Ground Current Paths and Second Harmonic Distortion
An amplifier datasheet can promise distortion figures in parts per million and still perform badly on a real board. The reason is usually not the device but the copper underneath it. High-current output stages deliver large, rapidly changing currents into a load, and those currents must return to their source through the ground network. If the return current shares copper with the input reference, the output stage modulates the input signal and the resulting error appears as distortion that cannot be corrected by any amount of component selection.
Where the Output Current Actually Flows
Consider a power amplifier driving a load through a bypass capacitor arrangement. Whether the amplifier draws its current from one supply rail or the other depends on the instantaneous polarity of the signal at the load. Current leaves the supply, passes through the bypass capacitor, flows through the amplifier into the load, and returns from the load ground through the earth plane and the bypass capacitor network back to the supply that provided it.
That return is the critical part. The common assumption that ground current takes the path of least impedance is misleading. Current divides among all available paths, and the share that flows in each path is proportional to its conductance. On a plane there is usually more than one low impedance route available at the same time: one directly to the bypass capacitor, and another that reaches the input resistor network first and excites it on the way.
Why the Division Matters
When bypass capacitors are placed at different locations on the board, the ground current from each half of the waveform reaches its own capacitor by a different route. The paths are not symmetric, and the effect has been described as a spatially non-linear ground. The practical consequence is that the positive half-cycle of the output current may take one route while the negative half-cycle takes another.
If a large part of one polarity component flows through the input circuit ground, only that polarity of the signal is disturbed. The other polarity is untouched. The input voltage then changes in a way that is not proportional to the signal, and the resulting error is not a simple gain change. Whenever one polarity is affected and the other is not, the disturbed waveform is no longer a sine wave, and the distortion appears at the second harmonic rather than the third.
This is worth pausing on, because it explains an otherwise puzzling measurement. Laboratory work with an ideal amplifier model driving a 100 ohm load, with the load current passed through a 1 ohm resistor and input ground voltage coupled onto only one polarity, produces a spectrum whose distortion is almost entirely second harmonic, measured at around minus 68 dBc. That order of coupling is easy to achieve on a real printed circuit board at high frequency, and it can destroy the excellent distortion performance of an otherwise well chosen amplifier without invoking any exotic device nonlinearity at all.
Bypass Capacitor Placement
The fix is to control the ground current path rather than to enlarge conductors. Rearranging the bypass loop so that the high-current return is kept away from the input device changes the distribution of the current and removes the modulation of the input reference.
The mechanics are straightforward. Each bypass capacitor should be connected to the ground plane with the shortest possible path, and the loop formed by the capacitor, the supply pin and the ground plane should be tight. The output stage’s return current should be given a dedicated, low-inductance route back to its own capacitor so that it has no reason to pass through the input area. In effect, the designer builds the high-current path deliberately and then arranges the input ground to be as far from it as the layout allows.
Where a single continuous plane is used, the current still divides between paths, so distance and geometry are the only controls available. Keeping the input stage, its feedback network and its ground reference physically separated from the output devices and their decoupling is the most reliable arrangement.
Non-Linearity of the Ground Itself
Ground copper is not an ideal conductor. It has resistance and inductance, and both vary with frequency and with current density. When a large transient current flows through a plane, the local voltage on that plane changes, and any circuit referenced to that point sees the change as part of its input. This is why the ground plane must be treated as a circuit element with a transfer function rather than as a universal zero-volt reference.
Two specific consequences follow. First, the plane should be as unbroken as possible under the sensitive circuitry, because a slot forces current to concentrate and increases the local impedance. Where a plane must be divided, the consequences are described in this article on power plane splitting. Second, the return path should be kept short, which is the general argument made in this discussion of copper flooding strategies.
Layout Rules for Low Distortion
Keep the input stage and its ground reference at the far end of the board from the output devices. Place the input connector so that the input signal enters at the quiet end of the board, not adjacent to the output terminals. Run the feedback network as a tight, short loop and route it directly back to the inverting input reference point rather than to the nearest available ground copper.
Decouple each supply pin at the device rather than at a distance, and use capacitor values chosen for the frequency range of interest, remembering that a small capacitor with a short loop outperforms a large capacitor with a long one. Keep the supply and ground connections to the amplifier symmetric so that the positive and negative current paths have similar impedance; an asymmetric supply network produces asymmetric modulation, which converts directly into second harmonic content.
Finally, verify the layout by inspection rather than by intuition. Trace the high-current return path on the finished artwork, following it from the output, through the load, back through the plane and into the bypass capacitor. If that path passes anywhere near the input reference, restructure the layout before the board is built. Additional guidance on routing geometry is available in this article on escape routing and pad fanout.
FAQ
Why does poor grounding cause second harmonic distortion rather than third? Because the disturbance usually affects only one polarity of the waveform. When the positive excursion is shifted and the negative excursion is not, the waveform becomes asymmetric, and asymmetry in the time domain appears as even-order distortion, dominated by the second harmonic.
Does a bigger ground plane solve this problem? Not by itself. A large plane lowers the average impedance but the current still divides among multiple paths, and the route through the input area remains available. The fix is to arrange the high-current return so that it has a dedicated, low-inductance path that keeps it away from the input reference.
Where should bypass capacitors be placed to protect a sensitive input stage? At the supply pins of the device they serve, with the shortest possible connection to the ground plane so that the loop area is small. They should not be shared between devices or placed at a distance from the load, since the trace inductance between the capacitor and the device removes most of the benefit.
