditional method, effectively suppresses the common-mode noise conducted or generated by the high-frequency transformer, and the circuit is easy to realize. In addition, since the voltage gain of the inverting active bandpass filter 110 at its center frequency and the impedance of the impedance circuit 120 jointly determine the common-mode feedback current gain, the greater the common-mode feedback current gain, the stronger the common-mode noise suppression capability, but the excessive common-mode feedback current gain will affect the stability of the floating system, so the common-mode feedback current gain should be adjusted to the optimal value according to the characteristics of the specific floating system.

[0080] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For example, the following adjustments are made: in the circuit shown in FIG3(A), the common mode reference ground (EG) is adjusted to be Vin, which is connected to the inverting input terminal of the first operational amplifier. The non-inverting input terminals of the first, second, and third operational amplifiers are respectively connected to the floating system reference ground (FG). That is, in FIG3(A) and FIG3(B), the common mode reference ground (EG) and the floating system reference ground (FG) are intermodulated.

[0081] In the connection mode of FIG3(A), the positive and negative 15V power supplies are referenced to the potential of the common mode reference ground, while in the adjusted connection mode as shown in FIG3(B), the positive and negative 15V power supplies are referenced to the potential of the floating system reference ground. Therefore, by adopting the adjusted connection mode, the noise suppression circuit can directly use the power supply of the floating system without the need for an additional power supply; while the embodiment using the connection mode of FIG3(A) cannot use the power supply of the floating system, but requires a set of external power supplies relative to the ground, which increases the difficulty in actual wiring.

[0082] In short, any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A noise suppression circuit, characterized in that: The noise suppression circuit is connected in parallel with an impedance from the floating system to the common mode reference ground, the two ends of the impedance are the floating system reference ground and the common mode reference ground respectively, and the noise suppression circuit comprises: An inverting active bandpass filter (110), the two input ends of which are connected to the impedance, and the center frequency or passband frequency of which is set to the frequency point or frequency band of the known target noise, for performing inverting amplification processing on the target noise; An impedance circuit (120) is connected between one input end and an output end of the inverting active bandpass filter (110) to attenuate the target noise; The noise suppression circuit composed of the inverting active bandpass filter (110) and the impedance circuit (120) is used as a whole to be equivalent to a capacitor whose capacitance varies with the frequency of the target noise; a capacitor C3 is connected in parallel between the input end and the output end of the inverting active bandpass filter 110, and the equivalent capacitance value of the input end of the inverting active bandpass filter 110 is: C×(1+G(f)), wherein C is the capacitance value of the capacitor C3 in the impedance circuit, G(f) is the response of the gain of the bandpass part and the gain product of the gain part to the target noise frequency, and when f is the center frequency, the corresponding G(f) value is the maximum value; The inverting active bandpass filter (110) comprises a bandpass part and a gain part; The bandpass part includes: a first operational amplifier U1, The floating system reference ground is connected to the inverting input terminal of the first operational amplifier U1; The non-inverting input terminal of the first operational amplifier U1 is connected to the common mode reference ground; The gain part includes: a second operational amplifier U2 and a third operational amplifier U3, The inverting input terminal of the second operational amplifier U2 is connected to the output terminal of the first operational amplifier U1, a fourth resistor R4 is connected between the inverting input terminal of the second operational amplifier U2 and its output terminal, and the non-inverting input terminal of the second operational amplifier U2 is connected to the common mode reference ground; The inverting input terminal of the third operational amplifier U3 is connected to the output terminal of the second operational amplifier U2, a sixth resistor R6 is connected between the inverting input terminal of the third operational amplifier U3 and its output terminal, and the non-inverting input terminal of the third operational amplifier U3 is connected to the common mode reference ground; The positive and negative power supply terminals of the first, second and third operational amplifiers are connected to the positive and negative poles of the power supply respectively, and the positive and negative power supply terminals are also connected to the common mode reference ground respectively.

2. The circuit according to claim 1, characterized in that The inverting active bandpass filter (110) comprises a bandpass part and a gain part, The bandpass part includes: a first operational amplifier U1, The common mode reference ground is connected to the inverting input terminal of the first operational amplifier U1; The non-inverting input terminal of the first operational amplifier U1 is connected to the floating system reference ground; The gain part includes: a second operational amplifier U2 and a third operational amplifier U3, The inverting input terminal of the second operational amplifier U2 is connected to the output terminal of the first operational amplifier U1, a fourth resistor R4 is connected between the inverting input terminal of the second operational amplifier U2 and its output terminal, and the non-inverting input terminal of the second operational amplifier U2 is connected to the floating system reference ground; The inverting input terminal of the third operational amplifier U3 is connected to the output terminal of the second operational amplifier U2, a sixth resistor R6 is connected between the inverting input terminal of the third operational amplifier U3 and its output terminal, and the non-inverting input terminal of the third operational amplifier U3 is connected to the floating system reference ground; The positive and negative power supply terminals of the first, second and third operational amplifiers are connected to the positive and negative poles of the power supply respectively, and the positive and negative power supply terminals are also connected to the floating system reference ground respectively.

3. The circuit according to claim 1 or 2, characterized in that: The bandpass part further includes: an RC series circuit consisting of a second capacitor C2 and a second resistor R2, connected in parallel with the first capacitor C1; a common connection end of the resistor and capacitor in the RC series circuit is connected to the output end of the first operational amplifier U1; One end of the first capacitor C1 is connected to the floating system reference ground via a first resistor R1 connected in series, and the other end is also connected to the inverting input terminal of the first operational amplifier U1; The passband frequency or center frequency f of the passband section is: The quality factor Q of the bandpass part is: Wherein, R1 and R2 represent the resistance values ​​of the first resistor R1 and the second resistor R2 respectively, and C represents the capacitance value of any capacitor C1 or C2 with the same capacitance value; The frequency point or frequency band of the inverting active bandpass filter is adjusted and set by adjusting the quality factor Q.

4. The circuit according to claim 1 or 2, characterized in that: The bandpass part further includes: a first forward diode D1 and a first reverse diode D2 respectively connected between the non-inverting input terminal and the inverting input terminal of the first operational amplifier U1.

5. The circuit according to claim 1 or 2, characterized in that: A second forward diode D3 is connected between the output end of the third operational amplifier U3 and the positive electrode of the power supply; A second reverse diode D4 is connected between the output end of the third operational amplifier U3 and the negative electrode of the power supply.

6. The circuit according to claim 1, characterized in that The impedance circuit (120) is a series RC circuit.

7. The circuit according to claim 1, characterized in that The impedance circuit (120) is a resistor.

8. A test circuit, comprising a common mode noise source V1, an impedance Z1, a floating system and an impedance Z2 connected in series at both ends of a common mode reference ground, characterized in that: It also includes the noise suppression circuit according to any one of claims 1 to 7, wherein the noise suppression circuit is connected in parallel with the impedance Z2.

Images