Why is ground voltage non-zero, and why does it increase when I plug in an LED?

Question

Here is the circuit (Arduino Due, 1kΩ resistor, LED):

Note how I bring the ground from the Arduino board to the breadboard, and also connect the ADC pin to the gnd rail on the breadboard.

Here is the code:

void setup() {
  Serial.begin(9600);
  #define ADC_FREQ_MAX 250000
  Serial.println(ADC_FREQ_MAX);
    }

void loop() {
  analogReadResolution(12);
  Serial.print(analogRead(0));
  Serial.print('\n');
}

Here is what I do:

• 600cs: I unplug the LED.
• 4000cs: I plug in the LED.
• 6500cs: I unplug the LED.
• 7750cs: I plug in the LED.

Here is the result:

I plug this same setup into an Uno, and I get no issue:

Why is ground voltage non-zero, and why does it increase when I plug in an LED? What issue with the Due could cause this?

Answer 1

In Breadboards with lengthy cables a 50 mV offset is acceptable and is not an issue. If it is affecting measurements, then you should consider shortening the cables and possibly use thicker cables. a 0.5 ohm resistance cable with 5mA current will give 2.5 mV drop. If there are other currents through the ground cable, assume Entire board current, then the drop will go up. For ex. 100 mA current now will cause a drop of 50 mV.

Answer 2

An Ideal ground is a perfect conductor that we measure all voltage relative to. Oops there is no such thing as a perfect conductor, so the ideal ground exists only in textbooks. In the real world it is just another conductor, with its own resistance (low but present), capacitance (low, but this depends on how close it is to other conductors), inductance (not an issue at low frequencies, but it can be a problem at high frequency and is also dependent on length and proximity), and so on. I don't even understand what implication the flux would have. Most of the time we can just ignore this as it is typically not significant. But this will blow your mind, grab a small capacitor and hook one leg to the ground on your breadboard near the middle and the other leg to a unused rail. Now run your experiment again. Wild, yes?