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EAGLE User Chat (English) GND V+ V-
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GND V+ V-

Former Member
Former Member over 9 years ago

Hello, I am rather new to designing things here so I got what might be a stupid question

 

I am going to use a chip that has a V+, V- and a GND pin, I'll be using a power supply that only has +/- poles of suitible voltage

 

I have concluded (correct me if I am wrong) that V+ and + on the power supply are to be connected but the negative one I am uncertain, should it go to V- or GND?

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  • autodeskguest
    0 autodeskguest over 9 years ago

    On 09/07/14 06:55, Zelos Malum wrote:

    I am going to use a chip that has a V+, V- and a GND pin, I'll be using

    a power supply that only has ± poles of suitible voltage

     

    I have concluded (correct me if I am wrong) that V+ and + on the power

    supply are to be connected but the negative one I am uncertain, should

    it go to V- or GND?

     

    We can't really answer that without more details. The datasheet for the

    chip you've chosen is where you need to look. In general, though, if the

    chip has split power rails then you need to give it split power.

     

    Depending on the amount of power you're dealing with, it might be

    possible to get away with a single rail supply connected to V+ and V-

    with a resistor divider providing the midpoint for "GND". You would

    almost certainly need a fair amount of capacitance in parallel with the

    resistors, though. And this is all very dependent on what else is in the

    circuit.

     

    Why not use a more appropriate power supply? Or a DC-DC converter to

    provide split rails?

     

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  • Former Member
    0 Former Member over 9 years ago in reply to autodeskguest

    http://www.mouser.com/ds/2/256/MAX4638-MAX4639-64806.pdf

    This is the part, I am fairly certain it does not need both connected as someone used this IC with nothing but a voltage source and a ground (as a passive device for a signal)

     

    Why not use a more appropriate power supply? Or a DC-DC converter to

    provide split rails?

    Could you explain what this means? I am fairly new and wether it is applicable or not I'd like to learn the idea.

     

    In general, though, if thechip has split power rails then you need to give it split power

    What is the differens exacly between split pweor and single/straight power? Which terminology is used for the contrast to split power?

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  • dukepro
    0 dukepro over 9 years ago in reply to Former Member

    On 07/09/2014 04:46 AM, Zelos Malum wrote:

    http://www.mouser.com/ds/2/256/MAX4638-MAX4639-64806.pdf

    This is the part, I am fairly certain it does not need both connected as

    someone used this IC with nothing but a voltage source and a ground (as

    a passive device for a signal)

     

    Why not use a more appropriate power supply? Or a DC-DC converter to

     

    provide split rails?

    Could you explain what this means? I am fairly new and wether it is

    applicable or not I'd like to learn the idea.

     

    The term "split rails" refers to power supplies of equal voltages but

    opposite polarity.  They are used when voltages both above ground and

    below ground are required to be output One such case is the  RS-232RS-232

    specification, which requires both +12V as well as -12V as the nominal

    signaling levels.  Older RS232 drivers required three power supplies:

    +5V for logic levels, and +12V and -12V for signaling levels.  Most

    modern RS232 driver IC's have a built-in charge pumps to generate the

    +12V and -12V, that way the IC can be run off of a single positive power

    supply, such as +3.3 or +5.0V.

     

    The "split" part comes from the fact that you have a potential

    difference between the two voltages and you split the difference by

    placing GND right smack in the middle of the two voltages.  Thus, when

    measured from ground, half of the potential difference is positive, and

    the other half is negative.

     

    The term "rail" is a bit historical.  In days gone by, most IC's were

    very power hungry, and it was typical for a circuit board to require 5

    to 20 Amps of current at +5V.  The traces on the circuit board could not

    be made wide enough to carry this much current, so designers would place

    a trace along the top edge of the board with mounting points along the

    trace for #4 or #6 sized screws.  Then an L-shaped aluminum or copper

    extrusion would be mounted to all these holes.  The mounted extrusion

    would be able to carry the high current along the edge of the board.  Of

    course, the ground trace would have to have the same design since it

    carried just as much current as the power supply.  These power rails

    also served to stiffen the circuit board.

     

    Some boards required both positive and negative voltages.  So, designers

    would place the positive rail across the top of the board, the negative

    rail across the bottom of the board, and the ground rail would be right

    down the middle of the board.

     

    Another example of the need for split power supplies (sometime called

    "dual supplies") is that of an op-amp.  Many op-amps can not output a

    voltage equal to GND or V+.  They get close, but they can be as much as

    a diode drop away.  i.e. the maximum output voltage is (V+ - 0.5V), and

    the minimum output voltage is (+0.5V).  For some applications, this

    "close but not there" is not good enough,  They really need to get to

    ground or below it.  In such cases, the designer would choose to use a

    dual supply op-amp, one that has power inputs for both V+ and V-, and

    place the ground at 0V.  Now the lowest output voltage possible is (V- +

    0.5V) which is well below ground.  The output voltage limits vary from

    device to device, but I used 0.5V here as an example - it is by no means

    the rule.

     

    Getting back to the MAX4638 analog multiplexer, you can use this device

    in either configuration - dual power supplies of +/-2.5V, or with a

    single power supply of +5V.  Look in the section "Applications

    Information" on page 9.

     

    HTH,

        - Chuck

     

     

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  • Former Member
    0 Former Member over 9 years ago in reply to dukepro

    Now that is very informative and good! From what I read there it says GND and V- should be connected togather then, do I understand this part right?

    For single-supply operation, V- should be

    connected to GND as close to the device as possible. 

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  • autodeskguest
    0 autodeskguest over 9 years ago in reply to Former Member

    On 09/07/14 14:25, Zelos Malum wrote:

    Now that is very informative and good!

     

    Chuck usually is image

     

    From what I read there it says

    GND and V- should be connected togather then, do I understand this part

    right?

    For single-supply operation, V- should be

     

    connected to GND as close to the device as possible. 

     

    Yes, that means that you should lay out your PCB such that the V- pin

    and the GND pin are connected together right where they are, and

    explicitly NOT wired separately back to the supply. The latter is

    sometimes done for various reasons, so this is telling you not to.

     

    This is a component designed for use in two different ways, which is why

    it falls outside the "in general" of my earlier reply, so long as you

    have no need to feed negative voltages through any of the signal paths.

     

    Regards,

    Rob

     

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  • Former Member
    0 Former Member over 9 years ago in reply to autodeskguest

    Thank you!

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  • xoke
    0 xoke over 6 years ago in reply to dukepro

    I have signed in just to say how much I appreciated this explanation!

     

    Tks a lot man.

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