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📐 - Designing / 📦-cob / BondTest72-Programmable-LDO
Between 2026-10-31 11:59 p.m. and 2026-12-01 12:00 a.m.
ADC_REF as per the below image, though this connection is optional and will restrict the Pi's ADC input range from 3.3V to 3.0V.
Further it provides a MCP4725 12bit DAC programmable via I2C; it's fed from the local 3.0V rail and it's output is buffered by a TP1562AL1. This op-amp is unity gain stable at arbitrary load capacitance; for supply/stock reasons the 2-channel version was selected and the otherwise spare channel is wired to buffer the 3.0V reference to somewhat isolate the I2C pull-up's from the analog supply; the channel couldn't have been left floating anyways so this was the next best option.
Unfortunately there seems to have been a BOM pick misclick/visual glitch/confusion, causing C1 and C3 to erroneously be populated with 100μF in correspondingly rather bulky 1206. Thankfully that shouldn't be an actual problem, because the only effect other than poor dynamics on the I2C pull-up's supply and strong low pass filtering of the precision reference, is that the DAC's power-on-reset mechanism for auto-loading a startup state from it's EEPROM will not reliably actuate. The mitigation for that is to start off with an I2C general call reset at the earliest decent opportunity for doing so; this reliably resets the DAC without reliance on VDD ramp rate for it's POR.BondTest72 r2 mainboard allows connecting it to the add-on.
ADC_REF (Pin 35).