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GND V+ V- [message #156209] Wed, 09 July 2014 05:55 Go to next message
Zelos Malum
Messages: 5
Registered: July 2014
Junior Member
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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Re: GND V+ V- [message #156210 is a reply to message #156209] Wed, 09 July 2014 06:59 Go to previous messageGo to next message
Rob Pearce
Messages: 480
Registered: September 2012
Senior Member
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?
Re: GND V+ V- [message #156212 is a reply to message #156210] Wed, 09 July 2014 08:46 Go to previous messageGo to next message
Zelos Malum
Messages: 5
Registered: July 2014
Junior Member
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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Re: GND V+ V- [message #156216 is a reply to message #156212] Wed, 09 July 2014 12:59 Go to previous messageGo to next message
Chuck Huber
Messages: 599
Registered: October 2004
Senior 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-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
Re: GND V+ V- [message #156218 is a reply to message #156216] Wed, 09 July 2014 13:25 Go to previous messageGo to next message
Zelos Malum
Messages: 5
Registered: July 2014
Junior Member
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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Re: GND V+ V- [message #156224 is a reply to message #156218] Wed, 09 July 2014 17:43 Go to previous messageGo to next message
Rob Pearce
Messages: 480
Registered: September 2012
Senior Member
On 09/07/14 14:25, Zelos Malum wrote:
> Now that is very informative and good!

Chuck usually is :)

> 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
Re: GND V+ V- [message #156227 is a reply to message #156224] Thu, 10 July 2014 07:08 Go to previous messageGo to next message
Zelos Malum
Messages: 5
Registered: July 2014
Junior Member
Thank you!

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Re: GND V+ V- [message #170143 is a reply to message #156216] Fri, 24 March 2017 13:24 Go to previous message
Felipe Rocha
Messages: 1
Registered: March 2017
Junior Member
I have signed in just to say how much I appreciated this explanation!

Tks a lot man.

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