MOS switched-capacitor analog sampled-data direct-form recursive filters

A new technique enabling the integration of audio frequency filters using standard MOS technology is described. This approach uses ratioed MOS capacitors, MOS amplifiers and switches to realize precision multiplication, summation, and delay functions. With these elements an analog sampled-data direct-form recursive filter, having the general biquadratic transfer function, was integrated in MNOS technology. This filter had a Q=19/spl plusmn/1 without external trimming and it could be electrically programmed into low-pass, bandpass, and high-pass responses. This biquadratic section can be used as a building block for higher order filters. The direct form switched-capacitor offers some useful advantages in comparison to the switched-capacitor integrator approach. These are the rejection of MOS amplifier noise and power supply noise below one-half the sampling rate, less silicon area especially when implementing high Q poles, and potential for multiplexing two or more filters.