Dual Power Supply for Op-Amps

Discussion in 'The Projects Forum' started by padhu84, Dec 21, 2009.

  1. padhu84

    Thread Starter New Member

    Dec 15, 2009
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    Hi Guys

    I need to power up an opamp with dual supply in the range of +/-1.8V. The application is for a hand-held system and hence this is a battery-powered system. I am contemplating using two LDO's ie. Positive and negative LDO's for generating +1.8V and -1.8V respectively. But the problem is the two LDO inputs requires voltage of opposite polatiry. Hence it seems like i need to use two separate set of battery cells to meet this.

    I have represented it as a block and attached it. Please let me know whether this implementation would work good and also whether there are any other alternates to overcome this separate batteries requirement

    PS: I tried using +ve to -Ve voltage converter but this generates unwanted noise, which surprisingly, could not be eliminated by power line filtering. So i am avoiding switching converters.
     
  2. padhu84

    Thread Starter New Member

    Dec 15, 2009
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    You there???

    Looking forward to your suggestions on this!! :cool::rolleyes:
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    So, +1.8v and -1.8v are the limits of the amplifier you're using?
     
  4. padhu84

    Thread Starter New Member

    Dec 15, 2009
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    Actually.. the power supply range is +/-0.9V and +/-2.5V. Hence decided on the voltage of 1.8V.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    So, why not just use two 1.2v cells per rail?, one pair for +v and one pair for -v?

    Or, choose a couple of 1.5v cells, and use one per side?
     
    Last edited: Dec 22, 2009
  6. Wendy

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  7. padhu84

    Thread Starter New Member

    Dec 15, 2009
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    I am not sure whether we get negative LDO's with such low input condition.

    I am planning to use this - LT1964- the adj version.
     
  8. padhu84

    Thread Starter New Member

    Dec 15, 2009
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    Actually.. i am doing just that rite. couple of 1.2V cells for each of V+ and V-. Or am i missing something here
     
  9. Tahmid

    Active Member

    Jul 2, 2008
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    Hi,
    You can try using ICL7660 from Dallas/Maxim. It creates dual supply from a single supply.
     
  10. padhu84

    Thread Starter New Member

    Dec 15, 2009
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    Hi Tahmid

    I have used the same chip in my earlier design but that is generating unwanted harmonics at certain frequencies. Thats the reason precisely i am going for negative LDO.

    @Sgt Wookie

    I am confused by your previous post. Can you help me bridge the gap???:confused:
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    Padhu84,
    What I am suggesting is that you don't need regulators if the total battery voltage is less than the opamp maximum voltage.

    Tahmid,
    Padhu84 did try using an ICL7660, but had noise problems. It is hard to avoid that when you are working on a breadboard. If he had it on a PCB with a ground plane and lots of bypass caps, that problem would likely go away; but he needed a "sure thing".
     
  12. padhu84

    Thread Starter New Member

    Dec 15, 2009
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    Oh, yeah. I gave it a thought too. But decided to slot in the LDO's just to make sure that opamp is supplied with a constant-regulated voltage rather than a supply that has its voltage trickling down slowly. I understand the the circuit operation is not impacted because of the range with which the opamp can operate but its just that i felt a lot more ' safe ' with the regulators.

    Thanks as always Sgt!!
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    Keep in mind that the regulators have their own "tax"; they are not 100% efficient, they are not free, and as the batteries run down, their output will always be less than the battery voltage.

    Using regulators in this situation would really not be a good idea. You will be adding cost, complexity, and reducing battery life for no good reason.
     
  14. padhu84

    Thread Starter New Member

    Dec 15, 2009
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    My main worry is because i am going to rechargeable Ni-MH batteries as the battery source. So, as you guys definitely know, eventhough the batteries are rated for 1.2V nominal voltage, everytime i recharge them, it is charged to beyond 1.2V and to 1.4V. Hence when i use two batteries in series, it goes to 2.8V, which is beyond the max 2.5V specified by the opamp.

    Is there any way to overcome this without using a regulator??The only way to over come is perhaps not recharge the battery completely. Its ok for me but if someone else is going to handle the system, he/she mite just forget this
     
  15. SgtWookie

    Expert

    Jul 17, 2007
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    OK then.

    Why don't you use a single 3.6v Li-Ion battery.
    Use an opamp to create a virtual ground.

    You can do this by using a resistive voltage divider across the +/- of the supply, with small caps to both power rails as the noninverting input of an opamp, that is wired for unity gain.
     
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