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The circuit which I am simulating here is a Strain Gauge Bend sensor module Y3 which I bought and is working well.

Now I want to integrate it within my circuit and it was better to add it in the design phase into my circuit instead of attaching it as an external module because I have a lot more modules to attach also. The circuit diagram which I got online for the device is below:

enter image description here

Now I want to simulate this in LTspice and I followed the procedure to install 3rd-party libraries for the LM358 op-amp using the PSpice libraries which I got online. The simulation for the TL431 however was successful; it was the op-amp which was the problem.

The library for the LM358 PSpice is from this TI website.

My schematic is as follows and in this version I replaced the op-amp with an AD820 because online forums showed that this might be a better approximation to the original one:

enter image description here

This simulation works well and does yield a somewhat similar result to what I expected but I can't add an input voltage of more than 3 V and the simulation does not show any output.

So the first thing I don't understand is why is this happening.

Next, when I simulate by replacing the AD820 with the LM358 3rd-party version, I get a time step too small error:

enter image description here

The above schematic is just a simple one where I use a UniversalOpAmp2.asy and change the directory to the new library which I got online and whose working is shown above.

Upon performing the simulation the error which I get is as shown:

enter image description here

I tried following a lot of videos online and other forums but the problem remains the same. I tried creating a symbol for this instead of using the UniversalOpAmp2 and changing directory, but the error remains the same and it would be very good if someone could help me solve this issue with a possible explanation on what was the mistake from my side so that I can learn.

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    \$\begingroup\$ Did you try running a simulation with the op amp in a circuit, not just on its own? \$\endgroup\$
    – Hearth
    Apr 4, 2023 at 3:02
  • \$\begingroup\$ @Hearth I tried running it in the original circuit and not just on its own but the error is still the same \$\endgroup\$ Apr 4, 2023 at 4:20
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    \$\begingroup\$ What happens if you add the following SPICE directive to the schematic?: .options cshunt=1p \$\endgroup\$
    – Ste Kulov
    Apr 4, 2023 at 5:16
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    \$\begingroup\$ Also, why is your VCC so low? It's 1V and through a silicon PN diode. That's rather low to operate an opamp off of. \$\endgroup\$
    – Ste Kulov
    Apr 4, 2023 at 6:49
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    \$\begingroup\$ @Willis Lin I will try to find online whether there is any equivalent version of what I am using. \$\endgroup\$ Apr 4, 2023 at 8:35

2 Answers 2

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Your second AD820 should be supplied with 1.25 V (VFF/2) at V+ input in place of VFF.

As this around RP1-C5 (1.25 V) in place of VPP ...
and your C7 should be disconnected and placed in parallel with your R4.

enter image description here

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Time step too small sometimes indicates a real problem with the circuit.

You also should have Vdd at least 3V (preferably a bit more) after the diode for the LM358 (and supply a diode model, not the default, such as 1N4148 if you insist on using a diode at all).

The main other problem I see is that the 1uF load capacitor should not be on the op-amp output- it will surely cause oscillation. If you want to slow the response, use a cap parallel to the feedback resistor.

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