Difference Amp vs. Instrumentation Amp

Discussion in 'General Electronics Chat' started by blah2222, Aug 2, 2012.

  1. blah2222

    Thread Starter Well-Known Member

    May 3, 2010
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    Hi, I am working on a project to collect some EMG signals from flexion of arms and legs and am not sure whether to go with a difference or instrumentation amp for my input stage. I am thinking either an INA106 (difference) or INA116 (instrumentation) by TI.

    Any thoughts on why one might be better than the other?

    I am also confused by the input differential parameter on the datasheets as the minimum voltages are shown in Volts, but these EMG signals are only a few miliVolts. How can an input to an op-amp have a minimum in Volts?! I must be reading this incorrectly.

    Thanks again,
    JP
     
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    An important difference would relate to the gain you require. The Diff amp has a limited gain on its own.

    In your case the small input signal issue would require some careful attention to noise performance when considering which device to use.
     
  3. blah2222

    Thread Starter Well-Known Member

    May 3, 2010
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    To my second question though, what is it referring to for MIN input differential voltage? Is that explicitly saying that it cannot handle less than +/- 1 V as shown in this datasheet: http://www.ti.com/lit/ds/symlink/ina106.pdf ?

    Why wouldn't that parameter be referenced in mV as that is normally how small these signals are?
     
  4. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    That is probably telling you that with a bipolar supply of ±15V the output will swing to the amplified limit without distortion with an input of ±1V. It might do better than that in practice but that is a guaranteed minimum differential input signal without loss of amplified signal integrity at the output.

    There's no reason you can't go down to milli-volt input levels with the proviso you could have issues when the signal of interest is associated with unwanted noise [common mode or differential mode] as well.
     
  5. crutschow

    Expert

    Mar 14, 2008
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    That's a good question. I don't understand what the ±1V minimum differential input voltage means either. It doesn't make sense to me. :confused: Perhaps it should have been in the MAX column.
     
  6. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    The real advantage of instrumentation amplifiers is extremely low DC drift and well controlled gain. You can make a great instrumentation out of some pretty mediocre op-amps, because the configuration cancels out most drift ills.
    Eric
     
  7. crutschow

    Expert

    Mar 14, 2008
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    But not offset voltage.
     
  8. t06afre

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    May 11, 2009
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