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 CO, resistor R, and the OP-07 op amp – must exhibit low drift for stable operation over temperature. The circuit provides linear operation well into the megahertz region.
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| Figure 1. | Using a capacitively loaded IC’s supply-current/frequency dependence, this F/V converter yields 0 to −10 V output with 0- to 10-kHz input frequencies. By paralleling more Schmitt triggers, you can use the circuit as a frequency summer. |
The average current (IAVG) from the 40106 Schmitt-trigger inverter’s ground pin (pin 8) is linearly dependent on the frequency at which CO is discharged into the op amp’s summing junction. The op amp forces this current to flow through the 13.33-kΩ feedback resistor, producing a corresponding voltage drop. The output voltage is
where f is the input frequency. Adjust the 10-kΩ potentiometer to calibrate the converter.
The 1- and 0.1-µF 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 higher-frequency 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 V0 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.
