In fact printed circuit board (PCB) is made of electrical linear material, that is, its impedance should be constant. So why is PCB introducing non-linearity into the signal? The answer is that the PCB layout is "non-linear" with respect to where the current flows.

Whether the amplifier draws current from this power supply or from another power supply depends on the momentary polarity of the signal on the load. The current flows from the power supply, through the bypass capacitor, through the amplifier into the load. Then, the current is returned from the load ground (or shield of the PCB output connector) to the ground plane, bypassing the capacitor and returning to the power supply that originally provided the current.

The concept of current flow through the minimum impedance path is not correct. The amount of current in all the different impedance paths is proportional to its conductivity. In a ground plane, often have low impedance path than a large proportion of the current flows through: a path connecting directly to the bypass capacitor; the other before reaching the bypass capacitor, an incentive for the input resistance. Figure 1 shows the two paths. The return current is the cause of the problem.

When the bypass capacitor is placed in a different position on the PCB, the ground current flows through the different paths to the respective bypass capacitors, meaning "spatial non-linearity". If a large portion of the current of a certain polarity flows through the ground of the input circuit, only the component voltage of the polarity of the signal is disturbed. And if the other polarity of the ground current does not disturb, the input signal voltage changes in a non-linear manner. When a polar component changes and the other polarity does not change, it will produce distortion, and the performance of the second harmonic distortion of the output signal. Figure 2 shows this distorted effect in an exaggerated form.

When only one of the polar components of the sine wave is disturbed, the resulting waveform is no longer a sine wave. With a 100Ω load over the analog amplifier, the load current through a 1Ω resistor, a ground voltage input is coupled in only one polarity of the signal, the results shown in FIG. Fourier transform shows that the distortion waveform is almost all-second harmonic at -68dBc. When the frequency is very high, it is easy to generate this degree of coupling on the PCB, it does not need too much PCB special non-linear effect, you can destroy the amplifier excellent anti-distortion characteristics. When the output of a single operational amplifier distortion due to the current path, by rearranging the bypass loop current flow can be adjusted, and keep a distance from the input device, as shown in FIG.

The problem of multi-amplifier chips (two, three, or four amplifiers) is more complex because it can not make the connection of the bypass capacitor away from all inputs. This is especially true for quad amplifiers. The four amplifiers have an input on each side of the chip, so there is no space to place the bypass circuit that can reduce the disturbance to the input channel.

Figure 5 shows a simple method of quad amplifier layout. Most devices are connected directly to the quad amplifier pins. The ground current of a 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 4 of the four amplifiers can be placed directly near its input; and (-Vs) the bypass capacitor can be placed on the other side of the package. (+ Vs) ground current can disturb channel 1, and (-Vs) ground current may not.

To avoid this problem, allow the ground current to disturb the input, but let the PCB current flow in a spatial linear manner. For this purpose, the bypass capacitor can be laid on the PCB in the following way: the current of (+ Vs) and (-Vs) flows through the same path. If the positive / negative current disturbance of the input signal is equal, no distortion will occur. Thus, the two bypass capacitors are arranged next to each other so that they share a ground point. Since the two polar components of the ground current come from the same point (the output connector is shielded or loaded) and flows back to the same point (the common connection of the bypass capacitor), the positive / negative current flows through the same path The If the input resistance of a channel is disturbed by the (+ Vs) current, the (-Vs) current has the same effect on it. Since no matter what polarity is, the resulting disturbances are the same, so there is no distortion, but will make the channel gain small changes, as shown in Figure 6.

To verify the above inference, use two different PCB layouts: simple layout (Figure 5) and low distortion layout (Figure 6). The FHP3450's four-operational amplifiers are 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 serious the distortion of the channel, the better the effect of improvement, so that the four channels in the performance close to equal.

If there is not an ideal quad amplifier on the PCB, it is 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 the different channels and produces different effects, but both are affected by each output, and this effect is measurable.

Table 2 shows the harmonics measured on other un-driven channels when only one channel is driven. The un-driven channel shows a small signal (crosstalk) at the fundamental frequency, but also generates distortion directly from the ground current without any significant basic signal. The low distortion layout of Figure 6 shows that the second harmonic and total harmonic distortion (THD) characteristics are greatly improved by almost eliminating the ground current effect.

to sum up
Simply put, on the PCB, the ground current flows through different bypass capacitors (for different power supplies) and the power supply itself, whose size is proportional to its conductivity. The high frequency signal current flows back to the small bypass capacitor. Low-frequency current (such as the audio signal current) may flow mainly through a larger bypass capacitor. Even if the frequency of the lower current may also "ignore" the existence of all bypass capacitors, direct flow back to the power cord. The specific application will determine which current path is the most critical. Fortunately, through the use of common ground and output side of the bypass capacitor, you can easily protect all the current path.

High-frequency PCB layout of the golden rule is the high-frequency bypass capacitor as close as possible to the package of the power pin, but compare Figure 5 and Figure 6 can be seen to improve the distortion characteristics and modify the rules will not bring much change. The improved distortion characteristic is at the expense of a high frequency bypass capacitor traces of about 0.15 inches long, but this has little effect on the AC response performance of the FHP3450. The PCB layout is important for taking full advantage of the performance of a high-quality amplifier, and the problem discussed here is by no means limited to high-frequency amplifiers. Similar to the frequency of audio and other lower signal on the requirements of the distortion is much more stringent. The ground current effect is smaller at low frequencies, but the ground current may still be an important issue if the required distortion index is required to be improved accordingly.