Hysteresis comparator circuit capable of externally setting threshold value

Abstract

The invention discloses a hysteresis comparator circuit capable of externally setting a threshold value, which comprises an input voltage ui, a positive and negative threshold voltage preset circuit, comparators A1, A2 and A3 and an A3 in-phase end voltage bias circuit, an output voltage uo, + / -Vcc and + Vcc are connected with-Vcc through a resistor R1, potentiometers P1 and P2 which are connected in parallel, a resistor R2 and form the positive and negative threshold voltage preset circuit, the sliding end of P1 is connected with the anti-phase end of A1, and the sliding end of P2 is connected with the anti-phase end of A3. The sliding end of the P2 is connected with the anti-phase end of the A2, the voltage ui is connected with the in-phase ends of the operational amplifier A1 and the operational amplifier A2 at the same time, the output end of the A1 is connected with the in-phase end of the operational amplifier A3 through the R3, the output end of the operational amplifier A2 is connected with the in-phase input end of the operational amplifier A3 through the resistor R4, the anti-phase input end of the operational amplifier A3 is connected with the working ground, and the output end of the operational amplifier A3 is connected with the in-phase input end of the operational amplifier A3 through the resistor R5. And the output end of the operational amplifier A3 outputs the output voltage uo.

Description

Technical Field

[0001] The present invention relates to a circuit for presetting a threshold voltage of a hysteresis comparator, in particular to a hysteresis comparator circuit capable of independently setting an external threshold value, wherein the threshold voltage of the hysteresis comparator has no relation with the output voltage of the comparator, but is preset only with an external power supply voltage and an external resistor. Background Art

[0002] A hysteresis comparator (hysteresis comparator) or Schmitt trigger is a special switch gate circuit. Different from ordinary switch gate circuits, the hysteresis comparator has two threshold voltages, namely the positive threshold voltage U + and negative threshold voltage U -- The input voltage that changes the circuit state when the input signal rises from low level to high level is called the positive threshold voltage, and the input voltage that changes the circuit state when the input signal drops from high level to low level is called the negative threshold voltage. The difference between the positive threshold voltage and the negative threshold voltage is called the hysteresis voltage. This voltage transfer characteristic curve is called the "hysteresis loop" of the hysteresis comparator. The hysteresis comparator is divided into the same-phase hysteresis comparator and the reverse-phase hysteresis comparator.

[0003] The accurate calculation of the threshold voltage of a traditional hysteresis comparator depends on the accuracy of the hysteresis resistor and the stable output voltage of the comparator. When powered by a single power supply, the output voltage of the comparator is either 0V or the power supply voltage; when powered by a symmetrical dual power supply, the output voltage is either +Vcc or the negative power supply voltage -Vcc.

[0004] The precision of resistors can be very high, such as 0.1% precision resistors, and even higher precision resistors can reach 0.01% precision, which is what electronic engineers call one ten-thousandth precision. Such resistors are generally thin film resistors, and resistors made of this material can generally meet the production process requirements.

[0005] However, according to the above description, whether the hysteresis comparator can stabilize the output voltage or output the correct voltage depends on the correct preset or calculation of the threshold voltage of the hysteresis comparator. Once the positive threshold voltage U + and negative threshold voltage U — If there is a calculation error, the output voltage of the hysteresis comparator may not be output correctly at the expected time; the untimely output voltage of the hysteresis comparator leads to the incorrect calculation of the threshold voltage. This interdependent relationship of accuracy may eventually cause the hysteresis comparator switch to malfunction, causing unnecessary losses to industrial and agricultural production.

[0006] The 555 timing circuit can also form a hysteresis comparator, whose threshold voltage depends on the power supply voltage Vcc and the three 5k resistors connected in series inside the chip. The rising edge threshold voltage is 2 / 3Vcc, and the falling edge threshold is 1 / 3Vcc. Therefore, the hysteresis comparator formed by the 555 circuit is obviously different from the traditional hysteresis comparator. The difference lies in the calculation or acquisition of the threshold voltage. The acquisition of the threshold of the 555 hysteresis comparator is only related to the power supply and the three resistors, and has nothing to do with the output value Uo of the 555 circuit; but the threshold of the hysteresis comparator formed by the 555 timing circuit is fixed (2 / 3Vcc, 1 / 3Vcc), which is very inflexible. If different threshold voltages are required in the circuit design, the 555 circuit is not suitable.

