Hello i am Alessandro, i am developing and audio system based on:
Musical Instrument (2Vpp) -> Input OpAmp (Single Ended) OPA134 -> ADC PCM1804 -> FPGA -> DAC PCM1793 -> Output OpAMP (differential) -> Musical Amplifier (100W)
The PCB has only two layer without separated AGND and DGND (saving costs etc), but it works good because i had only small floor noise that was very few audible at amplifier maximum output level without playing, and playing the SNR was very good.
I had no attenuation on input stage and no gain on output stage
I had to change the input stage because the ADC worked too near at its maximum level and sometime adc output saturated, so if the input signal was bigger (with some instruments happens), the sound clipped too early.
So i attenuated the input signal with the Input OpAmp and i added gain on the output OpAmp to match input/output signal, but the noise that before was little audible, became bigger and now the SNR isn't accetable anymore. I hear the noise when i play the instrument.
I checked power supply and they are ok, i checked clocks/data to the converters and they are ok, i set to GND all the digital FPGA pins to prevent switching noise on DGND/AGND, but the noise still persists.
I read on some papers that when you don't use all the full scale of the ADC converter attenuating the input signals, also few microvolt on noise input can degradate a lot the SNR.
I am thinking to change the single ended input stage into differential input stage to lower the inout noise because i tried everything, do you think it could work?
Can you help me? At this stage of the project i cannot change the pcb layers, i don't want to reach the maximum SNR of the converter but i need to turn down that floor noise i can hear.
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