This paper studies the tonal behaviour of the quantization noise in 2nd-order bandpass Sigma-Delta modulators. Closed-form expressions for the frequency of the idle tones are derived for different locations of the signal centre frequency. The analytical results are validated through experimental measurements taken from a 0.8 μm CMOS prototype realized using fully differential switched-current circuits.This work has been partially supported by the Spanish CICYT under contract TIC2001-0929, ADAVERE.Peer reviewed