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jkominek / piano-conversion

Hardware, and some firmware, for acoustic piano to MIDI controller conversion.

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ADC Board Rev 2

jkominek opened this issue · comments

commented

Place to record necessary changes for rev 2 of the board:

Necessary changes

  • Bring the JTAG header in 1-2mm from the edge of the board, so that the pcb stickvise can grab the board even with that header attached.
  • Silkscreen for +5V and GND on the external power header

Noncritical changes

  • Increase thermal relief on pin 3 of the MCP120, to ease hand soldering.
  • Consider just increasing the thermal relief everywhere, to ease hand soldering.
  • Move the power LED further away from the 8p8c jack, to ease viewing.
  • Bump up the size of the electrolytic cap footprints by 1, to better match the cap selection you get in variety packs.

Possible improvements

  • Consider swapping the location of the RS485/power input and the USB connector. As it is, the +5V power to the analog linear regulator is passing under the MCU, which will act as a source of noise.
  • I realized that I was probably wrong about the suitability of replacing the voltage reference with a simple linear regulator. I can probably do an error analysis to determine how any noise on the regulator will be converted into ADC errors. I should probably do so as penance for my overeager substitution.
  • The MIC5366-2.0YC5 linear regulator data sheet only calls for 1uF caps on input and output; the schematic lists 2.2uF. That's probably fine. Should definitely keep it in mind, though, and consider it as a spot for BOM reduction.
commented

First batch of changes made in f3f059a

commented

832f42c Restored the MCP1501 for the analog reference. Error analysis said that for a fixed Vref (2.5, 2.048, etc), the noise on Vref was passed through linearly to the output. Not surprising. Increasing Vref also reduced the noise, but bumping up to 2.5V doesn't seem worth it for the reduction in linearity that would likely cause in the CNY70's phototransistor output.

Switched to the SOT23-6 version as that's a JLCPCB part.

Reduced the specified output capacitance on the MCP1501 to 220pF.

I'm going to pass on rearranging the connections in this rev.