Precise voltage-frequency converter with low temperature drift

Abstract

The invention discloses a low-temperature-drift precise voltage-frequency converter which is characterized in that an operational amplifier A1, resistors R1, R2, R3, R4 and R7, a potentiometer P1, diodes D3 and D4 and a capacitor C1 form a multivibrator circuit, a pin 6 of the A1 charges the capacitor C1 through the P1, the R4 and the D3, and the capacitor C1 discharges to an output end of A2 only through the D4 and the R7; the diodes D1 and D2 form a temperature coefficient compensation circuit, the R6 and C3 form an integral circuit, the A2 and C4 form an integral comparator circuit, a pin 6 of the A1 is connected with a pin 3 of the A2 through the R6, the pin 3 of the A2 is grounded through the C3, and a signal output by the A1 generates direct-current voltage V2 through the integral circuit and enters the pin 3 of the operational amplifier A2; the direct current control voltage V1 enters the inverting input end of the operational amplifier A2 through the resistor R8, the output end of the operational amplifier A1 outputs an oscillation signal Fout through the resistor R5, and the frequency of the Fout signal is in direct proportion to the direct current control voltage V1.

Description

Technical Field

[0001] The present invention relates to a voltage-frequency converter (VFC) technology, in particular to a low-temperature drift precision voltage-frequency converter with a voltage / frequency transmission characteristic curve better than 0.5% and a temperature coefficient less than 0.01%°C. Background Art

[0002] A voltage-to-frequency converter (VFC) is a device that realizes analog-to-digital conversion. It can convert analog voltage into pulse signals. The frequency of the output signal is proportional to the input voltage. Its working principle is to pass the input analog voltage signal through an operational amplifier of appropriate proportion, output it to the voltage-frequency converter, reduce the pulse signal frequency linearly in proportion to the input voltage through a frequency divider, and then convert the square wave into a rectangular wave through a shaping circuit, and finally output a rectangular wave signal with a frequency proportional to the input voltage.

[0003] Voltage-to-frequency converters are widely used in measurement, control, automation and other fields. They can convert analog signals into digital signals, which is convenient for subsequent processing and application. For example, in the fields of motor control, variable frequency speed regulation, intelligent instrumentation, etc., voltage-to-frequency converters can convert physical quantities such as speed, flow, temperature, etc. into corresponding pulse signals, which is convenient for counting, measurement and control. In addition, in power systems, voltage-to-frequency converters can also be used to monitor and control the working status and operating parameters of the system.

[0004] The LM131 / LM231 / LM331 series of voltage-to-frequency converters are ideal for use in simple, low-cost circuits for analog-to-digital conversion, precise frequency-to-voltage conversion, long term integration, linear frequency modulation or demodulation, and many other functions, and are easily applied to all standard voltage-to-frequency converter applications.

[0005] Therefore, in actual industrial production, using the LM231 / 331 chip, a low-cost voltage-to-frequency converter can be constructed. Due to its high integration, the application is very simple and stable. When it is used as a voltage-to-frequency converter, the output will be a pulse train with a frequency linearly equal to the applied input voltage. In order to obtain a higher level of temperature drift accuracy, the LM231A / LM331A can be used, which can provide a 10Hz-11KHz change for a 0-10V input voltage change, with a maximum nonlinearity of only 0.03%.

[0006] Of course, voltage-frequency conversion is also an important teaching content in the teaching and experiments of analog and digital electronics in colleges and universities. In relevant textbooks and reference materials, the most commonly taught voltage-frequency conversion circuit is not the VFC circuit based on LM131 / LM231 / LM331, but the VFC circuit based on 555 timing circuit. Due to the high integration of LM131 / LM231 / LM331, it is not easy to draw inferences from one example to another in theoretical teaching, and the underlying principles of the chip cannot be taught.

