This is probably something really simple, but I wasn't able to find the answer yet.
I've been using a power supply with both COM and earth ground, like the first image below.
To power an op-amp, I was using COM for grounding.
Then, I came across a power supply like the second image below, which doesn't have COM.
Also, I've seen a power supply with no ground port whatsoever like the third image below.
I heard the earth ground is noisier. Why do some power supplies have both COM and earth ground, but others don't? And if I have to use one without COM or no ground port at all, what are my alternatives?
Best Answer
simulate this circuit – Schematic created using CircuitLab
Figure 1. Various options.
Photo 1
Photo 2
simulate this circuit
Photo 3
Figure 2. A dual supply can be used in multiple configurations.
This has two independent supplies but without the earth option. These can be used independently, as a symmetrical supply or, for example, as a +12 V and +5 V supply. Note the connection (or lack of) between them in each case.
From the comments:
I think you are confused. The Photo 2 PSU has only one output with + and - terminals. It is not a dual supply as shown in Photo 1. You can think of it as a variable voltage battery with an optional earth connection. You always connect the load to red and black and add an optional link from either to the earth terminal.
Have a look at my answer to Actual electric potential at terminals of battery and it may help your understanding.