Voltage, current, resistance: start with ground
Voltage is the difference between two points, and the reference point is ground. The three basic quantities for reading a circuit.
Voltage (V)
Voltage is not the value of one point; it is the difference between two points. When someone says "this pin is at 3.3 V", the words "measured with ground taken as 0 V" are left out. That is why the reading changes with where you put the black multimeter probe.
Current (A)
The amount of charge passing through a cross-section in one second. Current only flows around a closed loop. Current that goes out comes back along ground. Forget this return path and noise problems make no sense.
3D labRun what you have read so far→Resistance (Ω)
How hard it is for current to flow. At the same voltage, a larger resistance means a smaller current.
| Quantity | Unit | As a water analogy |
|---|---|---|
| Voltage | V | Pressure difference |
| Current | A | Amount of water flowing |
| Resistance | Ω | Narrowness of the pipe |
Ground is not one thing
On a schematic every ground symbol is the same point, but on a real PCB the ground wiring has resistance and inductance too. When a ground carrying large currents, such as a motor's, mixes with the ADC reference ground, ground itself moves by tens of mV and the readings jump.