[0007] Since the power supply Vcc of the comparator circuit is theoretically relatively stable, it is conceivable to preset the threshold voltage or threshold value voltage of the hysteresis comparator independently (or externally), that is, to calculate it independently of the output voltage of the hysteresis comparator, regardless of whether the positive threshold voltage U + and negative threshold voltage U — All are directly calculated based on the power supply Vcc and the resistor or potentiometer, so the accuracy of the threshold voltage is greatly improved, and the action of the hysteresis switch is more precise and flexible. Summary of the invention

[0008] The technical problem to be solved by the present invention is to provide a hysteresis comparator circuit which has a simple structure, low cost, reliable use and can independently set an external threshold value.

[0009] To achieve the above object, the present invention provides a hysteresis comparator circuit with an external threshold value, which includes an input voltage u i , positive and negative threshold voltage preset circuit, comparator A1 circuit, comparator A2 circuit, comparator A3 in-phase terminal voltage bias circuit, comparator A3 circuit, output voltage u o , power supply +Vcc, power supply -Vcc; the power supply +Vcc is connected to the power supply -Vcc in turn through the resistor R1, the parallel potentiometer P1 and potentiometer P2, the resistor R2, and constitutes the positive and negative threshold voltage preset circuit, the operational amplifier A1 constitutes the comparator A1 circuit, the operational amplifier A2 constitutes the comparator A2 circuit, the operational amplifier A3 constitutes the comparator A3 circuit, the sliding end of the potentiometer P1 is connected to the inverting input end of the operational amplifier A1, the sliding end of the potentiometer P2 is connected to the inverting input end of the operational amplifier A2, the input voltage u iThe non-inverting input terminal of the operational amplifier A1 and the non-inverting input terminal of the operational amplifier A2 are connected at the same time, and the resistors R3, R4, and R5 constitute the non-inverting terminal voltage bias circuit of the comparator A3. The output terminal of the operational amplifier A1 is connected to the non-inverting input terminal of the operational amplifier A3 through the resistor R3, the output terminal of the operational amplifier A2 is connected to the non-inverting input terminal of the operational amplifier A3 through the resistor R4, the inverting input terminal of the operational amplifier A3 is connected to the working ground, the output terminal of the operational amplifier A3 is connected to the non-inverting input terminal of the operational amplifier A3 through the resistor R5, and the output terminal of the operational amplifier A3 outputs the output voltage u o .

[0010] In the positive and negative threshold voltage preset circuit, the sliding end voltage of the potentiometer P1 presets the positive threshold voltage, and the sliding end voltage of the potentiometer P2 presets the negative threshold voltage, or vice versa, that is, the sliding end voltage of the potentiometer P1 presets the negative threshold voltage, and the sliding end voltage of the potentiometer P2 presets the positive threshold voltage.

[0011] In the comparator A3 circuit, the three resistors R3, R4 and R5 at the in-phase input end of the operational amplifier A3 have the same resistance value.

[0012] The comparator A1 circuit, the comparator A2 circuit, and the comparator A3 circuit belong to an integrated circuit IC1, model TL084, the power supply +Vcc is connected to pin 4 of IC1, and the power supply -Vcc is connected to pin 11 of IC1. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Attached Figure 1 , Attachment Figure 2 , Attachment Figure 3 , Attachment Figure 4 , Attachment Figure 5 It is used to provide a further understanding of the present invention and constitutes a part of this application. Figure 1 It is a traditional hysteresis comparator circuit; Figure 2 is the voltage transfer characteristic of the hysteresis comparator; Figure 3 It is a hysteresis comparator circuit with external threshold value; Figure 4 This is the waveform diagram of the hysteresis comparator with external threshold value; Figure 5 It is the voltage transfer characteristic curve of the external threshold hysteresis comparator (in phase). DETAILED DESCRIPTION

[0014] The embodiments of the present invention are further described below in conjunction with the accompanying drawings. First, a threshold voltage calculation formula of a conventional hysteresis comparator is introduced, and the creativity of the present invention can be seen through comparison.

[0015] The hysteresis comparator has hysteresis characteristics, that is, it has inertia, and therefore has a certain anti-interference ability. The circuit of the hysteresis comparator input from the inverting input terminal is as follows Figure 2As shown, positive feedback is introduced into the hysteresis comparator circuit.

