Frequency to Voltage

Frequency to voltage converters are available in a number of forms from a number of sources, but invariably require significant additional components before they can be put to use in a given situation. The LM2907, LM2917 series of devices was developed to overcome these objections. Both input and output interface circuitry is included on chip so that a minimum number of additional components is required to complete the function.

Simplify your F/V converter designs with versatile V/F ICs. Starting with a basic converter circuit, you can modify it to meet almost any application requirement. You can spare yourself some hard labor when designing frequency-tovoltage (F/V) converters by using a voltage-to-frequency IC in your designs. These ICs form the basis of a series of accurate, yet economical, F/V converters suiting a variety of applications.

The circuit in Figure 1 stems from a radio-controlled modeling application, which requires a voltage proportional to the width of the incoming servo pulses. The circuit is optimized for a positive-going pulse width of 1 to 2 msec, repeating at intervals of approximately 17 msec. The output produces a voltage of 0.95V for a 1-msec pulse to 2.25V for a 2-msec pulse. The circuit operates similarly to a PLL, but it locks onto the pulse width, rather than to the frequency, of the incoming signal.

The circuit in Figure 1 is a simple, low-cost voltage-to-time converter using the ubiquitous 555 timer chip. You can use the IC's monostable multivibrator as a voltage-to-time converter by connecting the analog-voltage input to the charging resistor, R, instead of connecting R to VCC. With this modification, the timer chip's output-timing cycle, tP, is proportional to the input voltage, VIN.

The AD650 is a versatile monolithic voltage-tofrequency converter (VFC) that utilizes a chargebalanced architecture to obtain high performance in many applications. Like other charge-balanced VFCs, the AD650 can be used in a reverse mode as a frequency-to-voltage (F/V) converter. This application note discusses the F/V architecture and operation, component selection, a design example, and the fundamental trade-off between output ripple and circuit response time.

The improved circuit in Figure 2 makes an end-run around these compromises. A low-cost sample-and-hold circuit such as LF398 can sample the F-to-V's output at the peak of its ripple, and hold it until the next cycle. The LF398 has fairly low output ripple (rms) but it does have some short duration noise spikes and glitches which can be removed easily with a simple output filter.

The circuit in Figure 1 converts pulse information to a clean dc voltage by the end of a single incoming pulse. In another technique, an RC filter can convert a PWM signal to an averaged dc voltage, but this method is slow in responding. Converting low-duty-cycle pulse information is slower yet. The circuit in Figure 1 uses two low-input-bias-current LT1880 op amps, IC2 and IC3, and an LTC202 quad analog switch,

The circuit in Figure 1 is a low-cost frequency-to-voltage (F/V) converter. Using a monostable (one-shot) multivibrator, the circuit accepts an open-collector square wave that varies in frequency from 0 to 10 kHz. The one-shot produces a pulse of a fixed width each time the input signal triggers it.

This circuit is designed to detect the narrow 1uS pulses produced by the above amplifier circuit. The clean logic type pulses produced by the discriminator are then sent to a frequency to voltage converter. The circuit is designed to process a pulse frequency of 10KHz that is frequency modulated by voice audio signals.

This PLL can operate over a wide frequency range, not just 1 or 2 octaves but over 1 or 2 or 3 decades. It naturally provides a voltage output which responds quickly to frequency changes, yet does not have any inherent ripple. Thus, it can be used as a frequency-to-voltage (F-to-V) converter

This Design Idea shows how you can use a frequency-to-voltage converter and a DDS (direct-digital-synthesizer) chip for precise digital-to-analog conversion. The DDS chip generates a precision frequency proportional to its digital input. This frequency serves as the input to a voltage-to-frequency converter, thereby generating an 18-bit analog voltage proportional to the original digital input. Figure 1 shows how the AD650 is configured for frequency-to-voltage conversion.

The high-accuracy frequency-to-voltage converter (FVC) in Fig 1 demonstrates how a synchronous, charge-balance, voltage-to-frequency converter (VFC) can function as a single-supply FVC given proper biasing and level shifting. This FVC can maintain a monotonic 0.01% linearity error over a 60-dB range of 9.7 kHz to 9.7 MHz; it operates from 12 to 36V power supplies. You can modify the circuit’s prescaler to adapt the circuit to considerably higher frequency ranges.