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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.

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  • \$\begingroup\$ The title of this question is a bit misleading. The ICL7017 inputs are "vacillating" too. So garbage in... garbage out... as someone would say. But the title is somewhat relevant in that the ICL is a non-trivial load and being a digital IC may induce noise in the rest of the circuit. \$\endgroup\$ – Fizz Oct 4 '15 at 20:53
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You have quite a bit of gain there and the op-amp you're using is far from being low noise. There are also opportunities for the ~200mA LED currents to cause issues or mains hum (though most mains hum should be rejected if the integration clock is correct). You are only seeing maybe 10-25uV referred to the input.

Start by determining the nature of the noise by using an oscilloscope on the outputs of the op-amps and looking to see if it's random looking noise or stepped when the LEDs switch.

Edit: Based on the comment, and from your video, from which I can see that your resolution is about 1mV and noise is around 20mV you should use a lower noise amplifier. You want an amplifier with noise in the 0.1-10Hz range that's maybe 1uV (or better). For example, the (quite old) OP07 is typically 0.35uV. You could also consider a more modern 'zero-drift' amplifier or an instrumentation amplifier. It's not enough to replace the gain-of-820 amplifier- the differential amplifier also is contributing noise. The final amplifier in your circuit probably isn't contributing enough noise to bother about.

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  • \$\begingroup\$ I looked the output of the last op amp with my oscilloscope and there were no particular pattern in it. It looked just like noise. I recorded the oscilloscope while I was checking the output of the op-amp. There is also a short video of the display so you can see what I'm talking about. drive.google.com/file/d/0B7s-vzSOB78SMUVZVWF0VFVadFk/… \$\endgroup\$ – Charles Vandal May 22 '15 at 17:38
  • \$\begingroup\$ Sorry, here's the video of the oscilloscope. drive.google.com/file/d/0B7s-vzSOB78SWUFMUEl6a3h4OEE/… Also, I changed the configuration of the last op-amp (the one which the output goes in the ICL). Here's the schema of it. i.imgur.com/ZxOIpvI.png So the video of the oscilloscope correspond to the output of this op-amp. It keeps switching like you can see. \$\endgroup\$ – Charles Vandal May 22 '15 at 18:00
  • \$\begingroup\$ Ok, I will change the op-amp and use an instrumentation amplifier and I will come back with the results. Since I don't have these op-amps it will takes some days before I can make these changes. I will let you know if it works. \$\endgroup\$ – Charles Vandal May 22 '15 at 18:49
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    \$\begingroup\$ Good news. I received the op amps OP07DP and I replaced all the other op amps. I also used an instrumentation amplifier for the first op amp. So, now the waving has considerably reduced. It's below 5 mV and sometimes it don't change at all. But I don't know what else I can do to get better results. The scale is now usable but like I said, sometimes it don't change but in most cases it keep switching about 2 to 5 mV. Thanks for your help, it is really appreciate. \$\endgroup\$ – Charles Vandal May 30 '15 at 21:27
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I'd suspect that your LCD drive frequency is getting into the offset input of your last op amp. You're only seeing a few millivolts, after all. Try replacing R17 with a pair of 3k resistors in series, with a 1 uF cap to ground from the connection of the two resistors.

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