| MS Seminar

Name of the Speaker: Mr. KISHAN (EE23S035)
Guide: Dr. Nagendra Krishnapura
Venue: ESB-210B (Conference Hall)
Date/Time: 25th November 2025 (Tuesday), 2:30 PM
Title: Design techniques for extending the frequency range of distortion suppression in opamp-based active filters.

Abstract :

High-resolution ADCs require a sine wave source of very low form factor to test their performance. Sine wave generators currently available have a large form factor or are very expensive, making them unsuitable for supporting multiple measurements simultaneously. This work explores one method of generating a sinusoid with very low Total Harmonic Distortion (THD) by designing a low-distortion filter that can filter out harmonics in a source. A Band Pass Filter (BPF) is used to suppress the harmonics of a -80dB THD sine wave to -140dB THD. Prior work in the literature demonstrates -140 dB THD up to 10kHz. This work extends the low distortion frequency range to 100kHz. A fully differential Tow-Thomas biquad filter is used to implement the bandpass filter because of its simplicity. The distortion of the input stage of the opamp and the capacitor nonlinearity are the dominant contributors. Additionally, nonlinearity cancellation methods that work well up to 10kHz fail at 100kHz due to additional phase shifts. Methods to mitigate these are proposed in this work. The design is implemented using 0.6μm technology from Texas Instruments. A single bandpass filter stage (including the nonlinearity cancellation circuit) consumes ≈ 40mA of current from a 6V supply. The BPF is implemented with Q = 7, providing -20dB suppression of HD2 and -27dB suppression of HD3. Three of these BPFs can be cascaded to get 60dB attenuation near the second harmonic. For a 10Vppd sine wave applied, the THD of the bandpass filter achieved from RC extracted simulations is ≈ −141dB at 100kHz.