for my first project I decided to make a scale using a load cell, the micro load cell 3133 from Phidgets. I also used an ICL7107 as A/D converter and the op amps are LM358.
I used a differential amp to get the voltage frome the cell and then I amplify and use another op amp to make the calibration of the cell. Then I use the ICL to show the load on a 7-segment display. I'm using 1.5 V piles to get 4.5V with which I'm powering my circuit.
I'm able to get the voltage from the load cell and to amplify it but the problem is that I seem to get very annoying noise from the last op amp because the display keep switching values (about 10 or 20 mV two or three times by second). I used decoupling caps (0.1 uF) for all the amps and for the IN of the ICL as for the ICL itself.
I find it kind of weird because when I first tested it without the ICL, I was reading values with my multimeter and they were right and weren't waving. But when I connected the last op amp with the A/D, I kept getting switching values. I tried many gains (from 5 to 800) but I always get this switching effect.
You can see the circuit on the following picture.
The decoupling caps aren't there but I can assure you I put it on the board.
I changed the last op amp configuration to make a straight line configuration for the calibration and we can't see it on the picture but I was getting the flickering before I changed it.
I made some tests when all was connected to the ICL and I realized that it was the output of the last op amp which was switching. I tried different op amp but I always got the same results.
I used the same ICL for a school project (a thermometer) and the display was fine. I don't know what to do right know and if anyone got a solution or have an idea I will be very grateful.
Thanks for your help.