Printed circuit board (PCB) wiring plays a key role in high-speed circuits, but it is often one of the last steps in the circuit design process. There are many problems in high-speed PCB wiring, and many literatures have been written on this topic. This paper mainly discusses the wiring problem of high-speed circuits from a practical point of view. The main purpose is to help new users pay attention to a variety of different problems that need to be considered when designing high-speed circuit PCB wiring. Another purpose is to provide a review material for customers who haven’t touched PCB wiring for some time. Due to the limited space, this paper can’t discuss all the problems in detail, but we will discuss the key parts that have the greatest effect on improving circuit performance, shortening design time and saving modification time.
Although this paper mainly focuses on the circuits related to high-speed operational amplifiers, the problems and methods discussed here are generally applicable to the wiring for most other high-speed analog circuits. When the operational amplifier works in a very high radio frequency (RF) band, the performance of the circuit largely depends on the PCB wiring. The high-performance circuit design that looks good on the “drawing” can only get ordinary performance in the end if it is affected by careless wiring. Pre-consideration and attention to important details in the whole wiring process will help to ensure the expected circuit performance.
schematic diagram
Although good schematic diagram can’t guarantee good wiring, good wiring begins with good schematic diagram. When drawing the schematic diagram, you should think carefully and consider the signal flow direction of the whole circuit. If there is a normal and stable signal flow from left to right in the schematic diagram, then there should be an equally good signal flow on the PCB. Give as much useful information as possible on the schematic diagram. Sometimes when the circuit design engineer is away, customers will ask us to help solve the circuit problems. Designers, technicians and engineers engaged in this work will be very grateful, including us.
Besides the common reference identifier, power consumption and error tolerance, what other information should be given in the schematic diagram? Here are some suggestions to turn ordinary schematics into first-class schematics. Add waveform, mechanical information about shell, printed line length, blank area; Indicate which components need to be placed on PCB; Give adjustment information, component value range, heat dissipation information, control impedance printed circuit, comments, and brief circuit action description.
location
Just as in PCB, location is everything. Where a circuit is placed on the PCB, where its specific circuit components are installed, and what other adjacent circuits are, all these are very important.
Usually, the positions of input, output and power supply are predetermined, but the circuits between them need to “exert their creativity”. That’s why paying attention to wiring details will pay off greatly. Start with the location of key components, and consider according to the specific circuit and the whole PCB. From the beginning, it is helpful to specify the position of key components and the path of signals to ensure that the design can achieve the expected working objectives. Getting the right design at once can reduce the cost and pressure-thus shortening the development cycle.
Bypass power supply
Bypassing the power supply at the power supply end of the amplifier to reduce noise is a very important aspect in PCB design-including high-speed operational amplifier or other high-speed circuits. There are two common configuration methods of bypass operational amplifier.
Terminal grounding: This method is the most effective in most cases. Multiple parallel capacitors are used to directly ground the power supply pin of the operational amplifier. Generally speaking, two parallel capacitors are enough-but adding parallel capacitors may bring benefits to some circuits.
Parallel capacitors with different capacitance values help to ensure that the power supply pins can only see low AC impedance in a wide frequency band. This is especially important at the attenuation frequency of the PSR of the operational amplifier. This capacitor helps to compensate for the reduced PSR of the amplifier. Maintaining a low impedance grounding path in many octave ranges will help to ensure that harmful noise cannot enter the operational amplifier. Fig. 1 shows the advantages of using multiple parallel capacitors. In the low frequency band, the large capacitor provides a low impedance grounding path. However, once the frequency reaches their own resonant frequency, the capacitance of capacitors will be weakened and gradually become inductive. That’s why it’s important to use multiple capacitors: when the frequency response of one capacitor starts to decrease, the frequency response of the other capacitor starts to play its role, so it can keep a very low AC impedance in many tens of frequency ranges.
Ground plane
Actually, the content that needs to be discussed is far more than those mentioned in this article, but we will highlight some key features and encourage readers to further explore this topic. At the end of this paper, relevant references are listed.
