3.14 Ground Bounce

Ground bounce often refers to the voltage measured between two ground points on a circuit board. It may also refer to the voltage drop on the ground pin of an IC. Since it is not possible to contact the die, the total voltage drop cannot be observed. Our point of view is to consider that energy supplied to the IC flows in fields on transmission lines. The short section from the board to the die is not a match to the impedance presented by the ground and power plane. The reflections on this short segment of line cause fields that would be sensed by a probe in this area. To attribute this field to the ground connection alone would be an error.

Measurements are usually made using an oscilloscope probe where the probe common is connected to one ground point on the circuit board and the probe tip is connected to a second ground point. The voltage that is sensed is interpreted as an IR drop in the ground plane. The measure is usually made when there is some sort of problem involving signal integrity and the source of difficulty is under consideration.

The presence of a probe in a circuit means that a short stub has been added. We have shown that even short stubs added to a transmission line can affect rise time. When the probe is used to measure points along a ground plane, the measurement has little effect on any logic transition.

Any measure of voltage is a measure of an electromagnetic field. Because the probe tip and the probe common (ground) form a loop, the probe will measure the field in this loop area. The field patterns in the area will probably be changed by the presence of the probe and its connections. In effect, the very field pattern that is sensed will be modified by the presence of the probe.

To begin a test, it is wise is to connect the probe tip to the probe common and make no connection to the circuit common. There should be no signal. Now, touch the probe common to the circuit common. Again, there should be no signal. Now, it is proper to move the probe tip to a second common point (ground) in the circuit.

The probe ground and probe tip form a loop. This loop can couple to any fields generated on the board. There is no simple way to orient this loop to avoid this coupling. Some of the signal may be the field associated with an IR drop, but usually, the largest component of the field will be associated with nearby logic transitions. At this point, it is important to note that the fields from transmission lines (traces and ground planes) are tightly controlled. The conductor geometry near component pins is not as well controlled, and this is often the source of field generation.

The real point to recognize is that there are IR losses in the ground plane. These losses are not simple to measure. Logic currents flow in a narrow paths under each circuit trace. The current flows where the E field lines terminate. For fast logic, in particular, the current does not penetrate very far into the conducting plane. The limited amount of copper that is used means that there is resistance that must be considered (see Section 4.11 for a discussion on transmission line resistances).