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The theory and practical implementation of an adaptive filter of the TX leakage in CDMA receivers are described. The filter uses a continuous-time signa-data LMS algorithm. It was designed as part of a 0.25 μm CMOS cellular-band receiver. It achieved the maximum rejection of the TX leakage of 28-30 dB, which was limited by the reference signal leakage to the LNA output. The LNA NF and gain degraded by 1.3 dB and 1.7 dB respectively after adding the filter.