A sigma-delta analog-to-digital converter-offers advantages such as noise shaping and high frequency operation. However, a sampling circuit needed to provide a highly oversampled discrete-time sample stream with low noise characteristics is difficult to design and implement. The present invention provides a sigma-delta mixer 300 with such a sampling circuit 310. The present invention discloses a sampling circuit using switched capacitors 307, 308, and 309 with low noise characteristics and at the same time is capable of providing a highly oversampled discrete-time sample stream.