A sigma-delta modulation based analog adaptive filter

In analog implementation, analog multiplier accuracy, circuit complexity, and lack of a lossless analog delay line have made it difficult to implement least-mean-square (LMS) analog adaptive filter structures. The authors propose an analog adaptive filter structure based on pulse density modulation to circumvent these problems by using a digital delay line and an analog tap value calculation of the adaptive algorithm. Sigma-delta modulation is used as an efficient conversion scheme to the binary representation, and the binary stream is processed using analog techniques. The adaptive filter can be realized using switched capacitor integrators for filter coefficients and only 1-b word length for the delay line. The high updating frequency of the filter taps leads to fast adaptation