[0007] The voltage-to-frequency converter based on the 555 timer can convert voltage changes into frequency changes. The frequency oscillation can be adjusted by changing the timing elements. This VFC circuit based on the 555 time base circuit is easy to teach from the basics and lay a good foundation for students' related circuit knowledge. However, the tolerance of its output frequency is less than 2%, which is still high.

[0008] It is also possible to design a precision-frequency converter using only two operational amplifiers. The first operational amplifier IC1 is used as a multivibrator. The oscillation signal is converted into a DC voltage average value V2 through an integration circuit. V2 is proportional to the oscillation signal frequency. The second operational amplifier IC2 is used as a comparison integrator circuit to compare and integrate the external control voltage V1 with V2. The output of the integrator will affect the pulse interval (not duty cycle) of the oscillator, and thus affect the oscillation frequency of the voltage-controlled oscillator. Since the circuit is always in balance, V1=V2. Since V2 is proportional to the oscillation signal frequency, the external control signal V1 is also proportional to the oscillation frequency.

[0009] The voltage / frequency transfer characteristic curve is better than 0.5% and the temperature coefficient is less than 0.01%℃. Compared with the voltage-to-frequency converter using 555, its linearity is higher and the temperature drift effect is much reduced. Summary of the invention

[0010] The technical problem to be solved by the present invention is to provide a low-temperature drift precision voltage-frequency converter with simple structure, low cost, reliable use, voltage / frequency transmission characteristic curve better than 0.5%, and temperature coefficient less than 0.01%℃.

[0011] To achieve the above-mentioned purpose, the present invention provides a low-temperature drift precision voltage-frequency converter, which includes a multivibrator circuit, a temperature coefficient compensation circuit, an integration circuit, an integration comparator circuit, a DC control voltage, and a zero adjustment circuit; an operational amplifier A1, a resistor R1, a resistor R2, a resistor R3, a resistor R4, a resistor R7, a potentiometer P1, a diode D3, a diode D4, and a capacitor C1 constitute the multivibrator circuit, the high level output by the output end of the operational amplifier A1 is sequentially charged to the capacitor C1 through the potentiometer P1, the resistor R4, and the forward diode D3, the diode D5 reversely cuts off the charging voltage of the capacitor C1, resulting in that the charging voltage of the capacitor C1 can only be discharged to the output end of the operational amplifier A2 through the forward diode D4 and the resistor R7; the diode D1 and the diode D2 constitute the temperature coefficient compensation circuit, the output end of the operational amplifier A1 is sequentially connected to the in-phase input end of the operational amplifier A1 through the forward diode D1 and the resistor R3, the reverse diode D2 and the forward diode D1 is connected in parallel; resistor R6 and capacitor C3 constitute the integration circuit, operational amplifier A2 and capacitor C4 constitute the integration comparator circuit, the output end of operational amplifier A1 is connected to the non-inverting input end of operational amplifier A2 through resistor R6, the non-inverting input end of operational amplifier A2 is connected to the working ground through capacitor C3, the oscillation signal output by operational amplifier A1 generates a DC voltage V2 equal to the average value of the waveform output by operational amplifier A1 through the integration circuit, and the DC voltage V2 proportional to the frequency of the oscillation signal output by operational amplifier A1 enters the non-inverting input end of operational amplifier A2, potentiometer P2 constitutes the zero adjustment circuit, pin 1 of operational amplifier A2 is connected to pin 5 of operational amplifier A2 through the resistor body of potentiometer P2, and the sliding end of potentiometer P2 is connected to the working ground; the DC control voltage V1 enters the inverting input end of operational amplifier A2 through resistor R8, and the resistance value of resistor R8 is 1MΩ; the output end of operational amplifier A1 outputs an oscillation signal Fout through resistor R5, and the frequency of the Fout signal is proportional to the DC control voltage V1.

[0012] In the multivibrator circuit, the power supply +Vcc is connected to the working ground through the parallel resistor R1, the resistor R3 and the resistor R2 in sequence. The connection point of the resistor R1 and the resistor R2 is the non-inverting input terminal of the operational amplifier A1 to generate a 10V reference voltage. The inverting input terminal of the operational amplifier A1 is connected to the working ground through the capacitor C1.

