Lowpass filter has improved step response

R4 and C3 provide lowpass-filtering to the original input signal, producing a 312-Hz cutoff frequency that reduces sensitivity to momentary glitches. The filtered input drives the window comparator's input. If that input is outside the ±50-mV window, comparator IC2A or IC2B asserts its output low. The low output drives Q5 into cutoff, causing its collector to assume a high impedance. Because Q5's collector no longer grounds capacitor C2, the filter's cutoff frequency increases by a factor of 10. When the system's output changes to within 50 mV of the input, the cutoff frequency throttles back to its quiescent state. Figure 2's oscilloscope photo shows the effect. The top trace is a step from 1.5 to 2.5V, the middle trace is the output with optimization circuitry enabled, and the bottom trace shows the filter's unmodified response. The optimized response includes a slight perturbation during the cutoff-frequency transition, but is five times faster than that of the unmodified circuit. The circuit in Figure 1 is configured for low cutoff frequencies, but you can rescale it for higher frequencies by changing C1 and C2. You can also modify R2 and R3 for different window values, for which the delta equals the resistance multiplied by 115 µA. The comparator must be an open-drain type.