Simple Voltage-to-Frequency Converter

Early this year I got my Digilent Analog Discover Kit. It is a wonderful PC based design and experiment kit complete with software measurement and resource tools perfect for students and professionals alike, but students surely will benefit the most out of it.The complete Discovery kit includes many useful exciting components that will enable you to design your own circuit in your living room!

One cute little component it has is the AD654, a low-cost IC voltage-to-frequency converter. It is a very simple to use IC that outputs a square wave with programmable frequency, set by just a single RC network. The scaling relationship as obtained from its datasheet is:

Below is a simple V-F circuit that takes its input from another IC included in the kit, the AD584, a precision programmable voltage reference.

Figure 1 V-F Converter

The AD654 accepts full-scale current up to 2mA, but the best linearity is offered at 1mA full-scale. The voltage input is converted to a drive current by the resistor installed at the pin 3 of the device, with the other end connected to the ground. No current flows into the AD654 positive input (pin 4), so it doesn't load my reference. I have configured the voltage reference to 2.5V, and have used a voltage divider to scale the output down for better linearity of the voltage-to-frequency converter operation. The AD584 programmable reference will be an excellent choice for its accuracy and flexibility, and can be easily configured to obtain a desired output. Making a voltage-to-frequency converter with good accuracy and linearity requires that the input be precise and has minimized error over time as well. A precision reference fits the job. The output frequency can be made adjustable by a trimming resistor or a potentiometer. A 1V input produces 10KHz square wave output. Snapshot from the kit's oscilloscope tool shows the output adjusted to 9KHz.

Figure 2 Digilent Analog Discovery Kit Oscilloscope Tool

Inputs to the voltage to frequency converter can come from a variety of sources in the real world applications, such temperature and other small signal measurements. Its output can be interfaced to a microcomputer that can determine the output frequency to accurately represent and interpret the input.

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