Hi all, I have a problem with my INA122.
I've done different test:
- 4 Res. 1/4w => output is always stable at 1.,20v +/- 0,05v
- 4 Regolable Pot. => output is instable and decreases or increases steadily, if i'm near to resistances there are additional noises (+/- 0,100/0,200v)
- 4 Strain Gage on the same piece of alluminium => output is instable, +/- 0,100v when i'm near or not to resistances
- 3 Strain Gage and a separated loadcell with 1 Strain Gage => output is very instable, +/- from 0,500v to 1,000v if i'm near to the loadcell, if I touch it, it stabilize and the noise is +/- 0,100v
Now, i've tryed to shield the circuit and i've noted this:
- 4 Res. => nothing change, it was already stable
- 4 Regolable Pot. => same problem, it's related to Pot. (this is why I have not used them in this circuit)
- 4 Strain Gage on the same piece of alluminium => If alluminium is outside the shield, there still same noise(+/- 0,100v), if inside the shield, no more noise( +/- 0,005v)
- 3 Strain Gage(inside the shield) and a separated loadcell with 1 Strain Gage => If loadcell is outside the shield, there still same noise( +/- from 0,500v to 1,000v), if inside the shield, little noise( +/- 0,020v)
Shield was connected to ground.
I've also tried to connect the body of loadcell to ground, i didn't notice any change.
Also, the output of wheatstone bridge is:
- 4 Res. => stable
- 4 Regolable Pot. => little instable couse Pot. inaccuracy problem
- 4 Strain Gage => stable at 1,8mV
- 3 Strain Gage and a separated loadcell with 1 Strain Gage => stable at 1,7mV
The loadcell is working, if i press the wheatstone bridge output change correctly.
So, my conclusion is: INA122 amplify all the noise that are on loadcell body or on straingage much more than the pressure on the load cell so it becomes unusable. Create a shield that cover circuit and loadcell is impractical, and there is anyway a bit of noise.
Now, what i can do? I need to create a precise digital balance with a self made amplifier that work with a wheatstone bridge and a selfmade lowcost loadcell.
As I said loadcell is correctly made and working, the INA122 seems working with 4 normal resistances so i deduce that INA122 is working(i've tryed 2 of them with the same results).
There are no supply interferences.
Maybe very precise is not really possible I guess, but working yes as i saw from other peoples.
The only method that came to my mind to solve this problem is to place a filter on wheatstone bridge output or another amplifier with and integrated filter. But I don't know what type and how.
Here's some picture and scematics:
(Images are big, I don't know how to resize them so rightclick on them and press "Visualize image")
(Near A, B, C, D in the last images are resistance, don't care about rectangles near them)
Please help me, I'm freaking out with this.