The ground plane acts as a common reference voltage, providing shielding, heat dissipation and reducing parasitic inductance (but it also increases parasitic capacitance). Although there are many advantages to using the ground plane, care must be taken in its implementation, because it has some limitations on what can and cannot be done.
Ideally, a layer of PCB should be dedicated to the ground plane. In this way, the best results can be produced when the whole plane is not destroyed. Never misappropriate the area of the ground plane in this special layer for connecting other signals. Because the ground plane can eliminate the magnetic field between the conductor and the ground plane, the inductance of the printed line can be reduced. If an area of the ground plane is damaged, unexpected parasitic inductance will be introduced to the printed lines above or below the ground plane.
Because the ground plane usually has a large surface area and cross-sectional area, the resistance of the ground plane is kept to a minimum. In the low frequency band, the current will choose the path with the least resistance, but in the high frequency band, the current will choose the path with the least resistance.
However, there are exceptions. Sometimes a small ground plane is better. If the ground plane is removed from under the input or output pads, the high-speed operational amplifier will work better. The parasitic capacitance introduced into the ground plane of the input terminal increases the input capacitance of the operational amplifier and reduces the phase margin, thus causing instability. As seen in the discussion of parasitic effects, the capacitance of 1 pF at the input of operational amplifier can cause obvious sharp pulses. Capacitive loads at the output, including parasitic capacitive loads, cause poles in the feedback loop. This will reduce the phase margin and cause the circuit to become unstable.
If possible, analog circuits and digital circuits-including their respective ground and ground planes-should be separated. The fast rising edge of will cause the current glitch to flow into the ground plane. The noise caused by these fast current glitches will destroy the simulation performance. The analog ground and digital ground (and power supply) should be connected to a common grounding point to reduce the circulating digital and analog grounding current and noise.
High-level PCB wiring is very important for successful operational amplifier circuit design, especially for high-speed circuits. A good schematic diagram is the foundation of good wiring; Close cooperation between circuit design engineers and wiring design engineers is fundamental, especially on the location of devices and wiring. The problems to be considered include bypassing power supply, reducing parasitic effects, adopting ground plane, the influence of operational amplifier packaging, and wiring and shielding methods.
1. In PCB design, capacitors such as bypass filter at the chip power supply should be as close to the device as possible, and the typical distance is less than 3MM.
2. The small ceramic bypass capacitor at the power supply of the operational amplifier chip can provide energy for the high-frequency characteristics of the amplifier when the amplifier is in the input high-frequency signal. The capacitance value is selected according to the frequency of the input signal and the speed of the amplifier. For example, a 400MHz amplifier may adopt 0.01uF and 1nF capacitors installed in parallel.
3. When we buy capacitors and other devices, we also need to pay attention to their self-resonant oscillation frequency. Capacitors with self-resonant frequency around this frequency (400MHz) are of no benefit.
4. When drawing PCB, don’t take other lines under the input and output signal pins of the amplifier and the feedback resistor, which can reduce the mutual influence of parasitic capacitance between different lines and make the amplifier more stable.
5. The high-frequency new energy of surface mount devices is better and the volume is smaller.
6. When wiring the circuit board, the wiring should be as short as possible. At the same time, pay attention to its length and width to minimize parasitic effects.
7. For the treatment of power cord, the parasitic characteristics of power cord are the worst DC resistance and self-inductance, so we should widen the power cord as much as possible when laying it.
8. For the amplifier, the current on the input and output connection lines is very small, so they are easily affected. Parasitic effects do great harm to them.
9. For the signal path over 1CM, it is best to use a transmission line with controlled impedance and both ends terminated (matched resistance).
10. Amplifier drives resistive load. In order to solve the problem of stability, a common technique is to introduce a resistor ROUT, and it is better to be close to the operational amplifier. Thus, the series output resistor is used to isolate the capacitive load.