[0013] The integrating comparator circuit, if the DC control voltage V1 is lower than V2, the output of the integrating comparator circuit A2 generates a positive slope sawtooth signal, the discharge rate of the capacitor C1 gradually slows down, and the period of the oscillation signal output by the operational amplifier A1 becomes larger and the frequency becomes smaller; if the DC control voltage V1 is higher than V2, the output of the integrating comparator circuit A2 generates a negative slope sawtooth signal, the discharge rate of the capacitor C1 gradually speeds up, and the period of the oscillation signal output by the operational amplifier A1 becomes smaller and the frequency becomes larger; if the DC control voltage V1 is equal to V2, the output voltage of the integrating comparator circuit A2 remains unchanged, and the period and frequency of the oscillation signal output by the operational amplifier A1 remain unchanged. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Attached Figure 1 , Attachment Figure 2 It is used to provide a further understanding of the present invention and constitutes a part of this application. Attached Figure 1 It is a voltage-to-frequency converter based on the 555 time-base circuit; Attached Figure 2 It is a precision voltage-frequency converter with low temperature drift. DETAILED DESCRIPTION

[0015] The embodiments of the present invention are further described below in conjunction with the accompanying drawings. First, the principle and structure of the VFC circuit based on the 555 time base circuit are briefly introduced to reflect the creativity of the present invention.

[0016] VFC circuit based on 555 time base circuit A voltage-to-frequency converter or VFC is an electronic circuit that converts a DC input voltage into a square wave pulse signal of a specific frequency. The voltage-to-frequency converter using a 555 time base circuit converts voltage changes into frequency changes. The frequency oscillation can be adjusted by changing the timing components. Such circuits are widely used in various practical applications and teaching applications, especially analog-to-digital converters and frequency modulation. The basic idea behind VFC is to generate a frequency that is linearly proportional to the input voltage. The schematic diagram is shown in Figure 1. Figure 1 shown.

[0017] This VFC converter circuit uses operational amplifier IC1 (TL071) as the input trigger amplifier of timer IC2 (555) and uses IC2 as an astable multivibrator. The circuit takes a 0 to -10 V DC voltage as the input signal V IN , and gives an output square pulse f with a frequency range of 0 to 1000 Hz out The capacitor C1 acts as feedback for the operational amplifier, so IC1 and C1 form an integrating circuit, and the current flowing through C1 can be calculated as follows.

[0018]

[0019] The input of the integration circuit based on the operational amplifier can be a step level signal, a square wave signal, or a sine wave signal. Figure 1 The input of the integration circuit is a step level Vin, and the output of the integration circuit needs a positive level, so the input level Vin needs to be Negative polarity voltage.

[0020] When the voltage level on C1 reaches 2 / 3 of the +Vcc of the timer IC2 power supply, the discharge tube inside IC2 is turned on, so the voltage on C1 reaches 1 / 3 of +Vcc. At this level, the discharge tube is turned off, and then C1 starts to charge again. When C1 is charging, IC2 outputs a high level, and when C1 is discharging, IC2 outputs a low level. Through this operation, the output frequency can be easily calculated.

[0021] Here P1 can be used to change the output frequency and input voltage. The tolerance of the output frequency is less than 2%. It can be seen that Figure 1 The VFC circuit shown in the circuit has a simple and easy-to-understand working principle, which is suitable for both commercial and teaching use. For commercial use, the frequency tolerance is relatively high, and the reasons are complicated, so I will not explain them in detail.

[0022] Precision Voltage-to-Frequency Converter with Low Temperature Drift Different from Figure 1 First, the DC control voltage is integrated, and then the integrated voltage is compared with the two threshold voltages of the hysteresis comparator based on the 555 time base circuit to generate oscillation. This design of low temperature drift precision voltage-frequency converter is as follows Figure 2 shown.

