C6713 DSK: AIC23 precision and antialiasing

Hi,

I have recently migrated from C6711DSK to C6713DSK. I am using a single
sinusoid as input to a loop program (i.e. output set to input). On the
C6711DSK the output resembles the input closely up until 3.4kHz where it
starts to attenuate due to antialiasing filter. However, on the C6713DSK I
have encountered a few puzzling issues. The CRO output has visible steps at
higher frequencies, f.e. at 2.5kHz there are roughly 16 steps. On the other
hand at low frequencies (say 500Hz) the steps are not visible. Please refer
to the photos on the following website:

http://picasaweb.google.com/Kamil.Wojcicki/C6713DSK

Is this 'normal'? Shouldn't the precision of the output be the same across
the entire frequency range of interest (0<->Fs/2=4000Hz)? This kind of
resembles subband coding... Could this issue be due to delta-sigma
technology on which AIC23 is based? Any clues are appreciated.

Kamil