PCB Tech - Amplifier and video filter circuit board design skills

In the toolbox of any circuit board designer, the integrated circuit amplifier is one of the most basic component modules, and it is one of the most versatile products on the market. The amplifier has multiple functions, such as driving ADCs, driving multiple video loads, acting as video or other types of filters, driving high-speed instrument signals, etc. They can also be used as oscillators, but they can become a problem in some practical applications, because the amplifier should only oscillate when the designer needs it. If the circuit board is not designed correctly, the amplifier will do its own thing and oscillate at will. So, what should designers do to avoid this harmful oscillation? Recall what we learned in the electronics course before, that is, oscillation is related to capacitance, inductance and feedback. Therefore, the key is to carefully design the circuit board to ensure that any extraneous capacitive and inductive feedback paths are reduced or eliminated. This article will propose 13 layout design guidelines.

Circuit boards, loads (especially capacitive loads), and/or layout design all bring invisible capacitance and inductance. In addition, the current flowing into the bypass capacitors around the circuit board may create different paths, resulting in distortion. Therefore, some technologies that claim to reduce distortion are actually counterproductive and run counter to the design rules for avoiding oscillations. (The job of a designer is never easy, and it is true.) So, when designing the layout of an amplifier or video filter, in order to maintain global balance and reduce distortion and oscillation, what matters need to be considered?

First look at the oscillator. When the amplifier is used to directly drive a capacitive load, the load will have a phase lag with the amplifier's output impedance, and the phase lag will cause pulse spikes or oscillations. Some amplifiers can directly drive capacitive loads, but some amplifiers need to add a small series resistance (Rs) at the output of the amplifier to improve the stability and settling time performance of the amplifier.

High-frequency amplifiers are easily affected by the distortion caused by the circuit layout. Even low-frequency amplifiers, such as audio amplifiers, have very strict distortion requirements. Distortion (THD) is the main indicator of audio quality. Therefore, reducing the distortion caused by the layout is critical.

The main rule of high-frequency PCB layout design is to place the high-frequency bypass capacitor as close as possible to the power supply pin of the package. However, experiments have shown that slightly extending the connection traces of high-frequency bypass capacitors can improve flatness and differential gain, thereby reducing distortion. Design rules are of course beneficial, and the designer’s experimental experience is also very useful to ensure that the rules are consistent with reality.

When designing a video filter driver on the circuit board, it is very important that the input coupling capacitor and termination resistor should be placed close to the input pins to obtain the best signal integrity. Figure 3 shows a typical AC coupling input configuration of a video filter/driver. In this configuration, 0.1uF ceramic capacitors are used to AC-couple the input signal. If the input signal is not lower than the ground potential, the clamp circuit will not start; but if the input signal is lower than the ground potential, the clamp circuit will set the lowest voltage of the sync terminal to just lower than the ground potential. The input level set by the clamp circuit, combined with the internal DC offset, will keep the output signal within an acceptable range, about 250mV.

In order to obtain the highest output signal quality, the series termination resistor must be placed as close as possible to the output pin of the component. This will greatly reduce the influence of parasitic capacitance and parasitic inductance on the output of the driver.

13 circuit board design rules:

1) RTM (read the product manual carefully). The amplifier's data sheet generally puts forward its minimum stable gain requirements. This index is very important. If the operating gain of the amplifier is less than the recommended minimum stable gain, oscillation may occur.

2) Use a ground plane. This is the best way to provide a low-inductive ground connection for the component.

3) Remove the ground plane under and around the amplifier, and remove the ground plane near the sensing pin. Remove the ground plane near the input and output pins of the high-speed amplifier to reduce stray capacitance. Similarly, it is helpful to remove the ground plane under and around the amplifier.

4) Use surface mount components. The pin inductance of this type of component is very small.

5) Keep the pin length as short as possible. Shortening the pin length can reduce the series inductance at the inverting input of the amplifier.

6) Avoid using slots. Avoid using sockets, or use flush-mount at most to reduce inductance.

7 Use the recommended feedback resistance value. This is very important when using current feedback amplifiers.

8) Do not use non-linear components (such as capacitors) in the direct feedback loop of the amplifier.

9) A feedback resistor is used to achieve unity gain configuration. Do not use standard voltage-follower circuits.

10) Use bypass capacitors. Adding a bypass capacitor to each power supply can help reduce the return current path impedance at the power supply pins, improve the power supply noise suppression capability, and perform high-frequency filtering on the power supply wiring. Most manufacturers recommend the use of 6.8uF Tan capacitors and 0.1uF ceramic capacitors. For best performance, capacitors should be placed in accordance with the following rules: 6.8uF capacitors should not exceed 0.75 inches from the power supply pins, and 0.1uF capacitors should not exceed 0.1 inches from the power supply pins. When the distance between the two increases, the filtering effect of the capacitor decreases due to the increase of the wiring inductance. However, this also needs to be weighed against distortion considerations, because experimental results show that a little bit longer this distance will improve distortion performance.

11) Adjust the bypass capacitor to reduce distortion. When a single operational amplifier produces distortion due to the ground current path, the bypass element can be adjusted to adjust the ground current to keep it away from the input element. This is very simple, just adjust the bypass capacitor to keep its ground connection away from the input.

12) For video filters, place the series termination resistor close to the output pin. Doing so can minimize the impact of parasitic capacitance on the filter output driver, thus avoiding oscillations at the output.

13) Place the input coupling capacitor and termination resistor close to the input pins to obtain the best signal integrity.

The circuit board layout has a great influence on the system performance. Therefore, in the layout design stage, it should be carefully monitored to avoid errors.