In fact, printed circuit board (PCB) are made of electrical linear materials, i.e. their impedance should be constant. So why does a PCB introduce nonlinearity into a signal? The answer is that the PCB layout is “spatially non-linear” relative to where the current flows.

Whether the amplifier receives current from one source or another depends on the instantaneous polarity of the signal on the load. Current flows from the power supply, through the bypass capacitor, through the amplifier into the load. The current then travels from the load ground terminal (or shielding of the PCB output connector) back to the ground plane, through the bypass capacitor, and back to the source that originally supplied the current.

The concept of minimum path of current through impedance is incorrect. The amount of current in all different impedance paths is proportional to its conductivity. In a ground plane, there is often more than one low-impedance path through which a large proportion of ground current flows: one path is directly connected to the bypass capacitor; The other excites the input resistor until the bypass capacitor is reached. Figure 1 illustrates these two paths. The backflow current is what’s really causing the problem.

How to reduce harmonic distortion in PCB design

When the bypass capacitors are placed at different positions on the PCB, the ground current flows through different paths to the respective bypass capacitors, which is the meaning of “spatial nonlinearity”. If a significant portion of a polar component of the ground current flows through the ground of the input circuit, only that polar component of the signal is disturbed. If the other polarity of the ground current is not disturbed, the input signal voltage changes in a nonlinear manner. When one polarity component is changed but the other polarity is not, distortion occurs and is manifested as the second harmonic distortion of the output signal. Figure 2 shows this distortion effect in exaggerated form.

How to reduce harmonic distortion in PCB design

When only one polar component of the sine wave is disturbed, the resulting waveform is no longer a sine wave. Simulating an ideal amplifier with a 100-ω load and coupling the load current through a 1-ω resistor into the ground voltage on only one polarity of the signal, results in figure 3.Fourier transform shows that the distortion waveform is almost all the second harmonics at -68 DBC. At high frequencies, this level of coupling is easily generated on a PCB, which can destroy the excellent anti-distortion characteristics of an amplifier without resorting to much of the special nonlinear effects of a PCB. When the output of a single operational amplifier is distorted due to the ground current path, the ground current flow can be adjusted by rearranging the bypass loop and maintaining distance from the input device, as shown in Figure 4.

How to reduce harmonic distortion in PCB design

Multiamplifier chip

The problem of multi-amplifier chips (two, three, or four amplifiers) is compounded by the inability to keep the ground connection of the bypass capacitor far from the entire input. This is especially true for four amplifiers. Quad-amplifier chips have input terminals on each side, so there is no room for bypass circuits that mitigate disturbance to the input channel.

How to reduce harmonic distortion in PCB design

Figure 5 shows a simple approach to a four-amplifier layout. Most devices connect directly to a quad amplifier pin. The ground current of one power supply can disturb the input ground voltage and ground current of the other channel power supply, resulting in distortion. For example, the (+Vs) bypass capacitor on channel 1 of the quad amplifier can be placed directly adjacent to its input; The (-Vs) bypass capacitor can be placed on the other side of the package. The (+Vs) ground current can disturb channel 1, while the (-vs) ground current may not.

How to reduce harmonic distortion in PCB design

To avoid this problem, let the ground current perturb the input, but let the PCB current flow in a spatially linear fashion. To achieve this, the bypass capacitor can be arranged on the PCB in such a way that the (+Vs) and (– Vs) ground currents flow through the same path. If the input signal is equally disturbed by positive and negative currents, distortion will not occur. Therefore, align the two bypass capacitors next to each other so that they share a ground point. Because the two polar components of the earth current come from the same point (the output connector shielding or the load ground) and both flow back to the same point (the common ground connection of the bypass capacitor), the positive/negative current flows through the same path. If the input resistance of a channel is disturbed by (+Vs) current, (– Vs) current has the same effect on it. Because the resulting disturbance is the same regardless of the polarity, there is no distortion, but a small change in the gain of the channel will occur, as shown in Figure 6.

How to reduce harmonic distortion in PCB design

To verify the above inference, two different PCB layouts were used: a simple layout (Figure 5) and a low-distortion layout (Figure 6). The distortion produced by the FHP3450 quad-operational amplifier using fairchild semiconductor is shown in table 1. The typical bandwidth of the FHP3450 is 210MHz, the slope is 1100V/us, the input bias current is 100nA, and the operating current per channel is 3.6mA. As can be seen from Table 1, the more distorted the channel, the better the improvement, so that the four channels are nearly equal in performance.

How to reduce harmonic distortion in PCB design

Without an ideal quad amplifier on a PCB, measuring the effects of a single amplifier channel can be difficult. Obviously, a given amplifier channel disturbs not only its own input, but the input of other channels as well. The earth current flows through all the different channel inputs and produces different effects, but is influenced by each output, which is measurable.

Table 2 shows the harmonics measured on other undriven channels when only one channel is driven. The undriven channel displays a small signal (crosstalk) at the fundamental frequency, but also produces distortion directly introduced by the ground current in the absence of any significant fundamental signal. The low-distortion layout in Figure 6 shows that the second harmonic and total harmonic distortion (THD) characteristics are greatly improved because of the near-elimination of the ground current effect.

How to reduce harmonic distortion in PCB design

This article summary

Simply put, on a PCB, the backflow current flows through different bypass capacitors (for different power supplies) and the power supply itself, which is proportional to its conductivity. The high-frequency signal current flows back to the small bypass capacitor. Low-frequency currents, such as those of audio signals, may flow primarily through larger bypass capacitors. Even a lower frequency current may “disregard” the full bypass capacitance and flow directly back to the power lead. The specific application will determine which current path is most critical. Fortunately, it is easy to protect the entire ground current path by using a common ground point and a ground bypass capacitor on the output side.

The golden rule for HF PCB layout is to keep the HF bypass capacitor as close to the packaged power pin as possible, but a comparison of Figure 5 and Figure 6 shows that modifying this rule to improve distortion characteristics does not make much of a difference. The improved distortion characteristics came at the expense of adding about 0.15 inches of high-frequency bypass capacitor wiring, but this had little impact on the AC response performance of the FHP3450. PCB layout is important to maximize the performance of a high-quality amplifier, and the issues discussed here are not limited to hf amplifiers. Lower frequency signals such as audio have much stricter distortion requirements. The ground current effect is smaller at low frequencies, but it may still be an important problem if the required distortion index is improved accordingly.