A 0.5 V 5th-Order Low-Pass Elliptic Filter
To demonstrate the capabilities and synergy of the proposed ultra-low voltage design techniques in Chapters 2 and 3, we designed a 5th-order low-pass elliptic filter with a 135 kHz cut-off frequency. For minimum sensitivity requirements, a leap-frog topology was used. The design in  was frequency-scaled to 135 kHz, such that the signal amplitude maxima at all OTA outputs are at the same level. The filter characteristics has a pass-band ripple of 0.1 dB, a stop-band rejection of at least 35 dB, and two zeros in the stop-band – at 180 kHz and at 280 kHz. To obtain an accurate transfer characteristic, the OTA should have substantial open-loop gain all the way to 280 kHz, the second zero of the filter. The proposed amplifier has a worst-case gain of 20 dB at 280 kHz, which is sufficient. The filter resistors and capacitors were scaled so that the total noise contributions from the OTAs and from the resistors, integrated in the pass-band, are equal.
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