ELIMINATE SALLEN-KEY STOPBAND LEAKAGE WITH A VOLTAGE FOLLOWER

摘要

The Sallen-Key lowpass filter is a form of the VCVS (voltage-controlled voltage-source) topology. Unfortunately, the interaction of the op-amp output impedance with the filter input degrades stopband performance (Figure 1). If the output impedance of the amplifier were zero, the stopband would continue to drop indefinitely with increasing frequency. This behavior corresponds to the ideal stopband response. The stop-band-leakage effect has been the topic of various articles. This article details a method of decreasing or eliminating the stopband leakage by modifying the feedback loop of the filter section itself, rather than by adding more filter sections. You can achieve 50-dB decreases in stopband leakage without loading or modifying the output of the filter circuit.