I am working with the ADAU1372 connected to an fpga evaluation board. I send and receive the data via I2S. Now I face the problem that I see random peaks on the DAC Output. For synchronization MCLK comes from the fpga board with 26 MHz. PLL activated on the ADAU1372 like described in the data sheet and works as master. First I thought the peaks arise out of problems in my IPCore on the fpga, but when I disconnected the GND between the two board the peaks nearly disappeared. I also tried to disconnect the USB power supply and use an external supply. Nevertheless the peaks are there. The fgpa board has a seperate power supply. I also recognized that I need to set the drive strengh to high. I placed no additional resistors in the data signals. With drive strengh on low there was no output on ADC_SDATA0 at all. Is it necessary to place an Isolator between the two boards for a proper functionality?