Analog and digital should be separate 5V supplies. Digital noise can cause problems with analog circuits.
I prefer to power with typically 7-8 volts although 12 is ok. The reason is the additional filtering etc I get with the onboard regulator and the reference is consistent. When powering through the USB you have some voltage drop because of the protection circuitry on most of the Arduinos. The USB works fine with the Nano and Uno when powered via an external supply. I generally see a noticeable contrast difference between the 5V and the USB on the LCD display if connected.
Hi,
If you are going to do some analog level processing with the op-amp, you may be better to have a separate 5V supply for it, keeping its gnd and arduino ground connected together.
That way you will be certain of a good noise free analog supply.
Something else to consider when powering the Arduino board via the USB port is all PC USB ports are not created equal. My own USB port delivers about 4.75 volts and not 5.00 volts. Therefore if I measure the 5V pin the best I will see is 4.75 volts. Additionally keep in mind depending on your PC USB that your available current is not infinite, it is limited.
Additionally consider what has been pointed out as to the use of separate supplies.
If you just need 10mA of 5V, you can pull them from the 5V rail. Add a bit of filter cap if you desire.
you can also supply the arduino from the 5V pin with a external 5V when you are not connected to a PC.
As I mentioned somewhere earlier, you can even power things like servos from the 5V pin. Yes, you are not supposed to power a servo with it -- you are supposed to power it with a bec -- but a bec is effectively a swtching dc-dc step-down voltage regulator. something not unlike the one found on the arduino.
the concern is that the servo, or whatever that is connected to it, will "pollute" the 5V rail, leading to voltage spikes/dips leading the processor to do undefined things like skipping instructions (think like skipping lines of code) and other stuff.
The traditional way is to simply put a bunch of filter cap on the line. And it does work, to most extent, as a way of smoothing out voltage. And that's good enough, apparently.
But yes, the more current drawn from the USB, the more the voltage drop because the on-board USB protection is simply a thermo-resetting fuse (gets more warm as more current flow through and trip once its hot enough). so you don't want to source large amount of current through the Arduino USB.
We can go a LONG way of talking about voltage drop (e.g. the long cable out from your PSU in your desktop, the various connector it get through to get to that usb port and the resistance in the usb cord itself and various other stuff)
and this is why things like the Apple 10W USB charger actually outputs 5.2V to compensate for the drop (when driving 2A loads)
And as much as it is complicated, we need to know that the mcu in the arduino will be quite comfortable as long as the voltage is, say, 2.7V-5.5V. (Arduino Leonardo). And add to it is that it consume little power, too, so you wont have to worry about voltage drops in most cases.
Whatever you are attaching, however, might be uncomfortable with the slightest fluctuations. In that case you should provide a dedicated power supply for it (pass the signal by connecting the ground and signal pin on both devices)
im not sure whether you can do a "data-only-cable" by not connecting the power pin on the USB. I might try that later. But if it work then you can have a good power supply for the entire setup and not worry about power flowing back into the usb port.
Yes, I once had a 3 week long battle with an SD card reader, in the end I found that the cards wouldn't work reliably on the 4.85V USB power they were getting. As soon as I connected the device to a PC power supply running 5.02V or something like that, everything ran like a charm.
However, without any details of your circuit, no conclusive answer can really be given.
That is entirely dependent on the protection circuit. I have seen from about 0.02 to about 0.8 volts depending on whatever is connected to it. Different designs have different protection schemes and some do change over time.
