The SEEED XAIO RP2040 is a fantastic little dev board, featuring the RP2040 MCU.

I needed a buffered and clamped 12 bit ADC for a volt-per-octave AD9833 based audio voltage controlled oscillator (VCO) I have completed.....The MCU driven VCO works--see the blog post here: https://audiodiwhy.blogspot.com/2023/03/ad9833-voltoctave-vco-new-jacks-n-pots.html; at its core the VCO uses the board you see here.

The op amp elements of this PCB were inspired by the buffering used by Mutable Instrument's famous designs.

The board's additional 12 bit ADC does not suffer from the RP2040's infamous DNL errors present in its ADC/mux subcircuit. So if you need a 12 bit ADC for your design that is better than the ADC built into the RP2040 this might be a good solution.

CALIBRATION, R8 and T1:

R8 and Trimmer T1 allow you to precisely dial in the source voltage going into the MCP3201 DAC. For synthesizer applications--what I initially designed this board for--this would need to be set so volt per octave control voltages correctly reach the MCU. I have used different values here--a total value of 100k is needed for volt/octave tracking, so, R8 82K, and a 25K trimmer for instance, or 68K and a 50K trimmer. Many combinations work, but you may need to adjust these two values based on your application. If you are building the AD9833 based synth VCO, try using 82K and a 25K trimmer, for instance.

USING THE BOARD:

Please note that voltages delivered to the RP2040 MCU by the bufffers are inverted to lower parts count in the best Mutable tradition.

If you use the board in one of your designs, your software will have to accommodate this voltage inversion.

You can add 100 mil jumpers to JD0, JD1, and JD2 to buffer the RP2040's ADC0,1 or 2's inputs or leave the jumpers off to use pins D0, D1 and D2 on the breakout board's edge connectors for GPIO.

If you don't use an op amp channel, please consider providing its input with a small voltage (say 500mV) so the Op amp stage doesn't go to its rail at output. You probably won't blow anything up if you just let unused inputs float, but managing unused op amp stages correctly is considered a best practice in analog design--learn more from the video here.

Alternately, if you have omitted JD0, 1, and/or 2, you can leave C6/R10 (ADC0); C7/R11 (ADC1); and/or C8/R12 (ADC2) off the PCB. In this configuration, the unused buffer will get the zener's approximate -10V signal which will keep the unused stage from flopping around at its output.

The board requires +/- 12V, and will probably work fine with +/- 15V, but to date I have only tested the board with Eurorack voltages (+/- 12V)

Current draw for the board without CV in is about 40mA.

There is a 5V voltage regulator on this buffer board, as well as a 3.3V rail brought to an edge connector, provided by an HT7333 LDO regulator.

The HT7333 can provide up to 250mA of current at 3.3V for external components, but I'd recommend sourcing less than that (probably not more than 100mA), and/or putting a heat sink on the HT7333 as needed.

The board provides 3.3V volt clamps for the RP2040's ADC inputs, but build mistakes (or anything stupid I have done as I revise this board) may keep the clamps from working. Voltages greater than 3.3VDC present to an RP2040 can ruin an MCU input pin or worse, so please test the buffered ADC inputs with a voltmeter before dropping in your SEEED RP2040 board.

For more detail, please see these Audio DIWHY blog posts:

https://audiodiwhy.blogspot.com/2023/01/improved-seeed-rp2040-buffer-board.html

https://audiodiwhy.blogspot.com/2022/10/ad9833-rp2040-vco-voltoctave-is-working.html

Revisions:

4-22-23 made signal flow from JP1 to ADC1 consistent with JP0 > ADC0 and JP2 > ADC2.

5-20-23 replaced 3.3V Zener with L7333 regulator--provides better consistency of 3.3V output for external loads

5-20-23 brought 3.3V regulated voltage from RP2040 to a test point on the board. The 3.3V edge connector pin is now sourced from the 7333 regulator instead of the 3.3V regulator on the SEEED board.