In fact, printed circuit board (PCB) is composed of electrical linear materials, that is, its impedance should be constant. So why does PCB introduce nonlinearity into the signal? The answer is that the PCB layout is “spatially nonlinear” relative to where the current flows.
Whether the amplifier gets current from this power supply or another power supply depends on the instantaneous polarity of the signal on the load. The current flows from the power supply, through the bypass capacitor, and into the load through the amplifier. Then, the current returns to the ground plane from the load ground terminal (or the shield of the PCB output connector), passes through the bypass capacitor, and returns to the power supply that originally provided the current.
The concept of minimum path of current flowing through impedance is incorrect. The amount of current in all different impedance paths is proportional to its conductivity. In a ground plane, there are often more than one low impedance path with a large proportion of ground current flowing through: one path is directly connected to the bypass capacitor; the other path forms an excitation to the input resistance before reaching the bypass capacitor. Figure 1 illustrates the two paths. The earth return current is the real cause of the problem.
When the bypass capacitors are placed in different positions of PCB, the ground current flows to their respective bypass capacitors through different paths, which is the meaning of “spatial nonlinearity”. If most of the component of a certain polarity of the ground current flows through the ground of the input circuit, only the component voltage of this polarity of the signal is disturbed. If the other polarity of the earth current is not disturbed, the input signal voltage changes in a nonlinear way. When one polarity component changes and the other polarity does not change, it will produce distortion and show as the second harmonic distortion of the output signal. Figure 2 shows this distortion in an exaggerated form.
When only one polar component of sine wave is disturbed, the generated waveform is no longer sine wave. If a 100 Ω load is used to simulate an ideal amplifier and the load current passes through a 1 Ω resistor, only one polarity of the signal is coupled with the input ground voltage, the result shown in Fig. 3 is obtained. Fourier transform shows that almost all the distorted waveforms are the second harmonic at – 68dbc. When the frequency is very high, it is easy to generate this degree of coupling on the PCB, which can destroy the excellent anti distortion characteristics of the amplifier without too many special nonlinear effects of the 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 circuit and keeping the distance from the input device, as shown in Figure 4.
Multi amplifier chip
The problem of multiple amplifier chips (two, three, or four amplifiers) is more complicated because it cannot keep the ground connection of the bypass capacitor away from all the inputs. This is especially true for four amplifiers. Each side of the four amplifier chip has an input, so there is no space for a bypass circuit to reduce the disturbance to the input channel.
Figure 5 shows a simple method for a four amplifier layout. Most devices are directly connected to four amplifier pins. The ground current of one power supply can disturb the input ground voltage and current of another 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, while the (- vs) bypass capacitor can be placed on the other side of the package. (+ VS) ground current can disturb channel 1, while (- vs) ground current may not.
To avoid this problem, we can let the ground current disturb the input, but let the PCB current flow in a spatial linear way. In order to achieve this goal, the bypass capacitor can be arranged on the PCB in the following way: make (+ VS) and (– VS) ground current flow through the same path. If the disturbance of positive / negative current to input signal is equal, distortion will not occur. Therefore, the two bypass capacitors are arranged next to each other so that they share a common ground point. Because the two polarity components of the ground current come from the same point (output connector shield or load ground) and return to the same point (common ground connection of bypass capacitor), the positive / negative current flows through the same path. If the input resistance of a channel is disturbed by (+ VS) current, then (– VS) current has the same effect on it. No matter what the polarity is, the disturbance generated is the same, so there is no distortion, but the gain of the channel will change slightly, as shown in Figure 6.
To verify the above inference, two different PCB layouts are used: simple layout (Fig. 5) and low distortion layout (Fig. 6). The distortion of fhp3450 four operational amplifier using Fairchild Semiconductor is shown in Table 1. The typical bandwidth of fhp3450 is 210mhz, the slope is 1100V / us, the input bias current is 100na, and the working current of each channel is 3.6Ma. It can be seen from table 1 that the more serious the distortion is, the better the improvement effect is, so that the four channels are nearly equal in performance.
How to avoid the distortion of PCB design
If there is not an ideal four amplifier on PCB, it will be difficult to measure the effect of a single amplifier channel. Obviously, a given amplifier channel not only disturbs its own input, but also disturbs the input of other channels. The ground current flows through all different channel inputs and produces different effects, but it is affected by each output, which is measurable.
Table 2 shows the harmonics measured on other channels that are not driven when only one channel is driven. The underived channel shows a small signal (crosstalk) at the basic frequency, but without any significant basic signal, it also produces distortion directly introduced by the ground current. The low distortion layout in Figure 6 shows that the second harmonic and total harmonic distortion (THD) characteristics are greatly improved because the ground current effect is almost eliminated.
Summary of this paper
In short, on PCB, the ground return current flows through different bypass capacitors (for different power supplies) and the power supply itself, and its size is proportional to its conductivity. The high frequency signal current flows back to the small bypass capacitor. Low frequency current (such as the current of audio signal) may mainly flow through larger bypass capacitance. Even the current with lower frequency may “ignore” the existence of all bypass capacitors and flow directly back to the power lead. The specific application will determine which current path is the most critical. Fortunately, all ground current paths can be easily protected by using a common ground point and a ground bypass capacitor on the output side.
The golden rule of high-frequency PCB layout is to place the high-frequency bypass capacitor as close as possible to the power pin of the package. However, comparing Fig. 5 and Fig. 6, it can be seen that modifying this rule to improve the distortion characteristics will not bring much change. The improved distortion characteristic is at the cost of adding about 0.15 inch high frequency bypass capacitor, but it has little effect on the AC response performance of fhp3450. PCB layout is very important to give full play to the performance of a high-quality amplifier. The problem discussed here is not limited to high-frequency amplifier. Lower frequency signals such as audio require much more distortion. The ground current effect is smaller at low frequency, but it may still be an important problem if the distortion index is required to be improved.