single or double supply opamp???(help)

Discussion in 'General Electronics Chat' started by saghar19, Dec 3, 2012.

  1. saghar19

    Thread Starter Member

    Jul 16, 2012
    I want to make an active buffer electrode for EEG recording,and I'm so confused about using single or double supply op amp:confused:.if i use single supply for op amp is there any problem?I'm worry about negative part of input voltage?what should i do?is it possible to use single supply or not?(as regards the input voltage amplitude is very low,about micro volt)
    i attached prototype circuit too.
    please help me soon!
  2. crutschow


    Mar 14, 2008
    If your signal goes plus and minus then you either need a dual supply op amp, or use a signal supply op amp and offset the input voltage to 1/2 the supply voltage so it can go plus and minus around this 1/2 point. Typically it's easier to use plus and minus supplies, if they are available.

    Note that either type of op amp can be used with either one or two supplies. The difference is that standard op amps only work with input and output signals that go within about 2V of the supply voltages, whereas rail-rail type op amps can work to the supply rails. There is a small subset of amps called single-supply that can work down to the negative rail (0V with a single positive supply) on the input and output but don't go all the way to the positive rail, such as the LM324.
  3. ramancini8


    Jul 18, 2012
    Since your schematic shows a single supply, I would go with a single supply amplifier. Look at "Op Amps for Everyone" on the Texas Instruments web site, and you will see a section that shows you how to design op amp circuits.
  4. saghar19

    Thread Starter Member

    Jul 16, 2012
    I have another question about this circuit
    in this circuit,output and positive rail are the same, so if the input be about micro volts is there any serious problem?I 'm really unclear on this circuit!!how does this buffer really work??i simulated that with two power supply and it works,but i can not analyze on paper!!please help!
    excuse me if my question is confusing!
    Thank you very much
  5. Jaguarjoe

    Active Member

    Apr 7, 2010
    Tying the (-) input, output and +V of the opamp all together probably is not gonna work. Back in the day we would evaluate op amps. One test was tying the output to the plus or minus rails and seeing if they would survive.
    If you are detecting microvolt level signals, the noise in the op amp will probably swamp that out.
    Look at Kistler Inst Co's Piezotron and PCB Piezotronic's ICP lines. They do the same thing but with a J-fet instead of the op amp. Works great.
  6. saghar19

    Thread Starter Member

    Jul 16, 2012
    Ok,but in simulation it works.I found this schematic on web,its an ieee transaction.But unfortunately doesn't answer with single supply.My question is about real function of this buffer and im using fet for the next step.
  7. saghar19

    Thread Starter Member

    Jul 16, 2012
    Oh my god!!any idea can help me,l want to know more and more!
    please tell,don't worry!!! ;)