Hello I am the guy building the PlainDAQ, unfourtunately I had to take a huge brek because my main job won't allow me to work on it
... after a 5 digit sum, to get your custom design certified, if you like to sell it. If you use a tm name like Bluetooth without a certification, to advertise it, the lawyers get after you.
PlainDAQ won't include any bluetooth and wifi capability I moved on from it because there is a version of pico with Wi-Fi already.
your github repro does not contain schematics in PDF, but native KiCad files. That makes it difficult for people to help you, by looking over the schematics. The repro got updated last month, so my guess would be thats the latest HW version.
I will upload it tomorrow. The hardware is not the latest I will have to make a few changes TBH.
you should begin your design, by plan to fulfill safety regulations. Its not expensive, nor difficult, to design the product according safety regulations. This safes our children, and enables you to make money from selling products. I see, you spend much time with your own software, but over several PCB releases, still have unprotected inputs. I would make the device compatible with a standard like Sigrok, (or Labview, or GNU Radio, or Octave). They have all the features you will need years to implement by yourself. But that is your choice, you seem to have fun with it. But still: you need to implement at least the basic safety standards.
Safety regulation issue is of the area that I am not really capable of, how should I get started to make something compliant. I haven't so much thought about making it compatible with already existing standarts, but it's a great idea, I was rushing to et something working, so I haven't had a chance to take a deep look into those issues.
your inputs are connected, with a 100 Ohm resistor, to the DG611 multiplexer. More then + or - 18V will kill this IC. If 220V comes on the input, smoke comes out in best case, and a splitter of an exploded resistor fly to the eye of a bystander and he is blind. Somebody suggested a 220k resistor in series with every input, what is a good start. That is a good start. But keep in mind, that the resistor must withstand the high voltage. A standard 0603 resistor may be spec`ed for only 50V. So use ether a high volt resistor, or connect multiple resistors in series. Safety standards call for 1000V. I would add at least a pair of TVS diode after the 220k resistor, to clamp the voltage to save areas for the DG611, +8V/-8V
I will add proper input protection, noted. THe new version had inputs directly connected to the opamps not the multiplexers, I had to use cheaper multiplexers because of the price and they are 5V rated.
Even you like your own software solution: When you want this project to become a commercial success, or more attraction in the open source world: consider to add compatibility to standard software: the RP2040 supports tinyUSB, and tinyUSB support USBTMC.
https://www.eevblog.com/forum/projects/usbtmcusb488-class-implementation-for-microcontrollers/
https://k1.spdns.de/Develop/Projects/pico/pico-sdk/lib/tinyusb/examples/device/usbtmc/
As beautiful your own software is: you will get 10x more attention, when backers can use Sigrok with your plainDAQ.
Your users can then choose between your software and standard software.
That's a glden advice for me and I'd like to really thank you for this suggestion. At this stage I don't even know what Sigrok is, but I will learn it.
The technical addvices you gave me are great. I am trying make it as noiseless as possible and as affordable as possible at the same time, trying to optimize both is not easy as you know
14-bit version is out of question at the moment I am only building the 12-bit because 14-bit is too costly for my price range. I will keep your advises in mind when building the new revision.
Thanks!