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| how to route codec (ADC/DAC), analog pins not all on one side |
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| loki42:
I just tested with all the switches removed and the noise performance is better. It's interesting because the switches sound fine when in bypass mode, when the signal is still going through them but not the codec, but when they are on and the codec is on then it's quite bad. I wonder if it's anything to do with the enable signal to the switches? That is coming from the digital side of the board right around. This was also the case with the relays though, but in the case the line was only high for a few milliseconds. I did change one other thing between this version and the previous relay based boards, which is that I switched to a different oscillator, from the same company as it was cheaper and had slightly better stats, but was in a larger footprint. |
| loki42:
Oh and I tried a resistor voltage divider instead of VCOM and it sounded really distorted. I wonder if that was an issue with the bias resistors or something else, or just that it exposes the noise in the 5VA supply more. |
| capt bullshot:
--- Quote from: loki42 on November 07, 2019, 07:49:22 am ---I just tested with all the switches removed and the noise performance is better. It's interesting because the switches sound fine when in bypass mode, when the signal is still going through them but not the codec, but when they are on and the codec is on then it's quite bad. I wonder if it's anything to do with the enable signal to the switches? That is coming from the digital side of the board right around. This was also the case with the relays though, but in the case the line was only high for a few milliseconds. I did change one other thing between this version and the previous relay based boards, which is that I switched to a different oscillator, from the same company as it was cheaper and had slightly better stats, but was in a larger footprint. --- End quote --- Yes, digital outputs can couple digital VCC noise into analog parts, they're rather low impedance coupled to the respective GND or VDD rail of the micro. So any noise that's on the digital supply will appear on the digital outputs. If the signal is low speed, this is rather easy to mitigate by an RC filter (in case GND to filter against is clean). DId you properly bypass (large cap to GND) your resistive divider instead of VCOM, did you measure if the divider output deviates from ideal midpoint due to loading? Excessive common mode voltage could lead to clipping of the differential signal, im my imagination this would sound distorted. WIthout bypassing, the resistive divider forwards AVCC noise to that virtual GND, that's why one bypasses this. |
| loki42:
I did bypass the resistive divider (47 uF) and the output seemed sensible, though I didn't measure how different it was to VCOM. I do have an apology though... so I still had worse noise with no switches in but I've now realised what the issue was. The negative output from the DAC wasn't properly soldered and actually wasn't connected, so the differential output wasn't doing it's thing, thus the significant noise introduction. I now have about the same noise as the relay version. So basically it's all good. I'm not sure how I missed that the pin wasn't soldered correctly! I'm not sure if the move to 4 layer was needed but things are working well, I am contemplating cleaning the AVCC rail more to remove the switcher noise but I'm not sure how important this is. I've seen some inductors with similar milliohm rating to the ferrite bead so maybe I should swap that out to create an LC filter designed to remove the switcher frequency? It's 650 kHz so far from the audio spectrum, so I'm not sure how much filtering it out will improve performance. |
| capt bullshot:
--- Quote from: loki42 on November 12, 2019, 12:21:09 am ---I did bypass the resistive divider (47 uF) and the output seemed sensible, though I didn't measure how different it was to VCOM. I do have an apology though... so I still had worse noise with no switches in but I've now realised what the issue was. The negative output from the DAC wasn't properly soldered and actually wasn't connected, so the differential output wasn't doing it's thing, thus the significant noise introduction. I now have about the same noise as the relay version. So basically it's all good. I'm not sure how I missed that the pin wasn't soldered correctly! --- End quote --- LOL, stuff like this just happens. I remember having spoiled a whole day of hardware debugging because I didn't notice the scope probe was set to 1:1 instead of 10:1. Fixed that with tape as soon as discovered, then bought better quality 10:1 (non-switchable) probes. --- Quote ---I'm not sure if the move to 4 layer was needed but things are working well, I am contemplating cleaning the AVCC rail more to remove the switcher noise but I'm not sure how important this is. I've seen some inductors with similar milliohm rating to the ferrite bead so maybe I should swap that out to create an LC filter designed to remove the switcher frequency? It's 650 kHz so far from the audio spectrum, so I'm not sure how much filtering it out will improve performance. --- End quote --- Depends. Switching noise on power rails can affect ADCs / DACs in rather subtle ways, I had a design using a slow ouput rate 24 Bit delta/sigma converter that was seriously affected by supply ripple way above the passband. The ripple coupled into the comparator node that was operating at high frequency and very sensitive threshold (that's the nature of delta/sigma conversion). Most probably, the outside amplifiers operating in the analog audio frequency range are less sensitive to 650kHz switching ripple. |
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