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I have an S-type load cell to measure weight up to 100 Kg. Its datasheet mentions 3mV/V against full range and excitation voltage.

When no weight is applied, the voltage is 9.9 mV (Excitation is 10 V). When I press the load cell manually, the voltage rises a few mV (like 14 to 17).

I designed a dual stage amplifier using an LM358 IC to amplify the voltage. The first stage is a differential amplifier with gain of 10 and the second a non-inverting amplifier, with a variable gain of 0 to 50 to achieve an output of around 10 V to feed to a PLC.

The circuit diagram is:enter image description here

The Problem:

I get a 4 V output at LC2_O/P and doesn't change if I press the sensor. However, if I apply somewhere between 0 - 0.1 V from my lab bench power supply, it shows a steady range of amplification.

Can someone please help?

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  • \$\begingroup\$ What is your VCC connected to? What are the absolute (Ground-referenced) voltage measurements at your inputs? \$\endgroup\$ – brhans Oct 3 '18 at 15:17
  • \$\begingroup\$ @brhans My VCC is connected to 24 V w.r.t ground. I'll have to check the individual voltages at my workshop this weekend. The voltage difference though, is 9.9 mV between green and white. \$\endgroup\$ – Ohbhatt Oct 3 '18 at 15:19
  • \$\begingroup\$ Looks like a duplicate of this question: electronics.stackexchange.com/questions/355479/… \$\endgroup\$ – Finbarr Oct 3 '18 at 16:02
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    \$\begingroup\$ Hey, @Finbarr I saw the problem, It's kind of similar to mine. So what I see as a problem is that my input impedance is low and to resolve it I would have to Either add voltage buffers to the inputs or replace LM358 with an Instrumentation amplifier. P.S. can you please suggest a good commonly available Instrumentation amplifier for this application. \$\endgroup\$ – Ohbhatt Oct 3 '18 at 16:13
  • \$\begingroup\$ Apart from the resistor values it's identical to yours. And the answers to that question will no doubt help you as well. \$\endgroup\$ – Finbarr Oct 3 '18 at 16:41
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If you replace the LM358 with an instrumentation amplifier then theoretically would work. Practically you will face one another problem, that is nulling.

Most instrumentation amplifiers requrire a REF input that has to be driven with a low impedance source, aka opamp. The purpose of REF is the null setpoint at zero load for your load cell, but this ref voltage gets only subtracted/added to the amplified signal. Therefore if the diferential signal at zero load is non-zero it gets aplified, so there is only little headroom left for the working range. The REF voltage is then subtracted/added to the already amplified signal.

The best approach is to subract/add tare (zero) before it gets amplified, thus having almost the entire supply voltage as operational range. Those are not classic instrumentational amplifiers

Fortunatelly there are some special instrumentation amplifiers, called indirect current feedback instrumentation amplifier.

enter image description here

More info: Link

Link 2

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  • \$\begingroup\$ First of all, I am very sorry for such a late response, I was busy with a family issue. I really appreciate your help and will surely look into it as soon as I find time. \$\endgroup\$ – Ohbhatt Oct 20 '18 at 10:42
  • \$\begingroup\$ I finally resolved to use the 3 op-amp Differential amplifier model using LM324. It is extremely cost effective (Rs.16) vs Rs.300 for common ICF Amplifiers. Thank you so much for your help, I got to learn more. \$\endgroup\$ – Ohbhatt Oct 24 '18 at 6:33

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