GND V+ V-

Question

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?

dukepro · Answer

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

autodeskguest · Answer

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