Integrated sensor interface circuits require power-efficient high-accuracy data converters. In many applications, the best choice is the incremental A/D converters (IADCs) incorporating extended counting. In this paper, we discuss the operation and design of single- and multi-stage IADCs. By using a direct-input feed-forward modulator, the IADC accumulates the residue voltage, and it is easy to implement a hybrid scheme of extended counting. Several hybrid schemes are reviewed and discussed. The energy efficiency c an be improved significantly by re-using the hardware to perform extended counting. A multi-step extended counting scheme is proposed for high-resolution power-efficient conversion. A design example with OSR=320 is described in this paper. By using multi-step extended counting, the SQNR performance is boosted to 108 dB, which is much better than that of a second-order IADC of the same OSR, 94 dB.