[0023] It can be seen that it includes a multivibrator circuit, a temperature coefficient compensation circuit, an integration circuit, an integration comparator circuit, a DC control voltage, and a zero adjustment circuit.

[0024] Multivibrator Circuit The op amp A1, resistors R1, R2, R3, R4, R7, potentiometer P1, diodes D3, D4, and capacitor C1 form a multivibrator. Ignore diodes D1 and D2 for now. The +Vcc power supply provides a reference voltage of 10V to the non-inverting input terminal of op amp A1 through resistors R1, R2, and R3. At the beginning of power-on, the initial voltage of capacitor C1 is "0", and op amp A1 outputs a +15V voltage, which charges capacitor C1 through D3, R4, and P1. The time constant is τ= (R4+P1) C1, and it is charged until the voltage of the inverting input terminal of op amp A1 is 10V and then increases to an infinitesimal value, and the output of op amp A1 is reversed to "0" V. The voltage at the non-inverting input terminal drops to about 5V due to the hysteresis caused by resistor R3.

[0025] Unlike ordinary multivibrators, due to the presence of reverse diode D5, capacitor C1 cannot discharge to the output "0" of op amp A1, but discharges to the output of integrating comparator A2 through diode D4 and resistor R7. The discharge rate is determined by resistor R7 and the output voltage of integrating comparator A2. The voltage of C1 gradually decreases until the voltage of the inverting input terminal of op amp A1 drops to 5V and then decreases to an infinitesimal value. At this time, the output of A1 reverses to "+15V". The above process is repeated, and the output waveform of op amp A1 consists of a series of positive pulses. It can be summarized as follows: when C1 is charged, A1 outputs a high level (duty); when C1 is discharged, A1 outputs a low level (0V).

[0026] Since the charging channel of C1 only has a resistor and a forward diode, the pulse duration (duty) T2 is a constant, and the pulse interval (0V time) depends on the output voltage of the integrating comparator A2.

[0027] Integrator Circuit Figure 2 There are two kinds of integrator circuits in the circuit, one is the traditional R / C integrator circuit; the other is a less common op amp-based integrator comparator circuit. The difference from the traditional op amp-based integrator circuit is that the in-phase terminal of the op amp is not grounded.

[0028] The integration circuit is a circuit configuration that integrates the input signal. It can be used for signal filtering, waveform shaping, analog computing, control systems, etc. The resistor R6 and capacitor C3 form an integration circuit. The output pulse voltage of A1 passes through the integration circuit to generate a DC voltage V2 equal to the average value of the output waveform of A1:

[0029] Where Vcc=15V, T is the pulse period, T=T1+T2, T1 is the zero-level time of the oscillation signal output by A1, and since T2 is a constant, V2 is proportional to 1 / T, that is, proportional to the frequency of the output pulse of op amp A1.

[0030] A capacitor C4 is connected between the inverting input and output of op amp A2, so A2 and C4 form an "integrating comparator circuit", which is neither a pure comparator circuit nor a typical integrator circuit (the non-inverting terminal of the typical integrator circuit based on the op amp is grounded). Based on the "virtual break" and "virtual short" characteristics of op amp A2, the input DC control voltage V1 is added to the inverting input terminal of comparator A2 through resistor R8, and the output voltage V2 of the R6 / C3 integrator circuit is added to the non-inverting input terminal of the integrating comparator A2. If the DC control voltage V1 is lower than V2, the output of the integrating comparator A2 will produce a positive slope sawtooth (linearly rising from 0). As mentioned above, the discharge channel of the capacitor C1 is the diode D4, the resistor R7 and the output end of A2. Therefore, C1 discharges at a slower rate, and slower and slower, making the low-level interval T1 of the multivibrator signal output by A1 longer, the period T becomes larger, and T2 is a constant. Then the output voltage V2 of the integrating circuit composed of R6 / C3 gradually decreases and gradually approaches the DC control voltage V1.

