This paper proposes a method to introduce BAW resonators in the loop of a continuous-time sigma-delta modulator. The method is based on the equivalence between the noise-transfer-function of a conventional bandpass discrete-time sigma-delta and the noise-transfer-function of a continuous-time BAW-based sigma-delta modulator with FIRDACs. The method is general and can be applied to BAW resonators with and without cancellation of the anti-resonance frequency. The noise-transfer-function and the signal-transfer-function of the BAW-based sigma-delta modulator are analyzed and compared with their discrete-time counterpart.