How to raise an analog signal with a positive offset ?

Discussion in 'General Electronics Chat' started by arry, Aug 11, 2010.

  1. arry

    Thread Starter New Member

    Apr 2, 2010
    26
    4
    I am trying to raise an analog signal with Vmin -1V and Vmax +1V with an offset of 2V.

    I made a circuit with LTSpice but results are awful :) . It is just a simple divider and I can't understand why it does not work.

    A pic of the circuit is attached.
     
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  2. peck68

    Member

    Nov 27, 2009
    73
    0
    I have never had any experience in this sort of thing yet,
    but what about an AC step up converter? Just a thing that just popped into my head :) with a ratio of 1:2 it will double it
     
    Last edited: Aug 11, 2010
  3. Potato Pudding

    Well-Known Member

    Jun 11, 2010
    684
    92
    Use a capacitor to couple your signal.

    A source is always a short circuit unless you specify that it has series resistance in which case you will need to match impedance or your signal can start to go invisible.

    I think your failure to offset was probably because the center of your divider was grounded through the source. You can also put a resistor between sugnal and ground and if it is large enough not to load the divider it will cause a loss of signal because your AC path becomes high impedance, the same as speccing the batteries with high internal series resistance - but external.

    If you don't want use a capacitor and don't mind cheating or you are simulating a source that is naturally offset then specify a offset DC value for your signal battery.

    You have the same node labelled as In and Out they are connected by wire. A capacitor can separate them so they are two nodes.
     
  4. AMIT_GOHEL

    Member

    Jul 13, 2010
    67
    7
    [​IMG]

    use clamper ckt and change value of capacitor as per ur requremene.
     
  5. arry

    Thread Starter New Member

    Apr 2, 2010
    26
    4
    If I use a cap , I won't be able to maintain DC characteristics of the signal.

    Yes, I put a series resistance of the batteries and signal started to disappear.
    What do I have to do ?
     
  6. Papabravo

    Expert

    Feb 24, 2006
    10,135
    1,786
    You cannot do what you are trying to do in this fashion as the simulator quite rightly points out. The voltage divider simply will not do the job because it is fighting the AC source.

    The proper way to do this is to use an operational amplifier to add the DC offset to the AC source. Google "summing amplifier"
     
  7. Potato Pudding

    Well-Known Member

    Jun 11, 2010
    684
    92
    The Input goes through a voltage divider to cut it in half, which counters the Amps Gain of 2.
     
  8. arry

    Thread Starter New Member

    Apr 2, 2010
    26
    4
    OK, thanks for the circuit. Obviously I will have to use an op amp.
    But what I do not understand is , having in mind the simple voltage divider, if I add a cap, it works, if I remove the cap it does not work. A cap works as a power source, so removing it and connecting the battery directly to the divider should be the same thnig, at least for me. But it is not, why ?
     
  9. Potato Pudding

    Well-Known Member

    Jun 11, 2010
    684
    92
    A Capacitor allows AC but blocks DC.

    Because it has the ability to store some charge a change in voltage like AC will just vary the charge on a capacitor. A constant voltage from DC will just equalize across the capacitor and then the current stops.
     
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