SSM2604 ADC noise

This is the first prototype we use SSM2604 with our DSP working, and we find there is ADC noise at the RIN and LIN, the input signal is 97Hz, 200mVpp sine, but the frequency we see is not stable, sometimes it jumps to 2 times frequency, that is 194Hz, the MCLK is about 12.29MHz, BCLK 3MHz, LRCK 48kHz,when we change the input signal to 40Hz, we can see the frequency jumps to 76Hz, and when signal input changes to 97Hz, 350mVpp sine, we increase the amplitude, the frequency is stable then. so the question is for the low frequency with low amplitude signal, especially frequency lower than 100Hz, amplitude lower than 200mVpp, the ADC capacity seems not enough, is there any solution to solve this issue? or any layout comments to improve for the signals, both digital and analog. Thanks.