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Not getting proper gain in opamp LM324N

  1. Feb 17, 2010 #1
    I used LM324N(4 opamps on 1 chip) from ON semiconductor to build an Instrumentation Amplifier circuit. I chose the values of the resistors so as to give a gain of 640. But when i checked the gain on the CRO the gain was around 175. What problem could this be?
    Datasheet link:http://www.onsemi.com/pub_link/Collateral/LM324-D.PDF
     
  2. jcsd
  3. Feb 17, 2010 #2
    What impedance is the output driving? What is your Vcc? Please post your circuit.

    Bob S
     
  4. Feb 18, 2010 #3
    I used this circuit:
    http://img195.imageshack.us/img195/9979/instamp.jpg
    I observed the output directly across the CRO.
    So don't have any idea about output driving impedance...
     
  5. Feb 19, 2010 #4

    berkeman

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    Staff: Mentor

    What voltages are you using for your power supply rails? What voltages are your inputs? (not super-important for experimenting, but what are you doing to tie off the 4th LM324?)
     
  6. Feb 19, 2010 #5
    that fig 13 equation is missing something
     
  7. Feb 19, 2010 #6
    LM324 has compatibility for split supplies and single supply.
    I used split supplies +12V and -12 V. Yeah I forgot to mention that I wasn't getting the output with +5V and ground. I'm sorry but I'm very new to op-amps. I left all pins of op-amp 4 open.
    What do you mean by tie off? Am I supposed to connect the pins of opamp which is not being used?
     
  8. Feb 19, 2010 #7
    I did the experiment on a trainer kit and rigged up the circuit using patch cords. Is there a chance that I'm not getting proper gain because of noise picked up by the cords?
     
  9. Feb 19, 2010 #8

    berkeman

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    It's good practice to tie off unused gates and opamps/comparators/etc. In this case, just ground the + input and configure it as a follower.

    On your gain question, what is your input signal (both differentially and common-mode)? With your target gain, what would that give you at the output?
     
  10. Feb 19, 2010 #9
    Could you also indicate the frequency and level of the input signal. The LM324 has only 1MHz unity gain bandwidth, along with fairly modest slew rate capability - see datasheet enclosed.

    Your circuit uses three amplifier cells, and I can't say exactly what the bandwidth would be by just looking at it, but I would guess that it won't be more than a few kHz for a gain of 640. Perhaps therefore what you are seeing is a frequency limitation.
     

    Attached Files:

  11. Feb 20, 2010 #10
    with lm324 you must ensure your circuit operating voltages stay well clear of the supply voltages. This is not a rail-to-rail op-amp.

    With an instrumentation amp circuit show, it is very important to match the like-value resistors as closely as possible.
     
  12. Feb 20, 2010 #11
    You should start out with C = a = b = 1, and R = R1 = 10k, and then increase C to 5, and a and b to ~10. This will give a gain of ~5 x 21 = 105. You should avoid any feedback resistors less than ~ 2K or or over ~100K. The gain-bandwidth product of the LM324 is ~1 MHz, so for a gain of 175, you should expect a small-signal bandwidth of ~5 KHz.

    Bob S
     
  13. Feb 21, 2010 #12
    What are rail to rail ICs. I tried to find out on the internet but didn't find a satisfactory explanation. Can anyone explain the exact meaning of rail to rail...
     
  14. Feb 21, 2010 #13

    berkeman

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    That refers to an opamp that can swing its output voltage very close to the power supply rails, and may also accept input voltages that are close to the power supply rails.

    So for example, if you are running the opamp with +/-12V supply rails, the inputs and outputs can swing very close to those supply rails.

    The original LM741-type opamps were not able to do this, because they were made up of bipolar transistors and needed some headroom from the rails at their inputs, and could not drive their output to more than a volt or two from the rails. The newer CMOS opamps, at least the more expensive ones, can have much closer to rail-to-rail performance.
     
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