A general technique for the design of undersampled LC bandpass modulators using feedback FIRDACs is proposed. The coefficients of the FIRDACs are used to increase the degrees of freedom in order to perform an exact equivalence between undersampled LC bandpass Sigma-Delta and high order discrete-time sigma-delta modulators. Using FIRDACs coefficients, it is also possible to simplify the circuit implementation by removing internal summing nodes and by decreasing coefficients spread. An undersampled 4th order LC sigma-delta is given as a design example. The effect of the undersampling ratio on the performance of finite quality factor LC sigma-delta modulators is also studied.