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    [Moved]: driving opamp and Adc with usb voltage...

    hi all.I've 2 question...
    1.I wanna supply an Opamp with (+ -)5 v but unfortunately I've only a 5v usb voltage available.
    how can i drive the opamp ???
    2.I have also an 16bit adc on this design
    for 2.5 volt reference I have used ADR421 bandgap voltage refrence.
    my problem is with analog input which is 5 v and i have to supply it with usb voltage.
    how can i handle this with lowest noise possible?(filtering or usnig LDO or any way you think )

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    Re: driving opamp and Adc with usb voltage...

    Here is a method for obtaining -5V from positive 5V pulses. A 555 IC drives a voltage multiplier. Diodes are oriented to produce negative output.



    The 7 ohm resistor can be whatever resistance may be needed to keep current flow below the safe limit recommended for the 555 IC.


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    Re: [Moved]: driving opamp and Adc with usb voltage...

    Brad's solution is nice, I like that.

    ICs like the 7660 turn input voltage to negative output with almost no loss and are easy to configure - two 10uF caps.
    "Super Charge Pump DC-to-DC Voltage Converter"


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    Re: [Moved]: driving opamp and Adc with usb voltage...

    thanks.
    what can I do for question 2?



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    Re: [Moved]: driving opamp and Adc with usb voltage...

    ADR421 has a very good rejection to ripple up to some undred of kHz, so probably you don't need further filtering unless you suppose to have higher frequency noise



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    Re: [Moved]: driving opamp and Adc with usb voltage...

    ADR421 has a very good rejection to ripple up to some undred of kHz, so probably you don't need further filtering unless you suppose to have higher frequency noise
    the concern is analog supply of Adc which is 5v and i have to supply it with usb voltage...
    what can i do for this?!



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    Re: [Moved]: driving opamp and Adc with usb voltage...

    OK, I think a LDO will do the job. For instance ADP333, but there are many other. This one has the benefit that it's stability is almost not affected by the ESR of the output capacitor.


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    Re: [Moved]: driving opamp and Adc with usb voltage...

    So what about dropout voltage ?
    for example if the usb voltage is 5v or less(4.8) the LDO voltage will be 4.8 or less(4.6) in typical .
    i am using AD7655 .can it be a problem for adc?
    what about this way?
    http://andybrown.me.uk/2015/07/24/usb-filtering/



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    Re: [Moved]: driving opamp and Adc with usb voltage...

    Hi amin5659,

    How about - if you aren't already - decoupling the op amp and decoupling the adc supply pins, as it seems you have a very tight margin to use an LDO. Usually 100nF can be enough, but I like to put 100nF polyester, 100nF X7R ceramic and 1uF polyester in parallel as close to the supply pins of all ICs as possible.
    Also, bypassing the USB power supply would be a recommended step to smooth out the USB ripple a bit, perhaps, if that is not already a part of your circuit design. I think you can put a very low Ohm resistor in series with the bypass caps, which would also help.
    Not sure you would really need an LDO, but I'm no expert, and if the adc is low power, you could maybe perhaps try to use a 5V ref (no dropout) on the supply pin, but that is not a very good method, I think.

    - - - Updated - - -

    ...another (convoluted) method: If your USB can output sufficient current, you could begin the circuit with a voltage doubler subcircuit, then follow that with a 5V regulator, to power the ADC and op amp, and if the ADC/op amp are low power enough any old 5V linear regulator would suffice unless an LDO were preferred for some reason.


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    Re: [Moved]: driving opamp and Adc with usb voltage...

    Hi,

    your AD7655 power supply specification is 4.75V ... 5.25V

    But USB power supply specification is 4.4V ... 5.25V (wikipedia)
    (I thougt USB Spec tells 4.5V ... 5.5V, but I may be wrong)

    My experience with USB supply is worse. Especially when drawing more than 100mA, use some longer cables and the use of passive HUBs.
    Especially cheap USB cables and cheap USB hubs seem not to take care about USB specification.

    ***
    My recommendaton: Use a 3V3 ADC and a LDO.
    Otherwise you need a SEPIC or buck/boost swtiching regulator to ensure proper power supply.


    **********
    OPAMPs usually are more tolerant against power supply fluctuations. Use bulk capacitors and fast ceramic capacitors .. additionally mabe inductors (donīt forget to damp resonance frequency) .. to stabilize supply voltage.

    Klaus


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    Re: [Moved]: driving opamp and Adc with usb voltage...

    thanks all.
    the main problem is usb voltage which is variable from one PC to another .
    and with bypassing Caps and ... the problem still remain.
    I have used AD7655 before and i got to 40 count noise in 64K that is OK for me .
    but I have used a 8v adapter and the ADC analog pins(5v) came from 7805 regulator with ferrite bead and decoupling capacitors which are necessary.
    My recommendaton: Use a 3V3 ADC and a LDO.
    my new duty is supplying this boards with USB without changing ICs.
    because of that I cant use another ADC
    =============================
    ...another (convoluted) method: If your USB can output sufficient current, you could begin the circuit with a voltage doubler subcircuit, then follow that with a 5V regulator, to power the ADC and op amp, and if the ADC/op amp are low power enough any old 5V linear regulator would suffice unless an LDO were preferred for some reason.
    Otherwise you need a SEPIC or buck/boost swtiching regulator to ensure proper power supply.
    So i have to use booster?!
    It seems little impractical because my board must be as small as possible.
    would you tell me about a simple and compact booster circuit which i can boost USB voltage for using LDO?
    ================================================== ===========
    If i forget about setting usb voltage to exactly 5 v and just confine to filtering USB voltage such as this :
    http://andybrown.me.uk/2015/07/24/usb-filtering/
    I will have to issues :
    1.the USB voltage :
    I think its allitle impossible for USB voltage to exceed from this range :4.75V ... 5.25
    what about you?
    2.the ADC analog pin's noise...
    do you think the filtered USB voltage (in the website) can be as well as 7805's output?!

    this way is very compact and easy!! and it's tempting me



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    Re: [Moved]: driving opamp and Adc with usb voltage...

    Hi,

    I have used AD7655 before and i got to 40 count noise in 64K that is OK for me .
    --> This is about 10 bit precision.

    Analog noise: Where does it come from? From power supply?


    Klaus


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    Re: [Moved]: driving opamp and Adc with usb voltage...

    no. I think it was signals noise that comes from senor.
    with connecting reference pin to channel i measured the numbers have 5 count noise ....



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    Re: [Moved]: driving opamp and Adc with usb voltage...

    Quote Originally Posted by amin5659 View Post
    So i have to use booster?!
    It seems little impractical because my board must be as small as possible.
    would you tell me about a simple and compact booster circuit which i can boost USB voltage for using LDO

    do you think the filtered USB voltage (in the website) can be as well as 7805's output?!
    Hi,

    a) have a look at "Boost Charge Pump (Inductorless)" - that type of IC may provide a good "real estate" solution.

    This was the first on the list on a manufacturer's product page for Charge Pumps, it looks like it could just fit your specifications:
    http://www.ti.com/lit/ds/symlink/lm2775.pdf

    b) It wouldn't be as good as a manufactured and perfected for years linear regulator if I tried to do it, but I'm pretty useless, so maybe it is as easy as the blog/website makes it look.


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