AN-C| Application Note

V/F Converter ICs Handle Frequency-to-Voltage Needs

V/F Converter ICs Handle Frequency-to-Voltage Needs
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. Figure 1 shows an LM331 IC (or LM131 for the military temperature range) in a basic F/V converter configuration (sometimes termed a stand-alone converter because it requires no op amps or other active devices other than the IC). (Comparable V/F ICs, such as RM4151, can take advantage of this and other circuits described in this article, although they might not always be pin-for-pin compatible). This circuit accepts a pulse-train or square wave input amplitude of 3V or greater. The 470 pF coupling capacitor suits negative-going input pulses between 80 us and 1.5 us, as well as accommodating squa
re waves or positive-going pulses (so long as the interval between pulses is at least 10 us).

National Semiconductor Application Note C Robert A. Pease August 1980

t = 1.1 RtC. The 1 uF capacitor filters this pulsating current from pin 1, and the current's average value flows through load resistor RL. As a result, for a 10 kHz input, the circuit outputs 10 VDC across RL with good (0.06% typical) nonlinearity. Two problems remain, however: the output at V1 includes about 13 mVp-p ripple, and it also lags 0.1 second behind an input frequency step change, settling to 0.1% of full-scale in about 0.6 second. This ripple and slow response represent an inherent tradeoff that applies to almost every F/V converter.

The Art of Compromise
Increasing the filter capacitor's value reduces ripple but also increases response time. Conversely, lowering the filter capacitor's value improves response time at the expense of larger ripple. In some cases, adding an active filter results in faster response and less ripple for high input frequencies. Although the circuit specifies a 15V power supply, you can use any regulated supply between 4 VDC and 40 VDC. The output voltage can extend to within 3 VDC of the supply voltage, so choose RL to maintain that output range. Adding a 220 k/0.1 uF postfilter to the circuit slows the response slightly, but it also reduces ripple to less than 1 mVp-p for frequencies from 200 Hz to 10 kHz. The reduction in ripple achieved by adding this passive filter, while not as good as that obtainable using an active filter, could suffice in some applications.

IC Handles the Hard Part
The LM331 detects an input-signal change by sensing when pin 6 goes negative relative to the threshold voltage at pin 7, which is nominally biased 2V lower than the supply voltage. When a signal change occurs, the LM331's input comparator sets an internal latch and initiates a timing cycle. During this cycle, a current equal to VREF/RS flows out of pin 1 for a time

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FIGURE 1. A Simple Stand-Alone F/V Converter Forms the Basis for Many Other Converter-Circuit Configurations

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2002 National Semiconductor Corporation

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