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SSM2603 noise in ADC path

Question asked by elodg on Jan 20, 2012
Latest reply on Jan 27, 2012 by elodg

I am validating a SSM2603-based design and noticed an audible hissing noise in supposed-to-be silent recordings. What the FPGA-based test project does is record 2s of stereo audio on linein (96kHz, 32-bit I2S format), play it back on headphones and repeat. Here is what I managed to find out so far:

- during playback of constant digital 0, bypass and sidetone disabled, no noise

- during playback of constant digital 0, bypass enabled, no noise

- playback of past recordings, noise appears

- noise appearing in ADC output (RECDAT, BCLK, RECLRC)

- noise frequency different in left and right channels: 3.2kHz left, 800Hz right

- noise amplitude 11-bits out of 24

- noise amplitude unrelated to input amplifier gain (INVOL)

noise invol 07.png

noise as seen in logic analyzer having the de-serialized 24-bit samples from the left channel plotted (INVOL = 0x07)

noise right invol 07.png

noise as seen in logic analyzer having the de-serialized 24-bit samples from the right channel plotted (INVOL = 0x07)

 

Configuration used:

0x0F = "000000000", -- Software reset
0x06 = "001110000", -- R6: Power up DAC, ADC, MIC, LINEIN
0x00 = "100010111", -- R0: Both-channel ADC mute disable, mid volume
0x02 = "101111111", -- R2: Both-channel headphone 6dB
0x04 = "000010000", -- R4: Sidetone disable and bypass disable, DAC to output, ADC input LINEIN
0x05 = "000000000", -- R5: DAC digital mute disable
0x07 = "000001110", -- R7: 32-bit word length, I2S mode, slave
0x08 = "000011100", -- R8: 96kHz ADC/DAC Sampling Rate, MCLK = 12.288MHz
--delay ~ 75ms
0x09 = "000000001", -- R9: Active
0x06 = "001100000"  -- R6: Power up Out, DAC, ADC, MIC, LINEIN

 

Schematic attached.

 

I am having trouble determining the source of the noise. From the test results described above, it seems to me that the noise is introduced by the ADCs. Any ideas on possible noise sources or tests I could conduct to narrow the issue down are welcome.

 

Message was edited by: elodg Added schematic attachment

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