ADAU1978 at 192 KHz: noise at high frequencies

Hi everyone,

I'm adopting ADAU1978 to acquire single ended MEMS microphones. When running at 192 KHz I notice a growth in the noise at high frequencies, starting from 30 KHz with a slope of about 3/4 dB per octave (I can send a picture of the FFT if needed) This happens even if the microphone is not connected and the 2 pins are tied to ground.

All the other sampling frequencies are working just fine (I tested 8, 16, 32, 48, 96 and 192 KHz).

I'm using the ADC in slave mode, without MCLK (PLL clock source is LR clock).

Here's the register configuration (including the reserved ones):

M_POWER          [0]   (0x01)
PLL_CONTROL      [1]   (0xD1)
RESERVED         [2]   (0x4A)
RESERVED         [3]   (0x7D)
BLOCK_POWER_SAI  [4]   (0xBF)
SAI_CTRL0        [5]   (0x04)
SAI_CTRL1        [6]   (0x50)
SAI_CMAP12       [7]   (0x10)
SAI_CMAP34       [8]   (0x23)
SAI_OVERTEMP     [9]   (0xF0)
POSTADC_GAIN1    [10]  (0xA0)
POSTADC_GAIN2    [11]  (0xA0)
POSTADC_GAIN3    [12]  (0xA0)
POSTADC_GAIN4    [13]  (0xA0)
MISC_CONTROL     [14]  (0x02)
RESERVED         [15]  (0xFF)
RESERVED         [16]  (0x0F)
RESERVED         [17]  (0x00)
RESERVED         [18]  (0x00)
RESERVED         [19]  (0x00)
RESERVED         [20]  (0x00)
RESERVED         [21]  (0x20)
RESERVED         [22]  (0x00)
RESERVED         [23]  (0x00)
RESERVED       [24]  (0x27)
ASDC_CLIP        [25]  (0x00)
DC_HPF_CAL       [26]  (0x00)

Thanks