A signal acquisition circuit detects a wanted signal in a composite signal containing the wanted signal and an unwanted signal, where the highest frequency in the unwanted signal is higher than the highest frequency in the wanted signal. A sensor captures the composite signal and an analog-to-digital converter samples and converts the composite signal to digital format, and a filter subtracts the unwanted signal from the composite signal. The sampled signal contains a first component containing the sum of the wanted signal and the unwanted signal sampled at a first rate at least equal to the Nyquist rate for the wanted signal but less than a second rate that is at least equal to the Nyquist rate for the unwanted signal, and a second component containing the unwanted signal sampled at the second rate. The analog-to-digital converter outputs to the filter a first digital signal containing the first component sampled at the first rate and a second digital signal containing the second component at the second rate. The circuit is useful for detecting a photoplethysmograph signal in the presence of ambient light in a pulse oximetry sensor.
