voltage amplification

Discussion in 'General Electronics Chat' started by anoup, Oct 25, 2010.

  1. anoup

    Thread Starter New Member

    Apr 27, 2009
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    I need to amplify the voltage from the strain gauge (0.1mV - 2mV) to Volts. I have tried it with a 741 and Lm324 but the saturates. Which op-amp can i use and if you do have a schematics that would be really great.
    Thanks
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    If you are using a strain gauge bridge, the output voltage is differential. You need an instrumentation amp to work with it.

    The old 741 and LM324 op amps are really poor performers, so do consider using more modern op amps.

    Can you provide a schematic of your circuit?
     
  3. anoup

    Thread Starter New Member

    Apr 27, 2009
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    I have tried that circuit.
    Can you please suggest me any instrumentation amp that i can use with the strain gauge.
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    It would be very helpful to have the schematic of the whole circuit and the parameters - things like excitation voltage, desired gain, power supply rails, things like that.
     
  5. anoup

    Thread Starter New Member

    Apr 27, 2009
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    The strain gauge used is an old 4-wired strain gauge found in the lab. The excitation voltage of the strain gauge 10V, and the signal from it is in terms of mV, voltage gain = 1000V/V because i want to amplify mV to Volts.
    The power rails is dependent on the op-amp used.(5V - 15V).
    I just need to amplify the voltage to have greater accuracy while taking values (voltage) for the different load.
     
  6. sage.radachowsky

    Member

    May 11, 2010
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    A couple of things:

    1. Your circuit is a simple single-ended amplifier. You may be able to use this with the differential output of your strain gauge, if the amplifier circuit is battery powered and "floating" -- so that simply one end of the differential pair becomes the ground for the amplifier circuit. I did just do this exact thing for a researcher who has a pressure gauge with the same kind of output. I made a floating pre-amp. However -- this is very important -- the gauge is basically a Wheatstone bridge, so your pre-amp circuit should not "pull" on the outputs. In other words, you need to have a very high impedance input to your circuit. So what I did was to buffer the input with a voltage follower, on the input that is in the feedback loop.

    2. There are "instrumentation amplifier" chips out there, which is a differential amplifier. However, I have found these not great for various reasons, like too high of an offset error. With your application, it is important to have a very low offset error, I think.

    3. Therefore, why not build your own diff amp using three op amps. You can get op amps with very very low offset errors, especially the "chopper" type op amps which are continuously recalibrating to have nearly zero offset error. Those might work well for you, because you want to amplify millivolt-level signals in a linear way. So, two of the op amps become voltage followers and the third forms the diff amp. You can also use the instrumentation amplifier topology.
     
  7. anoup

    Thread Starter New Member

    Apr 27, 2009
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    0
    The gauge is indeed a Wheatstone bridge. Can you please provide me with a schematic and the IC or op-amp to be used? or i can just use the instrumentation amplifier topology with LM 324?
    Infact I was trying the circuit using a potentiometer first before actually incorporate it with the strain gauge. But i never had the voltage gain i wanted (mV - V). Can you please help me?
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Read this Intersil Application Note: http://www.intersil.com/data/an/an1298.pdf

    It explains why you don't want to attempt to build one out of discrete opamps, and shows a number of circuits.

    Besides Intersil's instrumentation amplifiers, look at TI's INA114, INA118, INA128, INA129 amps.
     
  9. Jaguarjoe

    Active Member

    Apr 7, 2010
    770
    90
    I'll vouch for that. 40 years ago I spun my wheels for weeks trying to do that.
     
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