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DC coupling amplifier stages

Discussion in 'Electronic Design' started by Stretto, May 28, 2011.

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  1. Stretto

    Stretto Guest

    Is the only issue with DC coupling op amp stages the offset error? If so, in
    general, is this a big problem with modern op amps?

    How well does offset nulling work?

    Are op amps without nulling capabilities meant for AC coupling?

    Is the issue mainly a problem with high gain? Obviously high gain will
    increase the DC offset but is it something that should be worried about for
    low gain too?

    Does the CMRR reduces the DC offset significantly? Higher CMRR's will mean
    less DC offset? Can I expect by using simple circuit design(stuff found in
    basic textbooks) that the CMRR will reduce the DC offset by it's rated
    value? -120dB CMRR will reduce any DC offset by about 120db?

    The real issue with DC offsets has to do with the finite voltage swing
    capabilities? If we amplify the DC offset too much then the op amp cannot
    amplify the AC signals that may be outside than the rails? So DC offset
    problems simply reduce the headroom?

    Thanks
     
  2. Stretto

    Stretto Guest

    It's better to design your circuit so you don't need it. In general,
    leaving tweaks in your circuit costs more than buying more expensive
    components (or changing your system architecture) to render the tweaks
    unnecessary.

    -----

    Well, this is the problem. I'm designing some AC coupled stuff for audio but
    would like to forgo the coupling capacitors for various reasons. I'm worried
    about the dc offset issue as it could severely limit headroom. Since I've
    don't have any experience with dc offsets I'm not sure how big of an issue
    it will be. A few mV isn't a big deal at all but several hundred will be.
    The amplifier stages are rather low gain(< 50 easy). If a 10mV dc offset
    exists then that can become 500mV quite quickly.

    What I'm more worried about is some type of integration of the DC offset
    that might creep in, in some way which obviously can magnify the problem
    significantly. My 10mV dc offset maybe extreme though. In some of the op
    amp's I've looked at the input offset is typical 1mV but can reach 10mV.
    With any type of integration problems I could be seeing several volts rather
    quickly.

    I guess it's just something I'm going to have to get down and dirty with and
    work out.
     
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