[0031] If V2 is lower than the DC control voltage V1, the output of the integrating comparator A2 is a negative slope sawtooth signal (i.e., it tends to 0V), which makes the discharge rate of C1 faster and gradually accelerates, and shortens the low-level interval time T1 of the oscillation signal output by A1. T2 is a constant, so the period T of the A1 output signal becomes smaller, and the output voltage V2 of the integrating circuit composed of R6 / C3 becomes larger, gradually approaching the V1 voltage.

[0032] If V1=V2, the output voltage of the integrating comparator A2 remains unchanged.

[0033] It can be seen that the voltage-frequency converter circuit is always balanced, and V1=V2, because the output voltage of the R6 / C3 integration circuit is proportional to the frequency of the multivibrator signal output by the operational amplifier A1, so the frequency of the multivibrator signal output by the operational amplifier A1 is also proportional to the input DC control voltage V1 (V1=V2).

[0034] Precision, low temperature drift VFC implementation In order to improve the stability of the above VFC circuit, a slight improvement should be made. Due to changes in ambient temperature, the temperature coefficients of diodes D3 and D4 may produce errors, so the charging and discharging time of capacitor C1 will produce temperature drift with temperature changes. Therefore, in order to produce similar changes in the reference voltage at the non-inverting input of op amp A1, the same diodes D1 and D2 should be installed in series with resistor R3 to compensate for the temperature coefficient of the semiconductor device.

[0035] The potentiometer P2 between pin 1 and pin 5 of op amp A2 is a zero adjustment potentiometer, which can make the compensation voltage of op amp A2 "0", otherwise this voltage will cause zero error in the output of A2.

[0036] The charging channel potentiometer P1 of C1 can precisely adjust the VFC conversion ratio. Figure 2 The circuit component parameters shown have a conversion ratio of approximately 1kHz / V.

[0037] Although this voltage-controlled oscillator uses only two op amp circuits, its voltage / frequency transfer characteristic curve is better than 0.5% and its temperature coefficient is less than 0.01%°C.

[0038] Summarize Figure 1 The VFC circuit based on the 555 circuit is a common traditional design concept and is easy to understand. Figure 2 ) Although it doesn't look very complicated, I hope you understand Figure 2 The working principle is not very easy. Figure 2 In the figure, the charging channel of C1 based on the multivibrator circuit of operational amplifier A1 is a traditional design, but the discharging channel is relatively novel. It does not discharge to the output terminal "0" of operational amplifier A1 as is common, but discharges to the integrated voltage at the output terminal of integrating comparator A2. The larger the integrated output voltage of integrating comparator circuit A2, the slower C1 discharges; the smaller the output voltage of the integrating circuit of A2, the faster C1 discharges.

[0039] Therefore, two integration circuits appear in this design. The resistor-capacitor integration circuit composed of resistor R6 and capacitor C3 is a traditional design, and the integration value provides voltage for the non-inverting end of the comparison integrator circuit.

[0040] Another integration circuit is the integration comparator circuit based on op amp A2. It is neither a simple comparator circuit nor a typical integration circuit based on op amp (the non-inverting terminal is not grounded). It is relatively novel, but the underlying principle is still similar to the typical integration circuit. Each DC control voltage V1 of 0~10V enters the inverting input terminal of op amp A2, and the voltage V2 after the integration of the multivibrator signal based on A1 through R6 / C3 enters the non-inverting input terminal of op amp A2. After a period of convergence, the integration comparator circuit based on op amp A2 reaches V1=V2, and the oscillation frequency is proportional to the DC control voltage V1.

[0041] Therefore, this design can be used for commercial purposes, and can also be applied to the theoretical and practical teaching of analog electronic technology in colleges and universities. It can enable students to master more novel circuit designs that are different from the content of textbooks. It is practical, novel and creative.

