Abstract: A diode connected P-type CMOS transistor is operated in the sub-threshold area and, with a bypass capacitor, operates as a low pass audio filter. The equivalent resistance of the CMOS transistor in the sub-threshold range is very high—in the gigaOhm range. With this size resistor, a capacitor in the 1-25 pF range may be used to provide filtering capabilities with break points in the 1-10 Hz frequency range. Such a filter provides an effective low pass filter that attenuates audio frequency signals. The 1-25 pF capacitors use little chip area making the arrangement practical for integrating on an IC with the audio signals. In one embodiment, a digital signal and the audio signals share one pin, where the audio signal appears only when the digital signal is high. In this case, the audio signal filtered out from the digital high signal. The filtered digital signal drives digital circuitry while the audio signal is directed to other audio circuitry.

Abstract: A diode connected P-type CMOS transistor is operated in the sub-threshold area and, with a bypass capacitor, operates as a low pass audio filter. The equivalent resistance of the CMOS transistor in the sub-threshold range is very high—in the gigaOhm range. With this size resistor, a capacitor in the 1-25 pF range may be used to provide filtering capabilities with break points in the 1-10 Hz frequency range. Such a filter provides an effective low pass filter that attenuates audio frequency signals. The 1-25 pF capacitors use little chip area making the arrangement practical for integrating on an IC with the audio signals. In one embodiment, a digital signal and the audio signals share one pin, where the audio signal appears only when the digital signal is high. In this case, the audio signal filtered out from the digital high signal. The filtered digital signal drives digital circuitry while the audio signal is directed to other audio circuitry.