Hi all, I am working to develop a soundcard for the Pi5 that utilises the new multi-channel I2S capability. The design parameters are:
I selected TI's PCM3168 because the datasheet is more detailed than the one for the other main candidate, the AD1934, and it looked like the Linux driver support was much more complete. However, it has differential inputs and outputs which makes the surrounding circuitry more complicated. I am attaching the prototype design which I am pretty close to sending for fabrication. I would appreciate comments and suggestions from anyone who is interested. A few notes about the design:
- Decent performance for reasonable price (<-90dB THD+N for ~$100 parts)
- 8 channels out, 6+ channels in
- At least 96kHz sampling rate
- Full isolation from the Pi - so it will not use the Pi's power supply either.
- Single-ended inputs/outputs
I selected TI's PCM3168 because the datasheet is more detailed than the one for the other main candidate, the AD1934, and it looked like the Linux driver support was much more complete. However, it has differential inputs and outputs which makes the surrounding circuitry more complicated. I am attaching the prototype design which I am pretty close to sending for fabrication. I would appreciate comments and suggestions from anyone who is interested. A few notes about the design:
- Yes, those are electrolytic AC coupling caps. I know film caps are better, but for the prototype these will have to do. If anyone can suggest a good, not-too-expensive film cap, I'm all ears.
- Power input is +/-6V. There is a separate board (still under design) to supply this using two SMPS.
- I selected the OPA1679 as the op-amp, as the one in the datasheet (OPA4134) is too expensive. It has lower noise but higher distortion (although still very low). The slew rate is less than half of the 4134 and it is unity-gain stable. Importantly, it's about a quarter of the price! It seems like it should be OK, but TBH I have little knowledge in this area.
- Descriptions of the various schematics:
- rpi_soundcard.pdf - The top-level schematic with the PCM3168
- rpi_soundcard-Input.pdf - Input side, with 6 input filters into buffers into SE-to-Diff converters
- rpi_soundcard-Input-inFilt1.pdf - Input filter. RF filter, HPF, under/over-voltage protection
- rpi_soundcard-Input-LPF12.pdf - SE-to-Diff converter and LPF (straight from datasheet)
- rpi_soundcard-Output.pdf - Output side with 8 channels of diff-to-SE conversion and AC coupling
- rpi_soundcard-Output-out1234.pdf - 4 channels of diff-to-SE conversion
- rpi_soundcard-Output-out1234-of1.pdf - Single channel diff-to-SE converter
- rpi_soundcard-Output-outCap4.pdf - Output AC coupling
- rpi_soundcard-Pi_and_Isolation.pdf - Pi GPIO connector and isolation
- rpi_soundcard-power.pdf - Generates +5VA, VDD for PCM3168 and VCC+/VCC- for op-amps
Attachments
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rpi_soundcard.pdf341.3 KB · Views: 144
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rpi_soundcard-power.pdf155.5 KB · Views: 96
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rpi_soundcard-Pi_and_Isolation.pdf255.3 KB · Views: 77
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rpi_soundcard-Output-outCap4.pdf19.3 KB · Views: 85
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rpi_soundcard-Output-out1234-of1.pdf51.3 KB · Views: 59
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rpi_soundcard-Output-out1234.pdf97.2 KB · Views: 65
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rpi_soundcard-Output.pdf116.8 KB · Views: 71
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rpi_soundcard-Input-LPF12.pdf117.8 KB · Views: 95
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rpi_soundcard-Input-inFilt1.pdf32.8 KB · Views: 67
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rpi_soundcard-Input.pdf162.9 KB · Views: 86
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rpi_soundcard_pcb.zip216.9 KB · Views: 77