[0042] 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 low temperature drift precision voltage-frequency converter, characterized in that: The voltage-frequency converter includes a multivibrator circuit, a temperature coefficient compensation circuit, an integration circuit, an integration comparator circuit, a DC control voltage, and a zero adjustment circuit; an operational amplifier A1, a resistor R1, a resistor R2, a resistor R3, a resistor R4, a resistor R7, a potentiometer P1, a diode D3, a diode D4, and a capacitor C1 constitute the multivibrator circuit, the high level outputted by the output end of the operational amplifier A1 sequentially charges the capacitor C1 through the potentiometer P1, the resistor R4, and the forward diode D3, the diode D5 reversely cuts off the charging voltage of the capacitor C1, so that the charging voltage of the capacitor C1 can only be discharged to the output end of the operational amplifier A2 through the forward diode D4 and the resistor R7; the diode D1 and the diode D2 constitute the temperature coefficient compensation circuit, the output end of the operational amplifier A1 is sequentially connected to the in-phase input end of the operational amplifier A1 through the forward diode D1 and the resistor R3, and the reverse diode D2 is connected in parallel with the forward diode D1; the resistor R6, the resistor R7, the diode D8, the diode D9, the diode D10, the diode D111, the diode D12, the diode D13, the diode D14, the diode D15, the diode D16, the diode D17, the diode D18, the diode D19, the diode D210, the diode D211, the diode D222, the diode D23, the diode D24, the diode D25, the diode D26, the diode D27, the diode D28, the diode D29, the diode D210, the diode D29, the diode D211, the diode D212, the diode D213, the diode D214, the diode D229, the diode D214, the diode D215, the diode D2216, the diode D222 Capacitor C3 constitutes the integration circuit, operational amplifier A2 and capacitor C4 constitute the integration comparator circuit, the output end of operational amplifier A1 is connected to the non-inverting input end of operational amplifier A2 through resistor R6, the non-inverting input end of operational amplifier A2 is connected to the working ground through capacitor C3, the oscillation signal output by operational amplifier A1 generates a DC voltage V2 equal to the average value of the waveform output by operational amplifier A1 through the integration circuit, and the DC voltage V2 proportional to the frequency of the oscillation signal output by operational amplifier A1 enters the non-inverting input end of operational amplifier A2, potentiometer P2 constitutes the zero adjustment circuit, pin 1 of operational amplifier A2 is connected to pin 5 of operational amplifier A2 through the resistor of potentiometer P2, and the sliding end of potentiometer P2 is connected to the working ground; the DC control voltage V1 enters the inverting input end of operational amplifier A2 through resistor R8, and the resistance value of resistor R8 is 1MΩ; the output end of operational amplifier A1 outputs an oscillation signal Fout through resistor R5, and the frequency of the Fout signal is proportional to the DC control voltage V1.

2. A low temperature drift precision voltage-frequency converter according to claim 1, characterized in that: In the multivibrator circuit, the power supply +Vcc is connected to the working ground through the parallel resistor R1, the resistor R3 and the resistor R2 in sequence. The connection point of the resistor R1 and the resistor R2 is the non-inverting input terminal of the operational amplifier A1 to generate a 10V reference voltage. The inverting input terminal of the operational amplifier A1 is connected to the working ground through the capacitor C1.

3. A low temperature drift precision voltage-frequency converter according to claim 1, characterized in that: The integrating comparator circuit, if the DC control voltage V1 is lower than V2, the output of the integrating comparator circuit A2 generates a positive slope sawtooth signal, the discharge rate of the capacitor C1 gradually slows down, and the period of the oscillation signal output by the operational amplifier A1 becomes larger and the frequency becomes smaller; if the DC control voltage V1 is higher than V2, the output of the integrating comparator circuit A2 generates a negative slope sawtooth signal, the discharge rate of the capacitor C1 gradually speeds up, and the period of the oscillation signal output by the operational amplifier A1 becomes smaller and the frequency becomes larger; if the DC control voltage V1 is equal to V2, the output voltage of the integrating comparator circuit A2 remains unchanged, and the period and frequency of the oscillation signal output by the operational amplifier A1 remain unchanged.

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