ADC Board Rev 3

[jkominek]

Place to record necessary changes for rev 3 of the board:

Necessary changes

• [x] 4.5V regulator, LP2985AIM5-4.5/NOPB ("SOT-23-5")
• [x] 3.3V regulator, NCP115ASN330T2G ("TSOP-5" which is SOT-23-5)
• [x] 2.0V precision voltage reference, LM4120IM5-2.0/NOPB ("SOT-23-5")
• [x] Rearrange the analog voltage distribution. The phototransistors and op amps should be driven by the 4.5V. The 3.3V for STM32 VAA and op amps V+. 2.0V as Vref+.
• [x] Unify the ground planes, per STM32 EMC doc, and the people who seem to know the most about the subject.
• [x] Double check that we're keeping all of our decoupling cap traces as short as possible.
• [x] Tidy up analog area traces.
• [ ] Do anything else I can learn to do to improve EMC before I feel like having these boards built.
• [x] Adjust USBLC6-2SC6 footprint to also accept MAX3207EAUT.
  • This means shorting some pins together on the PCB. Make the traces for that swing out wide from the part, so they're cuttable when using the USBLC6-2SC6.
  • That'll also mean that you can just leave the part out entirely, which is probably good.

[jkominek]

Note to myself: We need to keep +3.3VA, as a requirement of the way the STM32 works. I want to bump Vce for the phototransistors up to at least 2V, preferably more. I don't want to run them off the input 5V. So we'll want analog voltage rails of 2V (vref), 3.3V (STM32 and opamps) and 4.5V (phototransistors). That'll leave us with 4.5V-2V=2.5V for Vce.

We'll say the conservative max for +4.5V will be 22 * ~5mA ~= 100mA. 2.0V will draw ~0mA, as it is just maintaining one of the high impedance inputs of an op amp. 3.3V will need to cover the STM32 and op amp circuitry. Maybe call it 50 to 100mA.

I'll abandon the interest in raising Vref; any improvement in "linearity" of the initial sensor signals is worth more.

[jkominek]

Unified ground nets, tided things, and then scooted decoupling caps closer in bbdd6db.

[jkominek]

More changes made in 5c3ecac.

continuing to peruse my copy of electromagnetic compatibility engineering looking for ideas on improving/reducing electrical noise on the ADC board.

[davidedelvento]

looking for ideas on improving/reducing electrical noise on the ADC board.

How about putting the op amp on the sensorboard? If I am not mistaken you are using parts which have 4 in one chip, so that's a perfect match for the 4x sensorboards.

[jkominek]

How about putting the op amp on the sensorboard?

That would require delivering a voltage source to the sensor boards.

I'm pretty sure it would also be worse; the current configuration was a deliberate design choice. The ADC input presents an extremely high impedance, so small amounts of stray charge can affect what it measures. By keeping the opamp output close to it, we make it easier for the op amp to control that voltage.

Whereas on the input side of the opamp, it shows ~0 impedance (because of the TIA configuration), so it would take a lot of stray charge to affect its inputs.

[jkominek]

dc4713c marked as rev 3 and ordered