1. In the toolkit of any circuit board designer, the integrated circuit amplifier stands out as one of the fundamental component modules and is among the most versatile products available today.

2. This amplifier serves various functions, including driving ADCs, managing multiple video loads, acting as video or other types of filters, and handling high-speed instrument signals, among others.

3. Additionally, they can function as oscillators; however, this capability can pose challenges in certain practical applications, as the amplifier should only oscillate when intended by the designer.

4. If the circuit board is not correctly designed, the amplifier may oscillate unpredictably. So, what steps can designers take to prevent this unwanted oscillation?

5. We should remember from our electronics courses that oscillation is influenced by capacitance, inductance, and feedback. Thus, the key lies in meticulously designing the circuit board to minimize or eliminate any extraneous capacitive and inductive feedback paths.

6. This article will present 13 layout design guidelines.

7. Circuit boards, loads (especially capacitive loads), and layout design all introduce hidden capacitance and inductance.

8. Moreover, the current flowing into the bypass capacitors around the circuit board can create various paths, potentially leading to distortion.

9. Consequently, some technologies that claim to reduce distortion may be counterproductive and conflict with the design principles aimed at preventing oscillations.

10. (Indeed, the designer’s role is never straightforward.) Therefore, when planning the layout of an amplifier or video filter, what key considerations should be taken into account to maintain global balance and minimize distortion and oscillation?

—

I hope this meets your needs!


First, examine the oscillator. When an amplifier is used to directly drive a capacitive load, the load will experience a phase lag relative to the amplifier’s output impedance, leading to pulse spikes or oscillations. Some amplifiers can directly drive capacitive loads; however, others may require the addition of a small series resistance (Rs) at the amplifier’s output to enhance stability and settling time performance.

High-frequency amplifiers are particularly susceptible to distortions caused by circuit layout. Even low-frequency amplifiers, such as audio amplifiers, must adhere to stringent distortion requirements. Total Harmonic Distortion (THD) serves as the primary indicator of audio quality, making it essential to minimize layout-induced distortion.

The primary guideline for high-frequency PCB layout design is to position the high-frequency bypass capacitor as close as possible to the power supply pin of the package. However, experiments indicate that slightly extending the connection traces for high-frequency bypass capacitors can enhance flatness and differential gain, thus reducing distortion. While design rules are certainly advantageous, the designer’s experimental experience is invaluable in ensuring these rules align with real-world conditions.

When designing a video filter driver on a circuit board, it is crucial to place the input coupling capacitor and termination resistor near the input pins to achieve optimal signal integrity. Figure 3 illustrates a typical AC coupling input configuration for a video filter/driver. In this setup, 0.1µF ceramic capacitors are employed to AC-couple the input signal. If the input signal does not drop below ground potential, the clamp circuit remains inactive; however, if it does fall below this level, the clamp circuit will limit the sync terminal’s minimum voltage to just below ground potential. The clamp circuit’s input level, combined with the internal DC offset, ensures the output signal remains within an acceptable range, approximately 250mV.

To achieve the highest quality output signal, the series termination resistor should be positioned as close as possible to the component’s output pin. This proximity significantly reduces the influence of parasitic capacitance and inductance on the driver’s output.

**13 Circuit Board Design Rules:**

1) **RTM (Read the Product Manual Thoroughly):** The amplifier’s data sheet typically specifies its minimum stable gain requirements. This parameter is critical; if the operating gain falls below this minimum, oscillation may occur.

2) **Utilize a Ground Plane:** This approach provides a low-inductive ground connection for components.

3) **Remove Ground Plane Under and Around the Amplifier:** Eliminate the ground plane near the sensing pin and close to the input and output pins of high-speed amplifiers to minimize stray capacitance.

4) **Opt for Surface Mount Components:** These components feature minimal pin inductance.

5) **Minimize Pin Length:** Shortening the pin length reduces series inductance at the amplifier’s inverting input.

6) **Avoid Slots:** Steer clear of sockets or at most, use flush-mount options to lessen inductance.

7) **Follow Recommended Feedback Resistance Values:** This is particularly crucial for current feedback amplifiers.

8) **Avoid Non-Linear Components in Direct Feedback Loops:** Do not use non-linear components (such as capacitors) within the amplifier’s direct feedback path.

9) **Employ a Feedback Resistor for Unity Gain Configuration:** Avoid standard voltage-follower circuits.

10) **Incorporate Bypass Capacitors:** Adding a bypass capacitor to each power supply pin can reduce return current path impedance, enhancing power supply noise suppression and enabling high-frequency filtering. Manufacturers typically recommend using 6.8µF tantalum capacitors alongside 0.1µF ceramic capacitors. For optimal performance, follow these guidelines: 6.8µF capacitors should be no more than 0.75 inches from power supply pins, and 0.1µF capacitors should not exceed 0.1 inches from power supply pins. Increasing the distance reduces the capacitor’s filtering effect due to rising wiring inductance; however, this must be balanced against distortion considerations, as experiments suggest that slightly longer distances may improve distortion performance.

11) **Adjust Bypass Capacitor to Minimize Distortion:** When a single operational amplifier generates distortion from the ground current path, adjusting the bypass element can help direct the ground current away from the input component. This is straightforward; simply adjust the bypass capacitor to keep its ground connection distant from the input.

12) **For Video Filters, Position the Series Termination Resistor Close to the Output Pin:** This placement minimizes the impact of parasitic capacitance on the filter output driver, thereby preventing output oscillations.

13) **Position Input Coupling Capacitor and Termination Resistor Close to Input Pins:** This arrangement maximizes signal integrity.

The circuit board layout significantly impacts system performance. Therefore, during the design stage, careful attention must be paid to avoid errors.

—

Let me know if you need any further adjustments!