Component Pre-distortion for Sal

　　This revision obsoletes the previous revision of this Application Note， and covers additional material. This Application Note shows a simple component pre-distortion method that works for many popular Sallen-Key （also called KRC or VCVS ［voltage-controlled， voltage-source］） filter sections. This method compensates for voltage-feedback and current-feedback op amps. Several examples illustrate this method. KRC active filter sections use an op amp and two resistors to set a non-inverting gain of K. resistors and capacitors placed around this amplifier provide the desired transfer function. The op amp’s finite bandwidth causes K to be a function of frequency. For this reason， KRC filters typically operate at frequencies well below the op amp’s bandwidth （f 《《 f3dB）。 ’Pre-distortion’ compensates for the op amp’s finite bandwidth by modifying the nominal resistor and capacitor values. The pre-distortion method in the Application Note compensates for the op amp’s group delay which is approximately constant when f《《 f3dB. One possible design sequence for KRC filters is： 1. Design the filter assuming an ideal op amp （K is assumed constant over frequency） • Select components for low sensitivities • Do a worst case analysis • Do a temperature analysis 1. Pre-distort the resistors and capacitors to compensate for the op amp’s group delay 2. Compensate for parasitic elements