LM331 Voltage to Frequency Converter Circuit

LM331 Voltage to Frequency Converter Circuit

A Voltage to Frequency (VTF) converter is a circuit, which converts a voltage level to a linearly proportional frequency. A special configuration of the LM331 IC performs this conversion. Interestingly, this same IC also performs the FTV conversion as shown in the previous pages of this article. In the VTF configuration, the input voltage "VIN" feeds pin 7, and a proportional frequency "fOUT" appears at the output pin 3.

A 25 kΩ preset in series with the 91 kΩ resistor at pin 2 provides the precise 100 kΩ resistance that is required. It has to be very precise to compensate for any tolerance errors, which is why a preset is used.

Simplifying the Formula

fOUT = (VIN / 2.09V) × (RS/RL) × (1/(Rt × Ct))

To keep the same linearity as before with the FTV conversion stage, RS = RL = 100 kΩ, hence RS / RL = 1.

The parameter V is the same as before, which is 12-volts.

The timing value of the resistor Rt = 4 kΩ, and capacitor Ct = 0.01 µF, which is the same as in the previous FTV circuit. Hence, the equation simplifies to that shown below.

fOUT = 996.81 × VIN


Calculating the Frequency Parameters

From this, we can calculate the frequency to expect for the maximum and minimum input voltages. Hence, when VIN = 8 V, fOUT = 7974 Hz, and when VIN = 0.53 V, fOUT = 528 Hz.

These frequencies are what one would expect and are approximately 16 fold to the input frequencies. This also proves that the equations work, and now all that remains is to build the circuit. Due to variations in component tolerances, the actual figures obtained from the circuit may vary, however they should be within the ballpark values. There is also the option of fine-tuning the whole circuit with presets, which should provide greater accuracy.

This Article Continues...

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