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An integrated lock-in amplifier for sensor applications is demonstrated. The circuit has been fabricated in a 0.7µm n-well CMOS process with analog features. The equivalent input noise is 17 nV/√Hz at 20 KHz, and the low-pass filter cut-off frequency can be varied in the sub-Hz range. The ability of resolving input steps of 100 nV at 20 KHz in the presence of a 32µV interference at only 5 Hz frequency offset is demonstrated.