How to reduce ground bounce in PCB

What is ground bounce in PCBA?

All ground points on a circuit board assembly should be at the same potential. For some reason, the voltage at the ground reference point of a single board can fluctuate. This phenomenon is called ground bounce.

Ideally, for an IC package mounted on a board:

How to properly ground a circuit board?

In large electrical systems such as substations and transmission equipment, the return path for the current is connected directly to the physical ground. This approach is called direct grounding. However, many electronic circuits lack this provision. Circuit board components in cars or spacecraft are isolated from the physical ground. Therefore, to provide a current return path, designers keep a point or plane at ground potential. This approach is called indirect grounding.

Different types of grounding in circuit boards are:

Case Ground

The common ground point connected to the metal casing of the device is called chassis ground. All ground points on the PCB are brought together and connected at this point. It provides shock protection and physical shielding for the device. This ground prevents current from flowing across the entire surface area of ​​the chassis. Therefore, no ground loops are formed that cause EMF.

Signal Ground

The signal ground is a point on the board where the signal is measured. This reference point is usually located on the surface of the board and connected to the internal ground plane. Noise can be injected on the signal ground point. This is why these points are placed on the PCB itself instead of being isolated to different locations.

Cleaner signal grounding is an important parameter when it comes to boards handling precise low voltages, such as medical PCBs. In these boards, even small noise signals can create signal integrity issues.

Analog and digital sections of the same board are allowed to share the same signal ground.

Ground Plane

The ground plane is a layer of PCB stackup maintained at 0V potential. It acts as a reference plane and a sink for return currents circulating through the board. Signal ground points from the surface are connected through vias. The ground plane provides effective signal return, voltage return, and reduces noise and interference. It is a good practice to keep the ground plane continuous. This will avoid the formation of ground loops that provide alternative current return paths, thereby disrupting operations.

Earth

Ground is a direct connection from a circuit to the physical ground. This type of grounding is prominent in electronic appliances. Short circuits in electrical interconnects or live components in contact with the chassis/cover of the equipment are dangerous. Operators can get an electric shock if they touch the chassis. Therefore, all high voltage circuits require grounding.

What causes ground loops?

Ground loops are multiple current return paths caused by ground potential differences throughout the circuit. Therefore, the current circulates through alternative paths that are shorter than the required path. Ideally, all ground points and planes on a PCB assembly should be at the same potential. But in real-world scenarios, there is inductance in the leads that connect the IC to the board. This inductance creates a small voltage drop and becomes the cause of ground bounce.

The value of inductive parasitic inductance is very small but it is not negligible for devices like transistors. These devices are high-speed switching circuits made up of logic gates. The signal voltage of transistors switches between high 1 and low 0.

When the IC voltage is 1, it means that the output voltage is approaching Vcc. Similarly, if the voltage is 0, it means that the output voltage is approaching the ground potential (0V). Due to the above inductance, the ground voltage starts fluctuating or bounces to other values ​​other than 0V. This is the reason behind ground bounce.

Ground bounce causes the circuit to misinterpret a low (0) as a high (1). As a result, the operation of digital logic devices throughout the board is affected.

Apart from its impact on operation, ground bounce is considered as noise on the PCBA. It therefore reduces signal integrity. Therefore, designers take necessary guidelines to eliminate ground bounce.

How does ground bounce occur?

The generation of ground bounce can also be clearly explained using the circuit given below.

The figure shows a circuit with a CMOS connected to an output load with C rise capacitor and R rise resistance. The other parameters are as follows:

LA: Intrinsic inductance of the package power lead

LB: Intrinsic inductance of the package output lead

LC: Intrinsic inductance in the CMOS package ground lead

R1: Output resistance of the CMOS IC

Now let us look at the scenario where the output changes from high (1) to low (0). In this case, current flows from the load side and the load capacitor discharges. This current (I) flows through the inductor LB and LC generating a voltage V=L(dv/dt). This generated voltage causes the internal CMOS ground to have a different potential from the external load ground. Therefore, the internal reference ground has a higher potential compared to the zero potential of the external ground. This difference manifests as noise in the circuit and affects the operation of the switching devices in the circuit.

The ground bounce of the circuit observed through an oscilloscope is shown below.

The top waveform represents the output of the switching device I/O pin.

The bottom waveform shows the spike (noise) due to ground bounce. How to Reduce Ground Bounce

PCB designers are always looking for tips to reduce ground bounce. The most common method is to place a bypass capacitor on the circuit. The bypass capacitor effectively bypasses voltage spikes and power supply noise. It is connected between the VCC and GND pins of the IC package. What does the bypass capacitor do?

Capacitors behave differently in AC and DC circuits. In a DC circuit, it is charged to the supply current and then completely blocks the current flow. In an AC circuit, it provides an easy path for transient signals to go to ground. Therefore, ground bounce or noise as a transient signal is bypassed directly to ground. The DC signal will be blocked by the capacitor and therefore flows through the circuit and stimulates operation.

Capacitor Design Parameters

Lead inductance is a critical factor
Multilayer ceramic chip capacitors (MLCC) are commonly used
Maximum capacitor current depends on maximum pulse slew rate
Amount of current consumed when switching from low to high

Where should bypass capacitors be placed on the PCB?

Bypass capacitors are placed as close to the power pins of the component as possible. This capacitor acts as a local charge storage for switching components such as transistors. Extra voltage spikes are stored in this capacitor instead of circulating through the circuit.

As a result, all ground points will be at the same potential and there will be no ground bounce noise. Here are more design guidelines related to bypass capacitors:

Use wide and short traces and vias to connect bypass capacitor pads to power and ground pins. This minimizes inductance and improves current flow.

Integrate SMD capacitors.

Add vias close to capacitor pads.

What is the difference between bypass capacitors and decoupling capacitors?

Capacitors are widely used as bypass and decoupling components. These terms are extremely common in circuit board design, so let’s take a look at the difference between the two.

Bypass capacitors

Decoupling capacitors

Bypass noisy signals to ground

Separate two circuits from each other

Place between supply voltage and ground pins

Place in parallel with supply and load

Short AC signals to ground

Create low impedance path for high frequency signals

Layout design rules to reduce ground bounce
Use via pads where design allows.

Reduce signal return path distance. Reduced distance will reduce parasitic capacitance. For this purpose, it is best to place components directly above their ground points.

Do not use sockets or wire wrap boards.

Never share ground vias or traces for ground connections. It is recommended to use separate vias and traces to connect to the ground plane.

Do not connect capacitors directly to outputs.

Implement low voltage differential signaling (LVDS) as the I/O standard. This standard provides high bandwidth and high noise immunity.

Select short lead packages to reduce series inductance. BGA is also recommended.

Use a solid ground plane to reduce IR losses and inductance. Avoid ground split planes.

Use lower switching components if the design allows.