Design Ideas: July 7, 1994
The circuit in Fig 1 uses two ICs, a few passive components, and a standard DMM to measure frequency. In the circuit, a switched-capacitor network, analog switch IC2 and capacitor C1, simulates a frequency-dependent resistor.
The network's resistance is the inverse of the input frequecy, fIN, multiplied by C1's value. If the circuit applies a constant voltage (VREF) across the frequency-dependent resistor, then the current through the resistor is directly proportional to the frequency.
IC1 supplies this 10V reference voltage. Capacitor C2 is a lowpass filter and should be much larger than C1. Table 1 contains values for C1 that provide convenient scale factors. (DI #1448)
| Table 1—Convenient C1 values | |
|---|---|
| Frequency range (kHz) | C1 (mF) |
| 0 to 0.1 | 1 |
| 0 to 1 | 0.1 |
| 0 to 10 | 0.01 |
| 0 to 100 | 0.001 |