Add and remove DC Bias from a signal

Discussion in 'General Electronics Chat' started by Dollarday, Jul 20, 2012.

  1. Dollarday

    Thread Starter Member

    Jan 25, 2012
    32
    1
    Hi all :)

    Quick question: I need to add and then remove DC bias to a data signal.

    To remove the DC bias is easy (I plan to use a high pass filter consisting of a resistor and capacitor)

    However, how do I add the DC bias to the signal?

    I did a bit of reading and the following circuit was found:
    http://www.ecircuitcenter.com/circuits/opsum/opsum.1.gif
    (The picture is from the level shifter from http://www.ecircuitcenter.com/circuits/opsum/opsum.htm)

    However, this is described as a inverting summer,which would invert my data signal (Which I do not want)

    I would also like to work from a single supply OP-AMP, so I want to keep my signal above ground. Any suggestions?

    Regards,
    Dollarday
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,421
    3,355
    Try this:

    [​IMG]

    Your signal will be attenuated somewhat.
     
  3. ramancini8

    Member

    Jul 18, 2012
    442
    118
    You can use a non-inverting op amp configuration to add a dc bias.
     
  4. Dollarday

    Thread Starter Member

    Jan 25, 2012
    32
    1
    Ramancini8, If I use a non-inverting op-amp configuration, wouldn't my signal be inverted
     
  5. crutschow

    Expert

    Mar 14, 2008
    12,988
    3,226
    By definition a non-inverting configuration does not invert the signal. It's "non-inverting".;)
     
  6. ramancini8

    Member

    Jul 18, 2012
    442
    118
    Be careful, the non-inverting inputs interact because they work into an infinite impedance, while inverting inputs do not interact because they work into a virtual ground. See "Op Amps for Everyone"; this book is on the Texas Instruments web site.
     
  7. crutschow

    Expert

    Mar 14, 2008
    12,988
    3,226
    You can also add a DC bias by using a series capacitor and a resistive divider to the supply voltage at the cap output. The equivalent resistor and capacitor values just have to be selected so they don't attenuate the frequencies of interest.
     
  8. Dollarday

    Thread Starter Member

    Jan 25, 2012
    32
    1
    If I use a inverting summer and then an inverting op-amp after that, would the inputs still interact?
     
  9. MrChips

    Moderator

    Oct 2, 2009
    12,421
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    Here is a summing amp:

    [​IMG]
     
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  10. ramancini8

    Member

    Jul 18, 2012
    442
    118
    The two inverting op amp solution works well, but insure the op amps are not in a feedback loop if they are both the same type. In the non-inverting summer shown in the previous post the positive op amp input is assumed to be infinity and the signal and offset inputs are assumed to be driven by zero impedance sources. So, when considering the signal input, R4 is in parallel with R5, and the parallel combination forms the lower resistor in a voltage divider with R3. Considering the offset input, R3 is in parallel with R5. You can write two equations for this situation and solve for exact resistor values, but changes will require more calculations.
     
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  11. ruiseixas

    New Member

    May 15, 2016
    1
    0
    Easy:
    [​IMG]
     
  12. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,516
    1,246
    If the source of the signal is a relatively low impedance such as an opamp or gate output, and the destination is a relatively high impedance such as an opamp or gate input, then you can add DC bias with a coupling capacitor and two resistors. This is what is happening in post #9.

    ak
     
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