Frequency comparator draws 8 µA
The circuit in Figure 1 includes all the necessary components to build a simple, low-power frequency comparator. The circuit centers around the operation of IC1, which integrates an amplifier, a comparator, and a voltage reference.
The circuit feeds a digital signal to a bandpass filter comprising R1 through R4, C1, C2, and IC1's internal op amp. D1 rectifies the op amp's output signal, and the output of D1 charges C4. When the input frequency is close to the center frequency of the bandpass filter, the voltage on C4 exceeds 1.2V and the comparator output goes high. When R2 equals 910 kiliohms, the filter's center frequency is 1 kHz±100Hz. When R2 is 0, the center frequency is 1 kHz±200 Hz. R5 sets the comparator's hysteresis; C3 filters the reference voltage.
The values of R3, R4, C1, and C2 must be within ±1% and be temperature-stable. The supply voltage can vary between 2.7 and 7V. A change in the input's logic level requires that you adjust R1 to ensure the same bandpass frequency. (DI #1995)