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I have an absolute pressure sensor: MPXA4115A that give me an analogical signal between 4.307 and 4.846 V. To get an altitude measurement I give it in input to an Arduino Due board which admits a maximum of 3.3 V. I am thinking to employ a voltage divider in order to scale the signal between 2.843 and 3.198 V. Now, I would like to gain resolution expanding this signal range on the entire Arduino Due's voltage range that is between 0 and 3.3 V. I am new in this matter and reading around other questions I think that what I need is a differential amplifier or an instrumentation amplifier but I am not able to do the correct choice. Before writing this question I tried to make an electrical scheme using the software LTSpice. I have imported the model of the inamp MCP6N11 but the analysis does not work and the following error message occurs:

Analysis: Time step too small; initial timepoint: trouble with node "u1:_u1:21"

More in detail I replicated the standard circuit showed in the MCP6N11 data sheet (page 27):

enter image description here

Here is the circuit:

enter image description here

I don't know if the error in the analysis is due to the fact that in the imported .txt file for the MCP6N11 is recommended to use PSPICE (other simulators may require translation).

So my question is: what kind of amplifier I have to choose?

Thank you for any suggestion.

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    \$\begingroup\$ What is your question? \$\endgroup\$ – Spehro Pefhany Dec 4 '16 at 19:53
  • \$\begingroup\$ Sorry! I edited my question now.. The question is what kind of amplifier I have to choose. Thanks \$\endgroup\$ – Iacopo De Angelis Dec 5 '16 at 9:30
  • \$\begingroup\$ Try adding Rser, maybe even Cpar to the supplies. Rser should be smalle enough to not have adverse effects, while large enough to not cause numerical errors due to large dynamic range. For your case, try Rser=1m (with possible CpaR=1n). There can also be the possibility that the model isn't very "cooperative" with LTspice. \$\endgroup\$ – a concerned citizen Dec 5 '16 at 16:47
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Theory vs reality first: Unless you use a rail to rail op amp, you will need more voltage than the maximum output you are looking for to power the op amp. Simulation doesn't always account for real world numbers. Rail to rail op amps are nice in that they output voltages closer to their supplied voltage, but they even have limitations. Powering an op amp to ground typically won't provide an output of 0V when 0V is expected. That's one of the reasons why lots of op amps operate using +/-12V or more. This allows the voltage to swing past zero. Please be careful though that you don't exceed the maximum voltage input of your Arduino.

For the schematic: You will want to use a "Zero and Span Circuit" to get the maximum resolution. The 'zero' will move 4.307V - 4.846V down to 0V - 0.539V. The 'span' will then amplify 0V - 0.539V to 0V - 3.3V. Please be careful though that you don't exceed the maximum voltage input of your Arduino.

For the op amp: This might be possible with the MCP6N11, but I've only done it with multiple stage amplifier circuits. Texas Instruments 741 or the OPA various series.

Please be careful though that you don't exceed the maximum voltage input of your Arduino.

enter image description here

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  • \$\begingroup\$ Hi, thank you for your answer. I was wondering if you can give me a schematic example in order to realize it in practice. Even choosing a different amplifier and not the MCP6N11. \$\endgroup\$ – Iacopo De Angelis Dec 6 '16 at 16:21

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