Frequency to Voltage Converter Circuit

The previous post explained a simple single chip voltage to frequency converter circuit using the IC VFC32, here we learn how the same IC could be used for achieving an opposite frequency to voltage converter circuit application.

Using the VFC32 Configuration

The figure below depicts another standard VFC32 configuration which enables it to work as a frequency to voltage converter circuit.

The input stage formed by the capacitive network of C3, R6 and R7 make the comparator input compatible with all 5V logic triggers.The comparator in turn toggles the associated one-shot stage on every falling edge of the fed frequency input pulses.

Circuit Schematic

The threshold reference input set for the detector comparator is around –0.7V. In case where the frequency inputs may be lower than 5V, the potential divider network R6/R7 can be appropriately adjusted for changing the reference level and for enabling proper detection of the low level frequency inputs by the opamp.

As shown in the graph in the previous article, the C1 value may be selected depending upon the full scale range of the frequency input triggers.

C2 becomes responsible for filtering and smoothing the output voltage waveform, bigger values of C2 help to achieve better control over voltage ripples across the generated output, but the response is sluggish to rapidly varying input frequencies, whereas smaller values of C2 cause poor filtration but offer quick response and adjustment with the fast changing input frequencies.

R1 value could be tweaked for achieving a customized full scale deflection output voltage range with reference to a given full scale input frequency range.

How the Frequency to Voltage Converter Circuit Works

The basic operation of the proposed frequency to voltage converter circuit is based of a charge-and-balance theory. The input signal frequency is calculated to be conforming the expression V)(in) / R1, and this value is processed by the relevant IC opamp through integration with the aid of C2. The result of this integration gives rise to a falling ramp integration output voltage.

While the above takes place, the subsequent one-shot stage gets triggered, connecting the 1mA reference current with the integrator input in the course of the one-shot operation.

This in turn flips the output ramp response and causes it to climb upward, this continues while the one-shot is ON, and as soon as its period elapses the ramp yet again is forced to change its direction and causes to revert to the downward falling pattern.

Calculating the Frequency

The above oscillating response process enables a sustained balance of charge (average current) across the input signal current and the reference current, which is solved with the following equation:

I(in) = IR (ave)
V(in) / R1 = fo tos
(1ma)
Where fo  is the frequency at the outputt is the one-shot period = 7500 C1 (Frarads)

The values for R1 and C1 are appropriately selected so as to result a 25% duty cycle on full-scale output frequency range. For FSD which may be above 200kHz, the recommended values would generate around 50% duty cycle.

Application Hints:

The best possible application area for the above explained frequency to voltage converter circuit is where the requirement demands a translation of a frequency data into a voltage data.

For example this circuit can be used in tachometers, and for measuring speeds of motors in voltage ranges.

This circuit can be thus used for making simple speedometers for 2 wheelers including bicycles etc.

The discussed IC can be also used for achieving simple, inexpensive yet accurate frequency meters at home, using voltmeters for reading the output conversion.