AD7960 incorrect data values

Ok, thanks for your replies so far. I think I need your help once again. I connected a wave generator to the adc and collected 8192 points of a sinusoid. The problem is that the measured results are not as expected and I haven't found a solution til now.

The problem is that I only get positive values and they are all smaller that 131072 in integer, so there is now problem with the two's complement. When I plot the results the curve looks like a sinusoid in a little part. There are many peaks and steps around. So could the problem be since I only connected Vin+ to my wavegenerator? Or is there something to swap with the endianess of the results?

best regards.