[0016] It can be seen from the limiting circuit at the output of the integrated operational amplifier A that u o =± U Z , the potential of the inverting input terminal of the integrated operational amplifier u N = u I , non-inverting input potential

[0017] make u N = u P , the obtained u I The value is the threshold voltage, so we get

[0018] Among them+ U T It is called the rising edge threshold voltage, U T It is called the falling edge threshold voltage.

[0019] Then, the output voltage is u i How does it change when it is equal to the threshold voltage? Assume u i Less than - U T ,So u N Must be less than u P ,thus u O =+ U Z ,so u P =+ U T Only when the input voltage u i Increase to + U T , and then increase an infinitesimal amount, the output voltage u O Only from + U Z Transformed into - U Z .

[0020] Similarly, assuming u iGreater than + U T ,So u N Must be greater than u P ,thus u O =- U Z ,so u P =- U T Only when the input voltage u i Reduce to - U T , and then decreases by an infinitesimal amount, the output voltage u O Will start from - U Z Transform to + U Z .

[0021] visible, u O From + U Z Transformed into - U Z and u O from- U Z Transform to + U Z The threshold voltage is different, and the voltage transfer characteristics are Figure 2 As shown, it can be known that the hysteresis comparator is an inverting hysteresis comparator.

[0022] Hysteresis comparator circuit with independently preset threshold values Most triggers with switching hysteresis (including Schmitt triggers or hysteresis comparators) have poor preset switching thresholds. This is because these thresholds and the output values ​​of the triggers affect each other, which in turn affects the switching action of the trigger, causing unnecessary losses to industrial and agricultural production.

[0023] The threshold voltage of the hysteresis comparator composed of 555 time base circuit is ± U T Although the output voltage of the timing circuit is independent u O , or with u O It has nothing to do with the 555 circuit, but its positive and negative threshold voltages can only be fixed at 2 / 3Vcc and 1 / 3Vcc, where Vcc is the power supply voltage of the 555 circuit, which is very inflexible.

[0024] A hysteresis comparator circuit with an external threshold value can be designed. By analyzing its working principle and describing in detail the characteristic relationship between its input voltage and output voltage, and comparing it with the above-mentioned traditional hysteresis comparator, it is found that the calculation or acquisition of its threshold value has nothing to do with the output voltage of the comparator, but can be calculated independently through the supply voltage and the resistor network. It is very flexible and the accuracy can also be guaranteed. Figure 3 As shown, it can be seen that it includes the input voltage u i , positive and negative threshold voltage preset circuit, comparator A1 circuit, comparator A2 circuit, comparator A3 in-phase terminal voltage bias circuit, comparator A3 circuit, output voltage u o . Obtaining the two thresholds of the hysteresis comparator with external threshold value By observation Figure 3 , the +15V power supply is connected to the -15V power supply through the resistor R1, the parallel potentiometers P1 and P2, and the resistor R2. It can be found that the rising edge threshold voltage U of the hysteresis comparator of the external threshold value T+ The falling edge threshold voltage U T- The rising edge threshold voltage U is obtained from the sliding end of the potentiometer P2. T+ Connect to the inverting input of op amp A1, the falling edge threshold voltage U T- Connect to the inverting input of op amp A2.

[0025] Based on the "virtual disconnection" characteristic of an ideal op amp, that is, the input currents at both input terminals of the op amp are "0", it can be considered that the rising edge threshold voltage U T+ And the falling edge threshold voltage U T- The calculation has nothing to do with op amps A1 and A2. These two threshold voltages are only related to the power supply ±15V, resistors R1, R2, and potentiometers P1, P2.

[0026] Therefore, the two threshold values ​​of the hysteresis comparator with independently preset threshold values ​​in this design are very simple and flexible to obtain. U can be obtained by adjusting the sliding end of the potentiometer P1 or P2. T+ or U T-- , as for whether P1 produces U T+ Or does P2 generate U? T+ , or P1 produces U T-- Or does P2 generate U? T-- ,analyze Figure 3 The two potentiometers P1 and P2 are in parallel, and the input current of the op amps A1 and A2 is "0" (virtual disconnection). It can be seen that both P1 and P2 can generate U T+ or U T-- , we can further know that the threshold voltage U is obtained at the inverting input terminal of the operational amplifier A1T+ Or U T-- , or the inverting input of A2 gets the threshold voltage U T+ Or U T-- , all can be arranged flexibly.

