Metastability Determines the Noise in Fast and Accurate A/d Converters

We have investigated metastability errors in algorithmic A/D converters. When the input signal is inside a window, VM, a metastability errors appear as a spike in the output code. These errors looks like white noise in the frequency domain, and can therefore be compared with the quantization noise. For an n-bit ADC, the probability of a metastability error in bit j is pMj = VM · 2 (n-j), error amplitude is AMj = 2 (j-2), and power contribution from these errors is P = VM ·2 (n-4)·(2(n+1) 2). For a metastability noise less than the quantization noise the relation VM < VQ/(2 (n-1)⋅3), where VQ is the quantization step, has to be fulfilled. The metastability problem determines the speed and gain requirements on the comparator. The relations are validated with measurements and simulations of an SA-ADC. A method for detecting and correcting these errors is indicated.