The builtin filter of Delta44The -3dB point of the builtin filter is at 46.3kHz and the attenuation is only 10dB at the Nyquist frequency 48kHz. Adequate attenuation, -60dB is obtained at 52kHz making the useful range 0 to 44kHz.
Low pass filter with notchSince the builtin filter is perfectly ok above 53 kHz the additional filter does not have to attenuate above this frequency. To get a very steep filter around 48kHz notches are used, see fig.1. Having the components at hand, the easiest way of producing notches is to use LC series links. Filter design in the audio range is conventional engineering and many other ways are possible to realise a filter with notches slightly above 48kHz. The useful frequency range is increased by about 2kHz by this filter and it is certainly questionable if it is worth the trouble of adding it to increase the total bandwidth from 88 to 92kHz (4.5%)