This slide illustrates a situation when a differential technique should be used to measure voltages which are referenced to ground. Often the quality and accuracy of these measurements will be greatly improved by using a differential technique. When measuring low amplitude signals, the noise and voltage gradients present in the ground distribution system can often be as large as or larger than the magnitude of the signal the user is attempting to measure. Voltage gradients result from load currents acting on the small parasitic resistance in the ground system. When the signal being measured is small, a few millivolts or less, the difference between ground near the signal source and ground where the probe is connected can be significant. Moving the probe ground connection near the source is not necessarily a good idea. The ground distribution system is usually very low impedance, far lower than the ground in the probe. Moving the probe ground connection in the circuit ground network introduces an alternate ground path which will result in large circulating currents in the probe ground lead. These will also result in a measurement error, as the oscilloscope measures voltages referenced to the ground potential at the input BNC connector, not the end of the probe ground. These problems are very common when dealing with high current coaxial shunts. Although the shunt is terminated in “ground”, the high current creates a voltage gradient in the ground plane. The shunt, being low resistance, will generate relatively low voltage, which can be masked in the ground gradient. If the currents in the ground distribution system contain high di/dt components, the parasitic inductance will result in gradients from the reactive losses.