Yes you are right about the under-sizing of the Load-cells. That is why the previous system had developed a lot of errors / problems over the years. I am not sure now as to how much was the drift, but from what i heard from my friend (who actually is not much of an electronics guy), he said the system had 2 primary issues.The weight would change / drift from batch to batch in the same day, specially near afternoon. Now this means there was a lot of drift caused by temperature change.

The load cells and summing modules were mounted in an open area, and absurdely enough, more than half the day, the sun shined directly on the amplifier & cells.

So now if i select the cells, i would atleast consider a safety factor of 1.25 for sizing the load cells. So if :

Total desired material weight = ~950 KGs
Dead weight of hopper = 250 KGs,
then total weight = 1200 KGs
Applying safety factor of 1.25 = 1200 * 1.25 = 1500 KGs
For 3x load-cells, each load cell capacity = 1500 / 3 = 500 KGs


As for the resolution, there is no doubt that 'Legal for Trade' lists resolutions to be 3000 ~ 6000 counts tops. However, a Weigh-Batching system needs much higher resolution than that, possibly around 10,000 counts. To achieve that much of 'displayable' resolution, the internal resolution should atleast be 10x of that, amounting to 100,000 counts or more. This translates to 17-bits, while it is better to have a 18- to 20 bits of resolution.

A 24-bit ADC, with Digital filters applied, would actually result in around 18 bits of useable resolution. But again, this is only ADC resolution. The rest of the Analog system should be able to deliver this 18 bits of resolution too.


You are right about the Passive amp with AD7730, AND that active amps are required mostly with PLCs because the PLCs have generic / universal ADCs that are targeted at general Industrial application, not specific for weighing. The previous system was using a 16-bit module on a S7-300, but since the distance between the load-cells and PLC cabinet was atleast 500 running foot of cable, there was an amp mounted on the hopper fixed-supports.

For an integrated solution, there might not be a need for the active differentiation and amplification of load-cell outputs provided an AD7730 is used. I can follow this approach, if all else fails. My main target was, to design an individual summing amp providing a voltage (0-5 or 0-10V) OR current (0-20mA or 4-20mA) so that i could use it in my project, and if teh results are good, i can replace the 'regular passive' amps on another system at the same plant with these to get better performance. the other plant is based on S7-300 PLCs.

However, if this is not possible, then ofcourse at the end of the day, i will have to make one completely integrated system, and then i can directly use the scheme suggested by Eric / Kai or Micheal to use a 4x channel ADC and connect all load-cells directly to it and do the summing / averaging within uC Code.