[0027] For ease of understanding, we temporarily stipulate that the rising edge threshold U generated by the sliding end of the potentiometer P1 is T+ , connected to the inverting input of op amp A1; the sliding end of potentiometer P2 generates a falling edge threshold U T-- , connected to the inverting input of op amp A2.

[0028] Calculation of two thresholds of hysteresis comparator with external threshold Since the resistor network including resistor R1, parallel potentiometers P1 and P2, and resistor R2 can generate threshold voltage independently, and the core of the resistor network is potentiometer P1 and potentiometer P2, firstly, P1 and P2 are connected in parallel, and secondly, both of them can adjust the resistance value, resulting in mutual voltage clamping, it is hoped to accurately calculate the threshold value U T+ or U T— The value of is more troublesome. In practice, two high-precision voltmeters can be directly connected between the sliding ends of P1 and P2 and the working ground, and the sliding ends of the two voltmeters can be adjusted until the values ​​of the two voltmeters meet the expected threshold voltage value. Of course, the sliding end of P1 outputs the rising edge threshold U T+ , P2's sliding end output falling edge threshold U T-- .

[0029] If you really need to calculate the threshold of the hysteresis comparator through a formula, you can use this idea to analyze and calculate.

[0030] Since the two potentiometers interfere with and clamp each other, and based on the "virtual disconnection" characteristic of the op amp, that is, do not consider the effect of op amps A1 and A2 on the current passing through resistors R1, P1, P2, and R2 at all, so first Figure 3 By finding the voltage at point A and point B, we can easily find the voltage output by the sliding end of potentiometers P1 and P2.

[0031] The voltage at point A can be calculated as follows (relative to -V CC That is -15V potential)

[0032] The voltage at point B can be calculated as follows (relative to -V CC That is -15V potential)

[0033] Then the voltage at the sliding end of the potentiometer P1 can be calculated as follows (1) Since the voltage U P1-2 For the working ground (GND) voltage, it is necessary to subtract -Vcc voltage 15V, where P 1-2 Refers to the lower half resistance of potentiometer P1, U P1-2 Refers to the sliding terminal voltage of the potentiometer P1, that is, the rising edge threshold voltage U of the hysteresis comparator T+ .

[0034] The voltage at the sliding end of potentiometer P2 can be calculated as follows: (2) The voltage U P2-2 For the working ground (GND) voltage, it is necessary to subtract -Vcc voltage 15V, where P 2-2 Refers to the lower half resistance of potentiometer P2, U P2-2 Refers to the sliding terminal voltage of the potentiometer P2, that is, the falling edge threshold voltage U of the hysteresis comparator T-- .

[0035] By comparing formula (1) and formula (2), it can be seen that the calculation of the rising edge threshold U T+ and falling edge threshold U T— The calculation formula is the same as that of the above-mentioned potentiometers P1 and P2, which fully demonstrates that both the potentiometers P1 and P2 can output the rising edge threshold U T+ and falling edge threshold U T— Value argument, if P1 output rising edge threshold U must be specified T+ , P2 output falling edge threshold U T-- , and because the rising edge threshold U T+ Must be greater than the falling edge threshold U T— , then you must pay attention to the sliding position of potentiometers P1 and P2.

[0036] If the adjustment result is opposite, that is, U T+ Less than U T— , then the rising edge threshold and the falling edge threshold can be used in reverse, which will not affect the final result.

[0037] We can use an example to demonstrate whether the above formulas (1) and (2) are completely correct. If we want to obtain the expected threshold voltage, U T+ =12V, U T— = -5V, the voltage at the sliding end of the potentiometer can be calculated by the above formulas (1) and (2). By formula (1), we know that P 1-2 =(12+15)*7 / 15=189 / 15=12.6K; According to formula (2), P 2-2 =(-5+15)*7 / 15=70 / 15=4.67K.

[0038] Because P1=10K, P in the above calculation 1-2 =12.6k is incorrect, so formulas (1) and (2) are not completely accurate. p1-2 The maximum value is: U p1-2max = (30 / 7) * 6-15 = 10.7V, that is Figure 3 The voltage of point A to ground is, therefore, in this design, the maximum rising edge threshold voltage U T+ Can only be set to 10.7V.

