@1.21Gigawatts Thank you (and all others) for your reply. Very informative indeed.
I had a crazy busy week with other projects so sorry for my late reply and thank you.
Lets dig some deeper
I tested te pcb and everything works except for the opamps. They just output a fixed value. One is at 3.3v and the other at 1.6 or something.
All other components seem fine.
Normally the wires from the loadcells would not be close to the plastic housing and inside an aluminium shell away from prodding fingers. So therefore I did not think to protect these inputs.
Would you agree with that (in other words, if the wires are stashed awaydo you think I still need to protect these inputs?)
If you still recommend protecting the inputs what are my options. I see you suggest a tvs diode and zener. I also have read about resistors in line with the inputs (to eat up the current I assume).
And for these parts, what are the variables I should consider (speed, max voltage, ...)
What will / might be the influence on the input?
The loadcells are polled around 10x second. So not extremely fast.
The housing is made of aluminium. This should act as some protection.
However. I did not connect it to the pcb ground in any way.
Should or shouldn't I do this and how?
Is it ok to connect it to ground at the battery compartment? Or is this too far away from the pcb?
I did not bother with making a separate analog and digital ground on this pcb since it is sooo small and in my eyes the traces are short. Was this a mistake?
(Maybe this is something @ChrisGammell would know)
Pickup of radio
So it picks up radio waves. Whenever it does that the measurement of the data is off scale. So does that imply that this is before the opamps (aka the loadcell wires) or after the opamp in the traces of 2cm to the ble module (on both sides fairly 'shielded' by groundplanes)
Unfortunately it would be very difficult to twistpair the wires after the fact. Next batch of loadcells I could ask the factory. Same for ferrite beads since there is no space for in the housing.
So my hopes are on shielding from the housing.
And maybe extra shielding by metallic mesh tape? Would that be an option? But should I then ground this mesh and how?
(Or would more affordable aluminium or copper tape also work?)
And/or a solution like a capacitor/filter between the inputs. Would this 10n capacitor work, and how? And what influence will it have (RC ?)
Which specs for the capacitor should I consider?
Well a lot of questions again. Hope to make this post more informative