Unwanted offset in output of op-amp

Discussion in 'General Electronics Chat' started by Lambda, Feb 26, 2012.

  1. Lambda

    Thread Starter New Member

    Feb 28, 2011
    28
    0
    This amplifier I'm working on is designed to work with small signals, less than 10mV. The TLO82 chip is being powered with ±10v. The problem I have is DC offset in the output. What an AC signal input, the output is only being driven positive, no part of the waveform crosses ground.
    With gain at 38db, the offset is 0.4 volts. The voltage of the output signal (DC component removed) is 0.4 volts. The signal and offset aren't constantly the same, as at 44db, the offset is 1 volt and signal is 0.8 volts. Input is 1khz, 5mV. Offset seems constant regardless of frequency.
    I can get a proper AC signal out only if I set the offset of the input signal to -5mV. This value doesn't change no matter what the input level is.
    Basically, the op-amp is acing like the input is offset 5mV positive when it's not, and the only way to counter it is to set the offset negative. What's going on here?
    This is the relevant part of my schematic:
    [​IMG]
     
  2. #12

    Expert

    Nov 30, 2010
    16,267
    6,778
    5mv is within the specs of the chip. You can use a TL081 chip to get offset adjustment pins available or build an external offset adjustment circuit.
     
    Lambda likes this.
  3. Lambda

    Thread Starter New Member

    Feb 28, 2011
    28
    0
    Oh, it was the chip. I probably should have read that datasheet a bit more carefully. OK then, I'll just use TL081 chips instead of TL082s. No big deal.
    Thanks.
     
  4. #12

    Expert

    Nov 30, 2010
    16,267
    6,778
    Well, that was easy! I was expecting some reluctance. Lucky me :)
     
  5. jimkeith

    Active Member

    Oct 26, 2011
    539
    99
    The way you have configured your amp causes it to amplify the offset as well as the desired signal--try this circuit that has unity DC gain and will not amplify the offset.
     
    Lambda and #12 like this.
  6. Lambda

    Thread Starter New Member

    Feb 28, 2011
    28
    0
    Jim, you're a lifesaver. I just added the 0.47uf cap into my current circuit, and it works great. Thanks, man.
     
Loading...