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In order to overcome the difficulty of realizing an optimal likelihood-ratio receiver, suboptimal approximations are used. One of the simplest approaches is to expand the likelihood ratio and truncate the expansion under the weak signal assumption. In this context, relations between the Taylor expansion and the Volterra expansion are studied. Emphasis is on the links between the Taylor expansion, with respect to the SNR and the detection criteria, and robustness, under a weak signal assumption. In particular, using two criteria derived from deflection, it is shown that the minimax robust filter is the matched filter. An application to underwater signal detection is studied, showing that the degradation resulting from the use of the locally optimal receiver is small.