Effect on the system’s time-domain response is a common problem when designing lowpass filters. The system may fail to recognize significant changes in time because pushing the cuttoff frequency lower slows the step response. The circuit below is presented to solve the problem:
On this circuit diagram, lower cuttoff frequency is allowed without sacrificing the step-response time. The delta (difference) between the filter’s input and output is monitored by window comparator. The filter increases its slew rate by increasing its cuttoff frequency an order of magnitude when the delta exceeds 50mV. Lowpass-filtered by R4 and C3 is the original signal which is produce a cuttoff frequency(312Hz) that reduces sensitivity to momentary glitches. The window-comparator input is drove by the filtered input. Comparator U2A or U2B will assert its output low if the input is outside the 50mV window. The low output drives Q5 into cutoff, causing its collector to presume a high impedance. The filter’s cutoff frequency increases by ten times because the Q5 collector no longer grounds capacitor C2. The cutoff frequency throttles back to its quiescent state When the system output changes to within 50mV of the system input. This circuit diagram is configured for very low cutoff frequency, but changing C1 and C2 can rescale the configuration to higher frequency, where the oscillation frequency fOSC (in kHz) is 30 x 103/COSC (in pF) and the cutoff frequency is fOSC/100. For different window values in which the delta equals the resistance multiplied by 115µA, we can modify R2 and R3. The type of comparator must be an open-drain type. [Source: maxim-ic.com]
Possibly Related Articles:
- Differential Low-Pass Filter and The Design Formula
Multiple Feedback (MFB) is the best filter topology to use for differential low pass filters. Here’s the circuit diagram: To design this filter, a corner frequency is chosen based on... - Second Order Butterworth Low-Pass Filter
A second order filters can be constructed using LC (inductor-capacitor) passive circuits, and they have been implemented for radio frequency for cheap and simplicity. For audio frequency range, unfortunately the... - Butterworth Second Order High-Pass Filter: The Circuit and The Formula
For high pass filter, this circuit is similar to low-pass filter circuit, but the position for resistors and capacitor are interchanged. Here is the circuit’s schematic and formula: Similar with... - First Order Low-Pass Active Filter: The Circuit Schematic Diagram and The Design Formula
The simple low-pass filter is shown in Figure below. The filter has a 20dB/decade roll-off after its corner frequency fc. Its low-pass frequency gain (f->0) is defined by R3/R1. This... - Transistor Hum Notch Filter
The function of this circuit is similar with the hum filter in our previous circuit, but it use transistors instead of op-amps. It uses BC109 transistors, but any low noise...
| 
