The system ground return, or GND, symbol is often taken for granted when drawing schematics. GND symbols are placed all over the schematic with the assumption that the different GNDs will all be at the same electrical potential on the printed circuit board (PCB). In reality, current flow through the GND impedance can create voltage differences between the GND connections on the PCB. Single-ended dc circuits are particularly sensitive to these GND voltage differences because the expected single-ended circuit is converted to a differential circuit causing output errors.
Let's use the standard non-inverting amplifier circuit shown below as an example. When the GND potentials of the input source, VIN, and the input resistor, RI, are equal, the familiar circuit gain of 1+RF/RI applies. Therefore, the 100 mV input signal is multiplied by a gain of 10 V/V and the output is 1V.