I made this circuit to amplify the output of a load cell.
simulate this circuit – Schematic created using CircuitLab
The instrumental amplifier has a Gain = 750.
I use a dedicated voltage regulator (LM350) because I want an output of max 5V.
The circuit seems to work well (there's a bit of background noise), but when I'm close to the circuit the output is greater than before.
The Output change a lot when I move my arms in the air.
The cable from Load Cell to circuit are close in an aluminum foil connected to GND.
Is this the correct way to read data form load Cell?
Do you know how to remove the influence of a body?
More details:
Thanks everyone for suggestion (it's only the second time I use electronics.stakexchange)
The schematic shows only a single part of the PCB circuit of my project.
In the real circuit, between load cells and amplifier, there are two demultiplexers.
They switch signal from 8 Load Cells to 1 amplifier. (1 demux manage the +In and the other the -In).
On PCB there aren't capacitors. I tried to add capacitors in the breadboard version.
The circuit and the load cells are mounted into a big aluminum case, not yet completely closed.
In this picture the cables are uncovered, but during tests they are covered with silver foil.
Every part of the aluminum case is connected to GND.
For @ANDY AKA
This is what the oscilloscope sees when I put my head near to the circuit in Breadboard version.
If I set AREF to 1V and move my arm near to the wires, you are right, the output do the opposite: it decreases.
Best Answer
It is not an schematic problem. It is implementation problem. What type of prototyping you are using? IMO, this schematic have to be prototyped carefully on a PCB, not on breadboards and any kind of rat-nest type of solder-less technologies.
Note, that such high gain, precision schematics are highly sensitive to all kind of EMI and must be constructed correspondingly.
Take special attention to the input circuits - make them with screened cables that are properly grounded and as short as possible.
Yes, the instrumental amplifiers suppress very efficiently the common mode EMIs but on gain 750 and bad construction, some differential EMIs can be inducted as well.
Edit (after PCB images has been posted): This PCB is not well designed. Routing the power and ground as such a thin tracks is wrong. You must use wide copper areas for the ground and tracks as wide as possible for the power tracks. Your PCB must be almost full of copper. Also the density of the PCB seems to be a little bit small. Higher part density will short the tracks and lower the possible influences.