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Choosing the right Op Amp for a differential amplifier

Antony Flores
Prodigy 30 points
Other Parts Discussed in Thread: INA326

I have designed a 3 Op Amps instrumentation amplifier with a variable gain (potentiometer) from 1V/V to 10,000 V/V . It seems to work fine in the simulations (TINA TI) with Op amps that are dual supply, but the problem is that I'm limited to build this with a positive single supply of 5V. I've tried different op amps single supply rail to rail, but they seem to not work at low input voltages. 

For example, If I put a 0.1V and 0.08V with a gain set to 100V/V, the reference value will be 0.02V multiplied by 100V/V. The expected output should be 2V, but I don't see this. The output values are really small and sometimes they don't even amplify at all. The problem is with low input voltages.

*Note: Both input voltages are DC, not AC.

Any advice on what should I be looking at? Any suggestion will be appreciated,

Thanks!

  • 0
    Prodigy 30 points

    Hello,

    I'm trying to design a difference amplifier with variable gain from 1 V/V to 1000 V/V preferably using a potentiometer. It is preferable that design has a linear gain, but it doesn't have to be perfect. For the op amps is preferably to use one single supply because only one +5V supply is given. A rail to rail was suggested do to the fact that low input voltage may be used. Also, something important is that both input voltages are DC.

    I have tried and simulated in TINA many topologies and different circuits from books and the internet, but none of them seems to work. It's kind of frustrating. Any help will be appreciated.

    Thanks!

    - A

  • 0
    TI__Genius 16025 points
    Hi Antony,

    Let me forward your question to the right group.

    Thanks,
    Anston
  • 0 in reply to Anston Lobo
    Prodigy 30 points
    Thanks! I'm new here hehe
  • 0
    TI__Mastermind 25815 points
    Hello Anthony,

    This is likely a common-mode vs. output swing issue. Even using rail-to-rail input/output (RRI/O) op amps for your instrumentation amplifier (IA) will not yield a RRI/O IA. For more information about this, please refer to this FAQ:
    e2e.ti.com/.../3373.understanding-instrumentation-amplifier-vcm-vs-vout-plots

    We have one RRI/O IA: INA326. Please note that it's not a traditional 3 op amp IA and I don't recommend that you build it discretely.
  • 0 in reply to Pete%20Semig
    TI__Mastermind 25815 points
    Hello Antony,

    It appears as though you've posted this question somewhere else, so I have merged the two threads.

    Please note that there is a discrepancy in the gain ranges you have posted (1,000 and 10,000).

    Finally, please understand that a discrete solution will suffer from resistor mismatch, which will limit the gain. We can typically trim the internal resistor ratios to within ~0.01% which yields good CMRR and gain error. Purchasing such resistors tends to be expensive if not impossible. Therefore I recommend simply using an integrated instrumentation amplifier.