A phase-domain analog-to-digital converter (PhADC) is a promising alternative to a pair of amplitude-domain in-phase and quadrature (IQ) ADCs for low power FSK/PSK demodulation, but the fundamental benefits and limitations of the PhADC over the IQ ADC have not been precisely quantified as yet. In this paper, analytical methods are proposed to comprehensively compare the PhADC and the IQ ADC. Phase signal-to-noise ratio (SNR) expressions of the two ADC types are formulated analytically to facilitate a quantitative comparison of them. In comparison with the IQ ADC, the PhADC is a more compact quantization and demodulation solution when interference accommodation is not required. Moreover, considering a flash ADC as an example of the low resolution (3-4 bit) IQ ADC, the PhADC has a lower theoretical energy limit than the flash ADC for a given phase ENOB. IQ offsets and amplitude mismatch impose unique nonlinearities on the PhADC due to the nonlinear amplitude-to-phase conversion. The understanding of this nonlinearity leads to a phase-domain mismatch and offset detection technique. Phase SNR is explicitly related to input noise for both ADCs, and to comparator offsets for the PhADC, respectively. All of the results prove that the PhADC is a promising quantization and demodulation solution.

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