This paper investigates the word length requirement of an analog-to-digital (ADC) converter for coherent and non-coherent detection of On-Off Keying (OOK) schemes. The paper presents closed-form expressions for coherent and non-coherent detection in terms of bit error rate in the presence of quantization noise (QN) and additive white Gaussian noise (AWGN) channels. The analytical models show that for coherent as well as non-coherent demodulation of OOK schemes in AWGN channels and affected by QN, a 4-bit ADC is able to provide close to optimum performance. As OOK is popularly being employed in event-driven radios, we extend our analysis to these radios. Our analysis shows that for event-driven radios (also referred to as wakeup radios) a 4-bit ADC is able to provide close to optimum performance. Furthermore, in the paper all the analytical models are in close agreement with the simulation results.