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About OPA2227

bingtong
Prodigy 105 points
Other Parts Discussed in Thread: OPA2227, OPA227
 Hello,dear sir or madam
I need some help about op-amp opa2227.Situation is like that, I give the system one 100uV sine input signal, after 4-stage opa227 magnified 100 thousant times, the waveform jitters and amplitude changes.
 I had simulated the circuit ,and it was correct when simulated.while the actual circuit can not work successfully.Tht output waveform  is as follows.Anyone could tell me what  happened about the circuit or the phenomenon?Tht output waveform picture and video are as follow,

  • 0
    TI__Genius 12631 points
    Please provide a schematic of your circuit showing power-supplies and any load resistance. Also, what is the impedance of the scope (50ohm or 1Mohm)?
    Does the distortion go away if you reduce the frequency?
    Is the distortion present at the stages 1-3 or only on the 4th stage ?
    -Samir
  • 0 in reply to Samir Cherian
    Prodigy 105 points

    Hello samir,

    Thank you for your duidance.The impedance of the scope is 1Mohm.I can not make sure the distortion will or will not go away with  the frequency reduced because my circuit contains a band-pass filter.While,on a small scale ,it does not improve.

    The distortion present at the stage 3,and it is worse at stage 4.Myschematic is as follow.

  • 0 in reply to bingtong
    TI__Genius 12631 points
    Hello Bintong,
    The main thing I see currently is that your 4th amplifier looks to be configured as a positive feedback loop. Please fix this as this could be your main issue.

    To simplify this debug I would suggest isolating the stages and then checking to see at which point the distortion is present. For example, when checking up to stage 3, I would uninstall C48 and place the scope probe at the output directly and see what happens. One very important thing here is to be mindful of the probe capacitance as that can cause issues in measurement. What is the capacitance of your probe? One thing I do is to take a through hole 100ohm-1kohm resistor and wrap one side of its leads around my scope tip. I then place the other side of the resistor leads on the point I need probed. This way I isolate the output of the amplifier from the probe capacitance through that series resistor. Does this make sense?

    -Samir