Reduced harmonic distortion PCB design method
Indeed printed circuit board (the PCB) , i.e., its impedance should be constant are constituted by an electric linear material. So, PCB Why would non-linear signal is introduced into it? The answer is: where is the current flowing with respect to, the PCB layout is the "spatial nonlinearity".
This amplifier is a current from a power source or from another power source, depending on the polarity of the signal applied load moment. Current flows from the power source, through the bypass capacitor, through the amplifier into the load. Then, current flows from the ground of the load ( or PCB shielded output connector ) back to the ground plane, through the bypass capacitor, to provide power back to the beginning of the current.
The concept of current flows through the path of least impedance is incorrect. Number proportional to the current in all its different impedances conductivity paths. 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. FIG 1 illustrates two paths. The return current caused by the real cause of the problem.
When the bypass capacitors on PCB different positions, the current flows to the respective bypass capacitor through different paths, i.e., the meaning of "Nonlinear Space" represents. If much of the input circuit flows to the polarity of a component of the current, the polarity of the voltage signal component of only the disturbance. And if the other polarity of the current applied to no interference, the input signal voltage is changed in a nonlinear manner. When no changes polarity while the other component of a polarity change, will be distorted, and the performance of the second harmonic distortion of the output signal. FIG 2 show the effect of this distortion in an exaggerated form.
When only one polar component disturbed sine wave, the waveform generator is no longer a sine wave. With a 100 [Omega] load over the analog amplifier, a current through the load . 1 [Omega] resistor, a ground voltage input is coupled only on a polarity signal, is obtained in FIG. 3 the results shown. Fourier transforms showed that almost all the waveform distortion of -68dBc at the second harmonic. When the frequency is high, it is easy PCB generated on the extent of this coupling, it does not need much means PCB special nonlinear effects, can ruin an excellent anti amplifier distortion characteristic. 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 4 FIG.
Multi-chip amplifier
Multi-chip amplifier ( two, three or four amplifiers ) the problem is more complicated, because it is not the bypass capacitor connected to the input of all the remote. Especially for four amplifiers. Each side has four input amplifier chip, there is no space to place the bypass circuit to reduce the input channel disturbances.
FIG. 5 shows a simple layout method of four amplifiers. Most devices connected directly to four amplifier pin. A power supply current to be input to the disturbance voltage power supply and ground current in the other path, resulting in distortion. For example, four-channel amplifier 1 on the (+ Vs) bypass capacitor can be placed directly adjacent to the input place; and (of -Vs) bypass capacitor on the other side of the package. (+ Vs) to be perturbations current channel 1 , and (of -Vs) to the current might not.
To avoid this problem, so that the input current disturbance, but let PCB current flows in a linear fashion space. For this purpose, it may be employed in the manner PCB layout bypass capacitors: making (+ Vs) and ( - Vs of the) ground current flows through the same path. When the positive / negative current equal to the input signal of the disturbance, it will not produce distortion. Therefore, the next two bypass capacitors are arranged so that they share a grounding point. Because the two polar component current from the same point ( the output connector to the shield or load ) , and are returned to the same point ( bypass capacitor is connected in common ) , the positive / negative current flows through the same path . If the input resistance of a channel is (+ Vs) current disturbance, the ( - Vs of the) current because they have the same influence. Because no matter what the polarity of the disturbance generated by the same, so no distortion, but will change the channel gain occurring small as 6 in FIG.
To verify the above estimation, using two different PCB layout: a simple layout ( FIG. 5) and low distortion arrangement ( FIG. 6) . Using Fairchild FHP3450 distortion generated four operational amplifiers in Table 1 shown, FHP3450 the typical bandwidth is 210MHz , the slope is 1100V / US , input bias current is 100nA , the operating current of each channel is a 3.6mA . From Table 1 it can be seen, the more severe the better channel distortion, improved results, so that . 4 channels are nearly equal in performance.
If the PCB on a not ideal four amplifiers, measuring the effect of a single channel of the amplifier will be very difficult. Clearly, a given amplifier channel inputs not only the disturbance itself, but also other disturbance input channels. The current flowing through all the different channels, and to generate different results, but are affected by each output, this effect is measurable.
Table 2 shows the drive only when a channel, measured on the other unaffected channels harmonic drive. No drive channel displayed in a substantially small frequency signal ( crosstalk ) , but without any significant signal is substantially a case, the distortion introduced by the direct current manner. FIG 6 low distortion display layout: virtually eliminated because the current effect, the second harmonic and the total harmonic distortion (THD) characteristics are greatly improved.
This Summary
Briefly, the PCB , the different currents flowing through the bypass capacitor refluxed ( for different power ) and the power supply itself, and its size is proportional to its electrical conductivity. Small high-frequency signal back into bypass capacitor. Low frequency current ( such as current audio signal ) may flow through the larger bypass main. Even if the lower frequency of the current may also be "ignored" the presence of all bypass capacitors, leads directly back to the source. Specific application will determine which current path is the most critical. Fortunately, by using the bypass capacitor and the common point on the output side, it can be easily fully protect the current path.
High frequency PCB golden rule of arrangement is that high frequency bypass capacitor as close to the power pin of the package, but comparing FIGS. 5 and 6 can be seen to improve the distortion characteristics of the modified rule will not bring much change. Distortion characteristic is improved by about 0.15 inches long traces high frequency bypass capacitor for the price, but this FHP3450 the AC response performance has little effect. PCB layout is very important to maximize the performance of a high-quality amplifier, the issue discussed here is by no means limited to the high-frequency amplifier. Similar audio and other lower frequency signals to be much more stringent distortion requirements. The current effects at low frequencies to be smaller, but if the required corresponding improvement required distortion index, the current may still be an important issue.