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I am planning to use the PP02 Omron pressure sensor. The user manual of the sensor (Usermanual link) suggests using LMV324 type of instrumentational amplifier with four op amps within in the schematic diagram. The problem with LMV324 (datasheet)is that it is not quickly available or as cheap as the other substitute. The other substitute is LM324 (datasheet), but I am not sure how one In Amp differs from another (are they easily interchangeable?). Secondly, how would the schematic diagram need to be change?

I need help with the following.

  1. Can LMV324 be substituted with LM324 without changing the rest of the circuit given in the user manual?
  2. If the circuit needs to be changed, please suggest the changes as I am not an electronics guy.
  3. Is it a good idea going for the LM324 as this project is prototyping with manufacturing in mind.
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    \$\begingroup\$ Can you update your question with more information about your circuit compared to the reference in the sensor manual? The big difference between these two op amps is that the LMV product has rail-to-rail output, which is likely important with a 5V supply. Note that neither the LM324 or LMV324 is an instrumentation amplifier. They are both quad operational amplifiers. The configuration of 3 amplifiers in the reference circuit is an instrumentation amplifier. \$\endgroup\$
    – Theodore
    Mar 29, 2022 at 16:42
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    \$\begingroup\$ The LM324 is not a direct replacement for the LMV324. I would have considered both to be very common parts, so if they are not easily available, what components are? To know whether LM324 is usable for you, we need to know 1. your available supply voltage, 2. your required accuracy (offset, gain). \$\endgroup\$
    – polwel
    Mar 29, 2022 at 16:54
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    \$\begingroup\$ Also note that the schematic form the sensor sheet specifies a 'reference voltage' node. How you implement this reference will greatly impact the accuracy of the measurement. \$\endgroup\$
    – polwel
    Mar 29, 2022 at 16:55
  • \$\begingroup\$ @Theodore Not built a circuit yet, I would be prototyping on Arduino so the voltage is 5V. I was surprised to find that they are using four Op Amps as I had started with the PP02 to replace my earlier HX710B sensor module using a MPS20N0040D-D sensor (datasheet softroboticstoolkit.com/files/sorotoolkit/files/…), and its recommended circuit which I found here (langster1980.blogspot.com/2014/11/…) needs only one Op Amp LM358. I have already ordered LM358 and was hoping to use this circuit on the PP02. Is it possible? \$\endgroup\$
    – tiktok
    Mar 30, 2022 at 3:03
  • \$\begingroup\$ @polwel I too was anxious of that 1volte reference voltage, where would I get that on an arduino prototype or in the end manufactured product. Please also read my comment to Theodore, I was thinking of using another sensor's circuit with just one Op Amp. Would it work? \$\endgroup\$
    – tiktok
    Mar 30, 2022 at 3:04

2 Answers 2

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For prototyping and proof-of-concept, the LM358 will do in a pinch, especially since you have already ordered it.

The more complex of the two circuits (circuit 1) is different in a few ways:

  • It sinks a constant current through the bridge with a current source. If your supply voltage is stable enough, you can just use that (which is what circuit 2 does). For the sensor you have in mind, you'll need to limit the current to 100 uA with an extra resistor.

  • Circuit 1 uses 3 opamps to form an instrumentation amplifier. A simple difference amplifier like in circuit 2 might be good enough for your purposes. The price you pay for simplicity is reduced common mode rejection and decreased input impedance (-> lower signal). This may or may not be a problem for you.

  • Circuit 2 does not really do away with the voltage reference. It simply uses GND as reference. That seems like an a odd and fragile way to do things.

EDIT: For the record, let me emphasize you will need to make some changes to the circuit such that the lower swing of the 324/358 can work. For instance, as suggested by Spehro Pefhany, in circuit 1 the bridge's output is biased to an uncomfortably high voltage. You'll want to adjust that. Your comments do suggest you are very much a beginner. If you feel uncomfortable, and don't want to waste time, don't bother, get the recommended part. If you want to learn, by all means.

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  • \$\begingroup\$ Thanks. This answer has done a lot to clear my mind. I have decided for now to use the dual op amp LM358 amplifiers and see where it goes. I am building the circuit now, I think I would need some more advice soon. I will come back seeking it once I have some concrete questions, but do please tell how can I get the 1 volte reference voltage out of the Arduino. \$\endgroup\$
    – tiktok
    Mar 30, 2022 at 16:16
  • \$\begingroup\$ Also please clarify, in the first circuit I see three ground connections, what is the third ground connected to? To a layman like me it seems to be hanging in air (please forgive my lack of knowledge, I am just a mechanical engineer trying my hand in electronics for the first time). \$\endgroup\$
    – tiktok
    Mar 30, 2022 at 16:23
  • \$\begingroup\$ All the ground connections are implicitely connected. I have to agree with Sphero, if you are unfamiliar with even the basics, please consider just waiting for the right part. If you have nothing else to do in the mean time, feel free to experiment. \$\endgroup\$
    – polwel
    Mar 30, 2022 at 17:39
  • \$\begingroup\$ About the voltage reference: voltage divider fed from the 5V rail will do in a pinch. Add a buffer amplifier. Really this should be a new question. \$\endgroup\$
    – polwel
    Mar 30, 2022 at 17:40
  • \$\begingroup\$ Ok...thanks. Will order the LMV324 \$\endgroup\$
    – tiktok
    Mar 30, 2022 at 18:11
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The LM324 likely does not have enough output swing to reliably work in this application circuit as-is. The high end of the bridge is already at as much as 3.2V and the internal nodes of the in-amp have to go beyond that.

It should be possible to redesign the circuit to work reliably with the LM324 (or two of the LM358), but I think it would be easier and better to source the recommended component, at least for your initial testing. If you need to make a huge quantity then redesign to cut cost.

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