What Is a Frequency to Voltage Converter?

VFC (Voltage Frequency Converter) is a device that realizes analog-to-digital conversion function. It converts the analog voltage into a pulse signal. The frequency of the output pulse signal is proportional to the input voltage. ^[1] Voltage-frequency conversion can also be referred to as voltage-frequency conversion. After the voltage signal is converted into a pulse signal, the anti-interference ability of the signal can be obviously enhanced, and it is also conducive to long-distance transmission. Through the counter interface with the microcontroller, AD conversion can be realized.

Voltage-to-frequency converters are also called voltage controlled oscillator circuits (VCOs), or voltage controlled oscillator circuits for short. Voltage-to-frequency conversion is actually a conversion technique between analog and digital. When an analog signal (voltage or current) is converted into a digital signal, the output of the converter is a series of rectangular waves whose frequency is proportional to the amplitude of the analog signal. Obviously the data is serial. This is different from the parallel output of current analog-to-digital converters, but its resolution can be very high. Serial output analog-to-digital conversion is very useful in digital control systems. It can turn the analog error signal into a pulse signal proportional to it, to drive the stepping servo mechanism for precise control. VFC voltage-frequency converter (vfc) is a circuit produced by Qingdao Transistor Research Institute. ^[1]

There are two common types of VFC: (a) multivibrator VFC; (b) charge-balanced VFC. Multi-vibrator VFC is simple, cheap, low power consumption and has unit MS output (it is very convenient to connect with some transmission media); the accuracy of charge-balanced VFC is higher than multi-resonance VFC. . ^[2]

The frequency of the voltage-to-frequency converter output signal is proportional to (or controlled by) the applied voltage. It can be concluded from this:

(1) If a DC voltage is applied, the voltage-frequency converter is a very convenient signal source for frequency adjustment;

(2) If a sine wave voltage is applied, the voltage-frequency converter is a frequency-modulated oscillator;

(3) If a sawtooth voltage is applied, the voltage-to-frequency converter becomes a frequency sweep oscillator. Therefore, voltage-frequency converters are widely used in circuits such as frequency modulators, frequency sweepers, phase-locked loops, and inexpensive and high-precision analog-to-digital converters and digital-to-analog converters. ^[3]

At present, a variety of monolithic and modular integrated voltage-frequency converters have been produced at home and abroad, and their models are ADVFC32, AD650, AD458, AD460, and so on. They have the advantages of few external components, high conversion accuracy, good temperature stability, and high operating frequency (up to 500kHZ). Therefore, the use of these integrated components, like the use of other dedicated analog integrated circuits, can make the entire system smaller, cheaper, better in performance, and easier to maintain. ^[3]

The voltage-frequency conversion (frequency-voltage conversion) method is widely used in phase-locked technology, analog-to-digital conversion, automatic zero stabilization technology, and multiple data transmission.

Figure 1 A / D converter structure

1. Analog-to-digital conversion

The AD652 plus an external clock and counter can be used as an analog-to-digital converter, the resolution of which depends on the clock frequency and gate time. The structure of the analog-to-digital converter is shown in Figure 1 on the right. The external clock is divided by 2N to obtain the gate time, and the binary data of the counting output is sent to the microcomputer for registration. In order to obtain high-resolution conversion data in a relatively short period of time, the clock frequency must be increased. For example, to obtain 16 resolutions, the clock frequency is 4 MHz, the gate time is 32.77 ms, and the linearity error is 0.02%. ^[2]

2. Auto-zero

The dynamic range of the voltage-to-frequency converter AD650 is very wide, with 6 orders of magnitude.

figure 2

For example, if its input full-scale voltage is 10V, its minimum accurately convertible voltage should be 10V. Under such operating conditions, the converter's input offset voltage becomes extremely important. If the offset voltage is constant, the conversion result will be shifted by a few hertz. If the offset voltage is changed, it is equivalent to having a small input and causing the output frequency to change, so that the dynamic range will be reduced. The circuit shown in Figure 2 can be used to automatically adjust the offset voltage. The AD582 is a sample-and-hold amplifier. Through the analog switch AD7512DI, the AD650 input terminal turns on the input signal v1 or ground, and the output of A ₁ (integrator) is adjusted by AD582 (AD650 pin 13). The holding capacitance of the AD582 is 0.1 F, so that the offset of the AD650 can remain unchanged for a long time. The above-mentioned auto-zero circuit is actually an amplifier servo loop. The amplifier under test is an operational amplifier A ₁ in a voltage-frequency converter. It is controlled by AD582 as a control amplifier, and its output is zero. Resistor (200k) and capacitor (1nF) are connected in parallel, as a single zero compensation of the servo loop, making the loop work stable. v _c is the control input of the circuit, during which the circuit is zeroed. ^[2]