Peter Winship EDN Using only six components, you can configure a circuit (Figure 1) whose output voltage is proportional to its input frequency. Moreover, only three of the components – capacitor C_{O}, resistor R, and the OP07 op amp – must exhibit low drift for stable operation over temperature. The circuit provides linear operation well into the megahertz region.
The average current (I_{AVG}) from the 40106 Schmitttrigger inverter’s ground pin (pin 8) is linearly dependent on the frequency at which C_{O} is discharged into the op amp’s summing junction. The op amp forces this current to flow through the 13.33kΩ feedback resistor, producing a corresponding voltage drop. The output voltage is where f is the input frequency. Adjust the 10kΩ potentiometer to calibrate the converter. The 1 and 0.1µF capacitors smooth the transients that result from the rapid switching. For the figure’s values, the output ranges from 0 to−10 V for inputs of 0 to 10 kHz. If you need higherfrequency operation, you must consider the effects of rapid switching on the CMOS_{ }inverter’s supply current. Because a CMOS IC’s power dissipation is proportional to frequency, you can simply add its supply current to the capacitor’s discharge current in the calculations. You can make a frequency summer by exploiting the fact that there are six Schmitt triggers per package: Attach a capacitor to each inverter’s output, apply a different frequency to each input, and obtain V_{0} proportional to the sum of the input frequencies: Moreover, you can extend the technique by paralleling additional ICs. With the figure’s component values, the F/V converter yields ±0.4% max nonlinearity for inputs of 0 to 10 kHz.
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