I recommend to try this solution first:
This is ready-to use project, using HX711 24-bit DelSig ADC, which is available elsewhere for under $1. Regarding the achieving accuracy of 1g using 20kg load cell (5E-5), please check the attached datasheet Appendix 1, which covers a demo project provided. Using 1kg load cell and a median filter, the short-term accuracy was about 30mg (3E-5), which matches your requirements (but is quite tight!).
Due to load cell signal creep (deformation + temperature drift) the scale should be "tared" after each weighting cycle.
I don't recommend using IIR filter (neither FIR) for weight load application. I find that median filter works best in this case (rejects load spikes and settles faster)
Though your approach is entirely doable, I don't recommend it for a beginner project. Another advantage of staying with HX711 is that less expensive processor (PSoC4) can be used instead of PSoC5.
HX711_v0_0_A.pdf 1.5 MB
Thank you for your help.
I'll try the solution with HX711 but the problem with that set will be the SPS. I think I'll need more than just 10 SPS.
I saw that there is a shielded version that some people say that works better. Maybe changing the rate to 80 SPS will end the problem...
Dou you have any ideas about the sample rate?
I recommend to start with existing design and follow up with improvements. For example, the HX711 allows for external clock input, so you can easily speed-up input A from 10Hz to e.g. 20Hz. I don't think the sampling rate is important here; technically you need only one good ADC measurement per load (1 Hz). Note that oversampling of Delta Sigma ADC will not improve S/N.
I am expecting that accuracy of the device will depend on simple things like quality of the load cell, load cell temperature stabilization, device shielding, power supply stability, weights loading mechanism, etc. ---- Not by the PSoC or ADC design implementations! My advise is to get some results ASAP, and then to roll hot potato over to the management: "options A, B, C will cost that, that and that and will take so much time, etc." (brutal force "project management").