1. In fact, the printed circuit board (PCB) consists of wire-like materials, meaning that its impedance should remain constant. So, why does the PCB introduce nonlinearity into the signal?
2. The answer lies in the “spatial nonlinearity” of the PCB layout relative to the current flow. Whether the amplifier draws current from this power supply or another depends on the instantaneous polarity of the signal at the load. The current flows from the power supply, through the bypass capacitor, and enters the load via the amplifier.
3. Afterward, the current returns from the load ground (or shielded PCB output connector) to the ground plane, passing through the bypass capacitor before returning to the original power source that supplied the current. The idea that current always flows along the path of least impedance is a misconception. The current distribution across different impedance paths is proportional to their conductance.
4. In the ground plane, there is typically a low-impedance path that carries a significant proportion of the current: one path connects directly to the bypass capacitor, while another path flows through the input resistor before reaching the bypass capacitor.
1. The ground return current is the primary cause of the problem.
2. Measures to reduce harmonic distortion in PCB design: When bypass capacitors are placed at different locations on the PCB, the ground current flows through different paths corresponding to each bypass capacitor, leading to what is known as “spatial nonlinearity.” If the majority of the ground current of a specific polarity flows through the ground of the input circuit, only the signal component of that polarity will be affected. If the other polarity of the ground current is unaffected, the input signal voltage experiences a nonlinear shift. This leads to distortion, which manifests as second-order harmonic distortion in the output signal when one polarity component changes while the other remains unaffected.
3. Measures to reduce harmonic distortion in PCB design during prototyping: When only one polarity component of the sine wave is disturbed, the resulting waveform is no longer a pure sine wave. To simulate an ideal amplifier, use a 100Ω load, passing the load current through a 1Ω resistor. Only the input ground voltage couples to one polarity of the signal, as shown in Figure 3. The Fourier transform analysis reveals that the distorted waveforms primarily consist of second-order harmonics at -68dBC. At high frequencies, this type of coupling is easy to occur on the PCB and can degrade the amplifier’s distortion performance, even without requiring complex nonlinear effects from the PCB itself.
4. When the output of a single operational amplifier is distorted due to the ground current path, the bypass loop can be rearranged to adjust the ground current flow and maintain proper distance from the input device.
5. Measures to reduce harmonic distortion in PCB prototyping: Multi-amplifier chips present a more complex issue, as they don’t allow the ground connection of the bypass capacitor to be placed far from all the inputs. This problem is particularly important when designing PCBs with quad amplifiers (two, three, or four amplifiers on a single chip).
6. Each side of the quad amplifier chip has an input terminal, leaving little room for a bypass circuit that could help reduce interference on the input channels.
7. Measures to reduce harmonic distortion in PCB design: Figure 5 illustrates a common approach to laying out a four-amplifier chip. Most devices are connected directly to the four amplifier pins. Ground current from one power supply can interfere with the input ground voltage and ground current from another channel, resulting in distortion. For example, the (+VS) bypass capacitor on channel 1 of the quad amplifier can be placed close to its input, while the (-VS) bypass capacitor can be placed on the opposite side of the package. The ground current from (+VS) may interfere with channel 1, while the (-VS) ground current may not.
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