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remove dc offset from signal going into an op amp

Intellectual 540 points

Replies: 15

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I have a signal that varies between 19V and 17V @ 250kHz. The signal operates in a burst pattern where it outputs 2048 values then stops for a few ms. I want to input this signal into an op amp and remove the dc level. I have a circuit that has a dc blocking capacitor between the signal and the op amp input. The op amp is set as a buffer, and also has a resistor to ground. So I have a high pass filter on the input of the buffer. The issue I have is that I am finding that the circuit takes a long time to settle if I select values that maintain the signal integrity. If I then change the values to speed up the settling time the signal starts to degrade. Are there any other options that will maintain the signal integrity whilst giving a fast settling time? I am open to using a completely different circuit. Thanks
  • Hello Jon,
    What is the part number of the op amp being used?


    Best Regards,

    Ed Walker
    Texas Instruments 
    Online Support - Systems & Tools

     

  • In reply to EdWalker:

    Hi Ed

    Its the OPA2625

    Thanks

    Jon

  • In reply to Jon Bean1:

    Hi Jon,

    can you show us a scope plot of the signal?

    Kai
  • In reply to kai klaas69:

    Hello Jon, would you consider a differential output. FDA's can remove DC offsets easily in a DC coupled signal path.

    Michael Steffes

  • In reply to Michael Steffes:

    And actually Jon, you would use a part like the THS4131 to level shift your input to 0V centered and then only use one side of the output as a single ended in to single out DC adjustment. 

    Would have to set up for a gain of 2 to get a net gain of 1 if only using one side of output. 

    Michael Steffes

  • In reply to Michael Steffes:

    Hi Michael

    Can you explain how I would connect the op amp.

    Thanks

    Jon

  • In reply to Jon Bean1:

    Hi Jon,

    again, can you show a scope plot of your signal?

    Kai
  • In reply to Jon Bean1:

    here you go Jon, see if this makes any sense,

    essentially using the FDA common mode loop to do a DC coupled level shift. It produces a differential output, but you can just use one side - works well it seems. Don't know your load, but match them on each output side.

    DC coupled level shift of 250kHz square wave from 18V centered to 0V centered.docx

    Michael Steffes

  • In reply to kai klaas69:

    Hi Kai

    Thanks for your reply. Having looked at Michael's solution I think that should perform as I want.

    Jon

  • In reply to Jon Bean1:

    Pretty cool yeah?? I had not done this previously, but can't let those common mode control loops go to waste - if you swings are not as known as you implied, added work here could adjust for input CM and Vpp variations.

    Michael Steffes

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