Hello.

I happened to read about load cell conditioning & problems some one was facing and other people (experienced, like Eric, Kai..) putting in their valuable feedback.

My query is related, but different. I am designing a 2-part system. The birds-eye-view is that there are 4x tanks / silos which will be feeding a weighing hopper. The system will cycle through all the silos / tanks till all the material is in the hopper, and then empty the hopper on a conveyor. The 2 parts system is basically 1) Summing / averaging amplifier AND 2) 8055 board with ADC to digitize the data and convert into real-world (KG / Lb) units.

The hopper is suspended on 3x Tension load cells (S-type). Hence the amplifier. The 8055 control system will be getting the summed / averaged load signal from this amplifier (0-5V or 0-10V). The running cable length between the uC Board and amplifier can be upto 50 feet, or max 100 feet.

Hopper dead weight = 250 ~ 300 KGs
Load-cells on hopper = 3
Load cell specs :
Type = Tension S-type
Capacity = 500 KG each
Output type = 2mV/V
Connection = 6-wire (EX+, Sense+, x-, Sense-, Out+, Out-) with sheild.
Total measuring range = 1000 ~ 1500 KGs
Minimum weight to measure = 0.1KG
Accuracy = +/-10% approx that of min. resolution

Thats the back ground and specs. Now i am actually stuck in designing the summing amp at the moment. Most commercial designs of load-cell summing amps that i have seen, including one on my friend's plant, have some low-noise / low-offset opamp as summing amp. The output of load-cells directly terminate on the resistors of the summing opamp. Moreover, the excitation is almost always fixed 10.0VDC with a 10- or 20-turn trimpot for excitation adjustment.

The circuit that i am (trying) to make, i am following a slightly different concept. Here are the specs i am planning to build :

Excitation : 5.0V
Excitation type = DC Excitation with sense lead compensation (Closed loop opamp + Vref based excitation),

I want to place an instrumentation amp on each load-cell input. Once all load-cells's outputs have been brought into single-ended (ground referenced) domain by the IA (and also amplified by a gain of 100), then these will be summed with a opamp in summing-amplifier configuration.

Q1) I need to know if i am on the right track. I know this approach, an IA for each load-cell, is costly upfront, but still.

Q2) I have researched a couple of IAs from Texas Instruments, Analog Devices and Linear, and i have made an excel sheet for comparison of the total error of these amps (ppm of full-scale on standard and elevated temperatures). Somehow i am seeing AD621 standing out in majority of the areas. I just need to know if that is really what i should use or any suggestion. I can send the Excel sheet to interested people.

Q3) Is the Fixed DC excitation OK, or should i consider AC excitation ?

Any help in this regard will be most appreciated.

Warm regards,
Zeeshan