  The problem of analytic continuation of the scattering data to the
negative-energy region to obtain information on asymptotic normalization
coefficients (ANCs) of bound states is discussed. It is shown that a recently
suggested $\Delta$ method [O.L.Ram\'{\i}rez Su\'arez and J.-M. Sparenberg,
Phys. Rev. C {\bf 96}, 034601 (2017)] is not strictly correct in the
mathematical sense since it is not an analytic continuation of a partial-wave
scattering amplitude to the region of negative energies. However, it can be
used for practical purposes for sufficiently large charges and masses of
colliding particles. Both the $\Delta$ method and the standard method of
continuing of the effective range function are applied to the $p-^{16}$O system
which is of interest for nuclear astrophysics. The ANCs for the ground $5/2^+$
and excited $1/2^+$ states of $^{17}$F are determined.
