change the DC bias of an AC signal ?

Discussion in 'Analog & Mixed-Signal Design' started by ralphnev, Aug 2, 2018.

  1. ralphnev

    Thread Starter New Member

    Jul 8, 2018
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    i have a fairly fast (~5Mhz) signal that is ~5-8VDC
    & reading this signal using a ADC that takes an input from 1-4VDC

    looking for solutions to shift the bias with little loss of fidelity of the signal?
    (for testing i had used a simple resistor divider but loose 1/2 the signal ...)
     
  2. ScottWang

    Moderator

    Aug 23, 2012
    6,415
    992
    If your reading from ADC is 1/2 of the input signal then you need to double the input values (multiple *2) or shift to left for one bit with assembly.
     
  3. ebp

    Well-Known Member

    Feb 8, 2018
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    777
    [EDIT] reread your question and now I'm not clear on what ~5-8VDC means. For the following I took it to mean that your signal swings between approximately +5 V and +8 V relative to circuit common ("ground"), so what you need to do is preserve the 3 volts peak-to-peak amplitude and just sift it 4 volts negative.

    You would typically do this with a differential amplifier with a gain of 1 and an offset of -4 volts, The signal goes into the non-inverting input and (functionally) +4 V goes into the inverting input. Given your ADC input span, this should be possible with an op amp with a single 5 volt supply since you don't need the amp to work very close to the supply rails. You would want to use good quality resistors. I would recommend 0.1% tolerance with 25 ppm/°C temperature coefficient if you need good stability and accuracy, but ordinary 1% types might meet your needs. You could likely use the voltage reference for the ADC for the offsetting voltage.

    e.g. (I think this is right - have to run off to so something & haven't checked properly)
    say 5V reference
    - reference through 1k to inverting input
    - 800 ohm feedback
    - signal through 444 ohms to non-inverting input (NII)
    - 556 ohms from NII to common (ground)

    gain of -0.8 for +5 V reference, so -4 V offset
    non-inverting gain is.556/(444+556) * 1.8 = 1
    Resistor values are for arithmetic simplicity - ratios are what is important & there may not be "real" resistors in these values

    You can find lots of info on single op amp differential amps on the web.
     
    ralphnev likes this.
  4. ralphnev

    Thread Starter New Member

    Jul 8, 2018
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    thank you / I'm mostly a digital guy / haven't worked with opamps in may years ...
    but i do understand above - now component choices ....
    is a video opamp appropriate ? ala AD8013 ?
     
    Last edited by a moderator: Aug 3, 2018
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