battery powered op-amp

Discussion in 'General Electronics Chat' started by sai, Jul 24, 2008.

  1. sai

    Thread Starter New Member

    Jul 24, 2008
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    Hi,

    I am trying to a power op-amp OP 627 with batteries. I used two 18V batteries in series with positive end to +Vs and negative to -Vs. I used the junction between the batteries to ground so it would be zero and the +Vs and -Vs terminals would be +18V and -18V respectively with respect to the ground.

    I am getting an AC output from the op-amp pk/pk 6V . I have built this thing on a bread board. I am not sure if its noise either.

    Can someone tell me how to go about powering the op-amp and if there is an explanation to the AC output.

    Thanks
    Lalitya
     
  2. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,574
    230
    Be aware that ±18v is the "absolute maximum" for this IC. If these batteries run a little higher than 18 volts you may do the chip in. :(

    How about a schematic of what you have on the bread board?

    Ken
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You are right at the maximum power supply limits for that amplifier.

    Double-check to make sure that the batteries are not putting out more than 18v.

    What does the rest of your circuit look like?

    If there is nothing connected to the inputs, the output will wander aimlessly.

    Connect the output to the inverting input, and connect the non-inverting input to ground. Then you should get a very steady unity-gain output, and you will be able to measure the offset voltage.
     
  4. sai

    Thread Starter New Member

    Jul 24, 2008
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    [​IMG][​IMG]Hi,


    I have attached the "circuit diagram". Couldn't draw it any better. Is this circuit correct? as in will it work? If no, how do I do it? I am a mechanical engineer with no background in electronic stuff. So I really need some help.


    The batteries are pumping 16V each.

    And I have made a summing amplifier. The input voltage is 5V and gain is unity( because the input resistance and resistance across non-inv and output terminal are same value).

    The output is +3.5V which is strange because its inverting input. When I add another 5V to the input the output doesnt change.


    Thanks to all replies . further help will bemuch appreciated
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    We cannot see the first two circuits. You tried to post them from your hard drive, and that will not work. You can see them, since they are on your drive. We cannot see them.

    You must attach them to your post, as you did with the basic battery/opamp connection. (See the attached for how one might typically draw such a circuit)

    Which terminal (pin) on the opamp are you calling "GND"?
     
  6. sai

    Thread Starter New Member

    Jul 24, 2008
    5
    0
    Hi,

    This time I managed to draw the entire circuit. Tell me if this should work. The one I am calling ground is the non-inv input
     
  7. sai

    Thread Starter New Member

    Jul 24, 2008
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    Oops There is the diagram
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    With the way you show that you have it connected, you should be reading about -10v at the output. It may be off a little due to the offset voltage of the operational amplifier itself, but less than 500uV (0.5mV). Differences in the resistors due to tolerance variations would likely throw it off much more than that. Measure each resistor to get an actual value.

    Please review your connections. It's an OPA627, correct? If so, then:
    Pin Function
    1 : Offset trim. Used in conjunction with pin 5.
    2 : -In (Inverting Input)
    3 : +In (Non-Inverting Input)
    4 : -Vs (Negative supply voltage)
    5 : Offset trim; used in conjunction with pin 1.
    6 : Output
    7 : +Vs (Positive supply voltage)
    8 : No connection

    If you wish to determine what the offset voltage of the opamp is, then:
    1) Remove all connections from the input and output terminals.
    2) Connect the noninverting input to ground.
    3) Connect the inverting input to the output.
    4) Measure the voltage from the output to ground.
    If you measure more than 0.5mV, there is a problem somewhere.

    If you wish to adjust the offset voltage yourself, you will need a linear 100k Ohm potentiometer. I suggest a 10-turn or more trimmer pot.
    Connect one end of the pot to pin 1, the other end to pin 5, and the wiper to +Vs (+16v). Adjust the pot until you read 0v between ground and the output pin.
     
    Last edited: Jul 24, 2008
  9. sai

    Thread Starter New Member

    Jul 24, 2008
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    0
    SgtWookie,

    That is the whole issue. I have checked over and over. I did expect some offset but its way off. Thanks a ton for looking into it patiently. I will try the potentiometer angle.
     
  10. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    If at some point you accidently wired +Vs and -Vs incorrectly, even momentarily, the op amp would be destroyed. They are generally pretty rugged, but won't take abuse like that.

    Also, since the inputs are FETs, they are very vulnerable to static electricity. Even a tiny "zap" will render them useless. This is a big problem in low humidity environments.
     
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