[0039] Similarly, we can calculate U p2-2 The minimum value is: U p2-2min = (30 / 7) * 1-15 = -10.7V, that is Figure 3 The voltage of point B to ground is, therefore, in this design, the minimum falling edge threshold voltage U T-- Can only be set to -10.7V.

[0040] To sum up, since the maximum rising edge threshold voltage U is known through calculation T+ and minimum falling edge threshold voltage U T— So, in practical applications, the simplest way is to use two precision voltmeters to obtain two threshold voltages as mentioned above. No matter which potentiometer is adjusted, as long as the threshold voltage is between 10.7V and -10.7V, it will be fine.

[0041] Analysis of Voltage Transfer Characteristic Curve of Hysteresis Comparator with External Threshold The rising edge threshold voltage U has been calculated above. T+ (output from the sliding end of P1) and the falling edge threshold voltage U T-- (Output from the sliding end of P2), when the input voltage u in Equal to the rising edge threshold voltage U T+ If we increase the infinitesimal value, comparator A1 outputs a high level of "+15V", and comparator A2 outputs a high level of "+15V", then the voltage at the in-phase terminal of op amp A3 must be greater than the voltage at the inverting terminal "0", and op amp A3 outputs a high level of +15V, that is, U OUT For high level +15V, such as Figure 4 As shown, U1 is the voltage at pin 1 of the output terminal of op amp A1, and U7 is the voltage at pin 7 of the output terminal of op amp A2.

[0042] When the input voltage u in Falling, less than the rising edge threshold voltage U T+ But it is greater than the falling edge threshold voltage U T--, comparator A1 outputs a level of "-15V", comparator A2 outputs a level of "+15V", since A3 output is still "+15V", the voltage at the in-phase terminal of op amp A3 is still greater than the voltage at the inverting terminal "0", and the output level of op amp A3 remains at +15V, that is, U OUT is +15V, such as Figure 4 shown.

[0043] When the input voltage u in Continue to decrease, down to the falling edge threshold voltage U T— Then the infinitesimal value is decreased, the output level of comparator A1 is "-15V", and the output level of comparator A2 is "-15V". Since the output of op amp A3 is still +15V at this moment, then when u in The voltage drops to the falling edge threshold voltage U T— The instantaneous current I5 (the current through the feedback resistor R5) flowing into the 10-pin (A3 non-inverting input terminal) at the moment of dropping to an infinitesimal value is calculated using the following formula, where U 10 This is the voltage of pin 10 of op amp A3. The following is similar and will not be repeated here.

[0044] (3) The current flowing out of the 10-pin terminal is divided into two parts, I3 and I4, where I3 is the current passing through resistor R3 and I4 is the current passing through resistor R4. The calculation formulas are as follows: (4) (5) For the in-phase terminal (pin 10) of op amp A3, according to Kirchhoff's current law, also known as the node current law, at any node in the circuit, at any time, the sum of the currents flowing into the node is equal to the sum of the currents flowing out of the node, so (6) Set the resistance values ​​of R3, R4, R5, and U OUT =+15V, U1=-15V, U7=-15V Substituting into equations (3), (4), (5), and (6), we can obtain the voltage of pin 10 of op amp A3: U 10 = -5V.

[0045] Therefore u in The voltage drops to the falling edge threshold voltage U T— At the instant when the voltage at pin 10 of op amp A3 drops to an infinitesimal value, -5V is less than the voltage at the inverting terminal of A3, "0"V. The output U OUT Reverse to -15V, such as Figure 4 shown.

[0046] when u in The voltage rises to the falling edge threshold voltage U T— Then the infinitesimal value is increased, the output of op amp A2 is +15V, and the output of op amp A1 is -15V. OUT It is still -15V. Based on Kirchhoff's current law, the pin 10 of op amp A3 is still -5V, and the output of op amp A3 remains -15V. Figure 4 shown.

[0047] when u in The voltage rises to the rising edge threshold voltage U T+ Then it goes up to an infinitesimal value, op amp A1 outputs +15V, op amp A2 outputs +15V, and the output of op amp A3 is still -15V. Still based on Kirchhoff's current law, the voltage at pin 10 of op amp A3 becomes +5V, which is greater than the inverting terminal voltage of A3 "0"V, and the output of op amp A3 is reversed to +15V.

