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First my system used four cheap resistive load cells (maks 5 kg, input 5 VDC) to measure COG of a quadrotor. The measurement pan dimensions are 60 cm x 60 cm and made from glass (glass weight=2080 gram), each load cell installed in each corner of the pan. The system used to measure weight variance from 0 g - 3500 g with Arduino Uno (custom-handmade) four-channel ADC 10 bit (A0-A3). Later data is transmitted to a PC via a WizFi210.

The load cell gives a linear differential output (after several adjustments with multiturns in a Wheatstone bridge) from 0.1 mV to 0.8 mV, and I need to amplify these outputs so my Arduino Uno can read these, and I used eight INA125s with a dual-power supply configuration.

First, four INA125s are used as instrumentation amplifiers with a gain of 100 and input from the load cell output, and the other four INA125s are used as operational amplifiers (negative Vin connected to ground directly) that amplify the instrumentation amplifier outputs.

The real problems are:

  1. My INA125s affect each other while I changed the amplifier setting of one INA125. Why that can be happen? What should I do?

  2. Between the Arduino Uno ADC inputs and the operational amplifiers I put capacitors (act as low-pass filters, value 9400 µF x2). Can these capacitors affect ADC reading performance?

  3. Why is my ADC reading in Arduino Uno really unstable? While using an oscilloscope it seems stable.

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    \$\begingroup\$ Please follow basic capitalization rules to make your question more readable. \$\endgroup\$ – JYelton Oct 4 '13 at 17:05
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I don't see any obvious reasons here, but, you talked about low amplitude and high gain amplification and this is where good decoupling, correct grounding comes into account. It could be that your problem doesn't come from a schematic (or principle) level but from a layout problem.

If this is done on a breadborad and not a PCB, make sure that your decoupling caps are very close to every power pins of your amplifiers.

The observed coupling between the INA could come from the power lines or the ground line.

The added capacitor will decrease the impedance of the signal that enters the ADC, which is good. But the value chosen is HUGE and may present very high ESR. IF your ADC samples at 500 KHz for instance, your capacitors won't be very effective at that speed. This could explain the noise you see.

Another source of the noise at your ADC is the wiring. You have to ensure that the signal and the GND are very short between your board and the arduino.

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  • \$\begingroup\$ i see thank you, that's really helpful. two more question please ( maybe that seems not important) First, are load cell produce ( very low amplitude) sinus wave? why that can be happen? Second, why switching frequency of switching power supply still exist in power rail? even i have used the capacitors to removes that ( while laboratory power supply don't). the configuration of my switching power supply are 2 power supply for + 12 V and -12 V that installed really close with each other, the grounds connected to the system casing. \$\endgroup\$ – user29937 Oct 3 '13 at 14:57
  • \$\begingroup\$ As far as I know, the load cell are variable resistors. They don't provides sine waves. If you measures sine wave, somewhere, that means that your amplifiers are unstable and this is the source of the sine wave. Second, your switching power supply will send noise to the power rails. It's your job to filter that noise. For instance : add a LC filter after the switching power supply. \$\endgroup\$ – Blup1980 Oct 4 '13 at 11:46
  • \$\begingroup\$ thank you. i had change the capacitor with low pass filter directly after instrumental amplifier ( it worked with low pass filter 50 Hz) and i set gain of instrumental amplifier 100 X. the additional gain (50x) i'll use operational amplifier ( replace INA 125 with Vin- connected to ground) \$\endgroup\$ – user29937 Oct 5 '13 at 15:12
  • \$\begingroup\$ i am newbie in INA 125. If the V in - grounded while V in + is the signal source 20 mV ( dual supply mode -12 V and +12 V), can i amplify the signal to 2 V? is there any side effect from this configuration? thanks \$\endgroup\$ – user29937 Oct 5 '13 at 15:25
  • \$\begingroup\$ I don't understand your last comments, and I think it would be better if you spend some time to add a schematics to your question. Just edit it, and use the embedded schematics editor. It would be much more easy to understand you issue. \$\endgroup\$ – Blup1980 Oct 6 '13 at 9:37

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