Hi,
There are different solutions. Ech has it´s benefits and drawbacks.
Junction box:
You didn´t draw the sensor excitation signals. Is it DC or AC?
DC is more simple, but with AC you automatically cancel out all thermocouple voltage errors.
Use low pass filters to reduce noise and increase ESD immunity.
For high signal quality (or long cabling) I recommend to put the amplifier as cloase as possible to the sensor. Amplify the signal. Then either do the ADConversion at the sensor or use differential analog signal wiring to the junction box.
It seems you want to add all four signals. Instead of one suming amplifier consider to use an ADC with 4 differential inputs. This makes signal processing more flexible. For example: if the sensitivity of all four sensors is not exactely the same then you can compensate for this.
Transmitting signals digitally makes it more immune against errors than transmitting analog signals.
Summing amplifier: (If you don´t convert 4 x digital signals)
I´d use (for high signal quality) 4 x the INA, and then use a differential OPAMP circuit with individual input resistors for each channel.
Signal processing with ADC and microcontroller:
Select a fixed, stable, continous sampling frequency that is an integer mutiple of lines frequency. Do digital averaging over one line full wave period (or multiple).
This cancels out almost all mains introduced errors.
If you expect high thermal drift and you want to compensate for it, then consider to install a temperature sensor.
Offset correction:
* the best quality is that you buy a sensor with already corrected offsets. --> if it is done by laseer trimming directely at the load cell it will result in very low future temperature drifts.
* I personally don´t like pots, because they will drift with time, temperature and mechanical vibration
* But if you use a pot, then place the pot where you expect abput constant temperature and/or no mechanical vibrtion. I see no big difference in your two modes.
My question is: Do you need analog offset correction at all? If the analog value is converted to a digital one and this data processed in a microcontroller, then do the offset cancellation in software.
For sure there are alot of factors that need to be considered.
* cost
* expected resolution, prcision and accuracy
* is there a way for automatic calibration
* cable length and cable type
* software effort
* environmental parameters: temperature, humidity, vibration...
* and so on
We can´t know about this, therefore you need to decide it on your own.
Klaus