# +2.5/-2.5 V Voltage Divider CAD Doubt

#### giovannelucas_

Joined Jan 5, 2023
34
Hi everyone,

I'm trying to design a voltage divider to power op amps. However, I'm in doubt if my schematic layout is correct as it uses 5V power from an Arduino instead of a double cell battery.

Another question is, for homemade etched copper pcbs with Perclorete. What should the trace width for power and signal be? 0.3mm for signal and 0.5 mm for power or 1 mm for both? How does eletromagnetic interference play a role in building noise in precision analog or digital signal applications?

Thank you.

#### dl324

Joined Mar 30, 2015
16,146
As drawn you have problems. You have both ends of the voltage divider connected to 5V. If you ever connect GND in your schematic to ground for the Arduino, you'll also have problems.

Instead of showing us your proposed solution, why don't you tell us what you're trying to accomplish?

#### crutschow

Joined Mar 14, 2008
33,355
Why do you think you need a dual supply?
Often you can use op amps with a single supply, and just offset the signal to the middle of the supply voltage.
What op amps do you intend to use?
Can they operate from 5V?

#### sghioto

Joined Dec 31, 2017
5,099
If you going to use a virtual ground the resistor values need to be smaller and observe polarity on C5.

#### giovannelucas_

Joined Jan 5, 2023
34
As drawn you have problems. You have both ends of the voltage divider connected to 5V. If you ever connect GND in your schematic to ground for the Arduino, you'll also have problems.

Instead of showing us your proposed solution, why don't you tell us what you're trying to accomplish?
Hi @dl324, I'm building an ecg amplifier. So the circuit needs to operate between +2.5V and -2.5V in order to reflect the negative parts of the ECG form. Right?

#### giovannelucas_

Joined Jan 5, 2023
34
Hi @sghioto,

Sorry for my question, How do I draw that 5V + and - symbol on Kicad? Is it on the power library or symbol library?
You're right about C5, I didn't realize the polarity was wrong.

#### sghioto

Joined Dec 31, 2017
5,099
Sorry for my question, How do I draw that 5V + and - symbol on Kicad? Is it on the power library or symbol library?
I don't know.
Here's another virtual ground using an op amp.

#### WBahn

Joined Mar 31, 2012
29,510
Your resistors need to be size so that your midpoint stays close enough to the desired midpoint voltage under the maximum current that it will need to support.

#### giovannelucas_

Joined Jan 5, 2023
34
I don't know.
Here's another virtual ground using an op amp.
View attachment 299816
Hi sghioto, I tried to make a modification on the schematic, similar to this one you showed me.
But when I go layout the board the OPAMPS still show powered by +5V and GND.

#### Attachments

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#### dl324

Joined Mar 30, 2015
16,146
But when I go layout the board the OPAMPS still show powered by +5V and GND.
Are you measuring with respect to what you've called the Arduino ground? Or your virtual ground?

If you measure between the V+ and V- pins of the opamp, you'll measure 5V. If you measure with respect to the virtual ground, you'll get 2.5V on V+ and -2.5V on V-.

#### giovannelucas_

Joined Jan 5, 2023
34
Are you measuring with respect to what you've called the Arduino ground? Or your virtual ground?
Hi dl324, the Arduino ground. It should show +2V5 and -2V5 on the pins when I go layout on kicad.

#### sghioto

Joined Dec 31, 2017
5,099
Hi sghioto, I tried to make a modification on the schematic, similar to this one you showed me.
Don't modify it use this circuit: Measure from ground to +5 volts and -5 volts

#### giovannelucas_

Joined Jan 5, 2023
34
Hi @sghioto, it's because I don't know how to draw that +5V with a green wire and + and - poles. I searched but didn't find that symbol. Would you mind showing me how to draw that symbol?

#### sghioto

Joined Dec 31, 2017
5,099
Would you mind showing me how to draw that symbol?
I just copied the schematic and modified it in Windows Paint. Had nothing to do with Kicad.

#### dl324

Joined Mar 30, 2015
16,146
the Arduino ground
Since you're "generating" the negative voltage from the Arduino supply, you can't have a negative voltage with respect to Arduino ground.
It should show +2V5 and -2V5 on the pins when I go layout on kicad.
Voltages are relative to whatever you're calling ground. You can have -2.5V relative to your virtual ground, but you can never connect your virtual ground to Arduino ground.

#### giovannelucas_

Joined Jan 5, 2023
34
Hi Dennis, thanks for the clarification. I understand the concept of the relation of voltage and ground, however, I don't know how to draw it on a schematic for a PCB. Do you have any schematic examples?

#### sghioto

Joined Dec 31, 2017
5,099
'm building an ecg amplifier. So the circuit needs to operate between +2.5V and -2.5V in order to reflect the negative parts of the ECG form. Right?
Will the Arduino recognize a -2.5 volt signal?

#### dl324

Joined Mar 30, 2015
16,146
I don't know how to draw it on a schematic for a PCB. Do you have any schematic examples?
Your virtual ground is just another net in the schematic. You can use any net name you want, but using GND will cause confusion, while Va wouldn't. You can avoid even more confusion by not using the ground symbol to represent it.