[0048] The above changes are repeated, forming Figure 4 The waveform shown, the voltage transfer characteristic curve is as follows Figure 5 As shown, it can be known that the hysteresis comparator is a non-inverting hysteresis comparator.

[0049] visible, Figure 3 The hysteresis comparator shown with Figure 1 The voltage transfer characteristic curve of the hysteresis comparator shown is exactly the same, which proves that it can fully realize all the functions of the traditional hysteresis comparator, and is more flexible and accurate than the threshold voltage of the traditional hysteresis comparator, which can further improve the effectiveness of the hysteresis comparator in actual production applications.

[0050] The present invention has three innovative points. The first is that the calculation of the hysteresis comparator is no longer related to the output voltage of the comparator; the second is that the calculation of the hysteresis comparator is only related to the external resistor network and can be flexibly adjusted; the third is that the acquisition points of the rising edge threshold voltage and the falling edge threshold voltage of the hysteresis comparator can be replaced, making the threshold voltage test more convenient.

[0051] The above embodiments are only used to illustrate rather than limit the technical solutions of the present invention. Although the present invention is described in detail with reference to the above embodiments, those skilled in the art should understand that the present invention can still be modified or replaced by equivalents. Any modification or partial replacement that does not depart from the spirit and scope of the present invention should be included in the scope of the claims of the present invention.

Claims

1. A hysteresis comparator circuit with an external threshold value, characterized in that: The hysteresis comparator circuit includes an input voltage u i , positive and negative threshold voltage preset circuit, comparator A1 circuit, comparator A2 circuit, comparator A3 in-phase terminal voltage bias circuit, comparator A3 circuit, output voltage u o , power supply +Vcc, power supply -Vcc; the power supply +Vcc is connected to the power supply -Vcc in turn through the resistor R1, the parallel potentiometer P1 and potentiometer P2, the resistor R2, and constitutes the positive and negative threshold voltage preset circuit, the operational amplifier A1 constitutes the comparator A1 circuit, the operational amplifier A2 constitutes the comparator A2 circuit, the operational amplifier A3 constitutes the comparator A3 circuit, the sliding end of the potentiometer P1 is connected to the inverting input end of the operational amplifier A1, the sliding end of the potentiometer P2 is connected to the inverting input end of the operational amplifier A2, the input voltage u i The non-inverting input terminal of the operational amplifier A1 and the non-inverting input terminal of the operational amplifier A2 are connected at the same time, and the resistors R3, R4, and R5 constitute the non-inverting terminal voltage bias circuit of the comparator A3. The output terminal of the operational amplifier A1 is connected to the non-inverting input terminal of the operational amplifier A3 through the resistor R3, the output terminal of the operational amplifier A2 is connected to the non-inverting input terminal of the operational amplifier A3 through the resistor R4, the inverting input terminal of the operational amplifier A3 is connected to the working ground, the output terminal of the operational amplifier A3 is connected to the non-inverting input terminal of the operational amplifier A3 through the resistor R5, and the output terminal of the operational amplifier A3 outputs the output voltage u o .

2. The hysteresis comparator circuit with an externally settable threshold value according to claim 1, characterized in that: In the positive and negative threshold voltage preset circuit, the sliding end voltage of the potentiometer P1 presets the positive threshold voltage, and the sliding end voltage of the potentiometer P2 presets the negative threshold voltage, or vice versa, that is, the sliding end voltage of the potentiometer P1 presets the negative threshold voltage, and the sliding end voltage of the potentiometer P2 presets the positive threshold voltage.

3. The hysteresis comparator circuit with an external threshold value according to claim 1, characterized in that: In the comparator A3 circuit, the three resistors R3, R4 and R5 at the in-phase input end of the operational amplifier A3 have the same resistance value.

4. The hysteresis comparator circuit with an externally settable threshold value according to claim 1, characterized in that: The comparator A1 circuit, the comparator A2 circuit, and the comparator A3 circuit belong to an integrated circuit IC1, model TL084, the power supply +Vcc is connected to pin 4 of IC1, and the power supply -Vcc is connected to pin 11 of IC1.

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