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, passes through the bypass capacitor, and enters the load through the amplifier. Then, the current returns to the ground plane from the ground terminal of the load (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 that the current flows through the path with minimum 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 through which a large proportion of ground current flows: one path is directly connected to the bypass capacitor; The other one excites the input resistor before reaching the bypass capacitor.
When the bypass capacitors are placed in different positions on the PCB, the ground current flows to their respective bypass capacitors through different paths, which means “spatial nonlinearity”. If a large part 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 will be disturbed. If the other polarity of the ground current is not disturbed, the input signal voltage changes in a nonlinear way. When one polarity component is changed while the other polarity is unchanged, distortion will occur, which is the second harmonic distortion of the output signal.
PCB design When only one polarity component of sine wave is disturbed, the generated waveform is no longer sine wave. Using a 100Ω load to simulate an ideal amplifier, and making the load current pass through a 1Ω resistor, the PCB design only couples the input ground voltage on one polarity of the signal, and the result shown in the figure is obtained. Fourier transform shows that the distortion waveform is almost 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 much special nonlinear effect of PCB. When the output of a single operational amplifier is distorted due to the ground current path, the ground current can be adjusted by rearranging the bypass circuit, and the distance from the input device can be kept.
The problem of designing PCB multi-amplifier chips (two, three or four amplifiers) is more complicated, because it can’t make the grounding connection of the bypass capacitor far away from all the input terminals. This is especially true for four amplifiers. Each side of the four amplifier chips has an input terminal, so there is no room to place a fibrechannel that can reduce the disturbance to the input channel.
A simple method of four amplifier layout is given. Most devices are directly connected to four amplifier pins. The ground current of one power supply can disturb the input ground voltage and ground current of another channel power supply, resulting in distortion. For example, the (+Vs) bypass capacitor on channel 1 of four amplifiers can be placed directly near its input; And 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.
To avoid this problem, the ground current can disturb the input, but the PCB current can flow in a spatial linear way. To achieve this goal, the bypass capacitor can be laid out on the PCB in the following way: the (+Vs) and (–vs) ground currents flow through the same path. If the positive/negative currents disturb the input signal equally, there will be no distortion. Therefore, two bypass capacitors are arranged next to each other so that they share a ground point. Because the two polarity components of the ground current come from the same point (the output connector shield 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 the (+Vs) current, the (–vs) current has the same effect on it. No matter what the polarity is, the generated disturbance is the same, so there will be no distortion, but the gain of this channel will change slightly.
To verify the above inference, two different PCB layouts are adopted: simple layout and low distortion layout. 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. As can be seen from Table 1, the more distorted the channel, the better the improvement effect, so that the four channels are nearly equal in performance.
If there is no ideal four amplifiers on PCB, it will be difficult to measure the effect of single amplifier channel. Obviously, a given amplifier channel not only disturbs its own input, but also disturbs the inputs of other channels. The ground current flows through all the different channel inputs, and produces different effects, but they are all affected by each output, which is measurable.
The design of PCB gives the harmonics measured on other undriven channels when only one channel is driven. The undriven channel shows a small signal (crosstalk) at the fundamental frequency, but without any significant fundamental signal, it also produces distortion directly introduced by the ground current.
Simply put, on the PCB, the ground return current flows through different bypass capacitors (for different power supplies) and the power supply itself, and its magnitude 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 a larger bypass capacitor. Even the current with lower frequency may “ignore” the existence of all bypass capacitors and directly flow back to the power supply lead. The specific application will determine which current path is the most critical. Fortunately, all the ground current paths can be easily protected by using the common grounding point and the ground bypass capacitor on the output side.
The golden rule of high-frequency PCB layout is to keep the high-frequency bypass capacitor as close as possible to the power supply pin of the package, but it can be seen from the comparison of Figure 5 and Figure 6 that modifying this rule to improve the distortion characteristics will not bring much change. The distortion characteristic is improved at the cost of adding about 0.15 inch long high frequency bypass capacitor trace, but this 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, and the issues discussed here are by no means limited to high-frequency amplifiers. Signals with lower frequencies, such as audio, have much stricter requirements for distortion. The effect of ground current is smaller at low frequency, but if the distortion index needs to be improved accordingly, ground current may still be an important problem.
