Blog  /  PCB Guard Ring And The Significance In A Circuit

PCB Guard Ring And The Significance In A Circuit

With technology slowly growing and evolving, new possibilities and more efficient techniques to make products are now coming to light. PCB or Printed Circuit Boards play a significant role in the IT world when it comes to working.

Whether phones, laptops, printers, or any other electronic machine, they all use some printed circuit board. Today we will dig deeper into one of the components that PCB consists of: the guard ring.

A guard ring stands in a somewhat precarious place when it comes to importance. It is not as crucial as a resistor or a battery, but it is not as useless as a diode. People who have some experience with printed circuit boards have heard of what a guard ring is, and yet, even some experts don't understand why someone would use it.

Another common problem with a guard ring is that people do not know how to use it. In this guide, you will learn about guard rings, what they are, what they are capable of, and other parts that work in conjunction with them.

 

1、What is a PCB Guard?

 

Guard rings are one of the essential additions to a printed circuit board. Their presence on the circuit board allows the minimal loss of either current or voltage, depending on the type of configuration of your guard ring. The PCB Guard, attached to a low-impedance voltage source, ensures that a circuit remains closed and no voltage or current escapes. Though the guard ring is only thin copper wire, it is still an essential part of the course.

PCB guards operate to ensure no loss of voltage or current to other conductors near the circuit. People often use guard rings on AC circuits, where the guard ring surrounds the surface current with a potential problem or is leaking.

 

Image 1: PCB Guard Rings

 

Ensuring the current in the guard ring is equal to the current within the circuit, the potential difference is rendered zero and stops the wind from leaking. Due to its linear path, a guard ring can only be used in AC circuits; Therefore, they often appear in electric motors and other AC circuit equipment.

Another name for these PCB guard rings is driven guards. This term is mostly used by engineers or experts in the field of electronics. Guard rings or driven guards are one of the most critical parts of an AC circuit, especially that of an ECG (Electrocardiography).

In a process such as electrocardiography, which requires precise readings and has tiny room for error, a guard ring is usually present.
The guard ring helps keep the circuit closed and avoids current or voltage from leaking depending on the impedance of the course.

 

PCB Guard RingsImage 2: PCB Guard Rings

 

While a guard ring is one of the most critical parts of an AC circuit, it is only as effective as the impedance in the course. A guard ring is always connected to a low-impedance voltage source to nullify the otherwise current leakage produced by high impedance nodes on the circuit. Since high impedance circuits allow more voltage to pass than the present, the current eventually leaks out of the course. This particular scenario would be devastating for a precise low- current measurement, as leakage of wind can cause significant variations in the final results.

Leakage current is widespread in AC circuits attached to other electronic devices, such as a diode or a transistor, through a capacitor. It makes the electronic devices conduct current, which causes variations in results. On a smaller scale, the leakage of AC will increase the entire circuit; on a larger scale, the course can entirely fail.

Due to the potential danger of current leakage, most experts add a guard ring to shield the circuit and avoid leakage current adequately. Thanks to the guard ring's potential difference, which is equivalent to the potential difference of the high impedance node, the resulting current is equal to zero. It leads to precise measurements in both an accurate low-current size and in the electrocardiography process.

Now that you understand how vital a guard ring is, you must also know that a guard ring is unnecessary. Right, a guard ring stops current leakage, one of the most significant problems that a person working with AC circuits could face, but current leakage is not always a big issue.

In larger equipment, such as generators, current leakage is nearly impossible to stop, which is why there is often a warning on AC generators not to touch them when they are functional. When there is no other electronic equipment to conduct the current, the current begins to flow into the metal case of a generator.

Now that you understand the use of a guard ring or a driven guard, it is time to move onto the layout of a guard ring and where it is placed.

 

2、Guard Ring Layout

 

The layout for a guard ring differs depending on its purpose and the circuit for which you are using it. In the case of a high-impedance AC analog circuit, there are certain factors that you must take into consideration when adding a guard ring.

 

Image 3: Red Solder Mask

 

2.1 Thermocouple effect

 

To get readings as precise as a microvolt, you will need to consider the thermocouple effect. The thermocouple effect is one of the most important factors determining a layout for your circuit and should be your highest priority.

A thermocouple effect revolves around three different phenomena, each interconnected. When two dissimilar metals connect, they create a thermoelectric junction. This thermoelectric junction begins to generate its small voltage that is temperature-dependent. Many refer to this process as the "Seebeck Effect," It is one of the significant causes of error in a low-current measurement circuit.

Many candidates can bring about thermocouple errors in the final reading. Components like sockets, relay contacts, resistors, switches, and connectors are just a few of the many factors that you will have to take into consideration while making your circuit layout. Even the copper wire of the guard rings can generate a thermal EMF of nearly 200nV/°C, which is just enough interference to disrupt the final reading by nearly 2nV.

To ensure minimal thermocouple interference, you must take special care of the layout and components you use. Different brands give different levels of thermocouple interference, with some giving almost no EMF during the time of readings.

After you get the right brand of components, you must also be careful about how many of you can add to the circuit. Try to avoid using features such as switches, sockets, and connectors whenever possible. However, if using one of these components is unavoidable, be sure to use a brand that has low thermal EMF interference.

After you consider the multitude of factors that can distort your readings, there is just one last thing that you must do to get proper and accurate readings, and that is to place the guard rings.

 

2.2 Guard Ring Placement

 

Since you already know and understand the purpose of guard rings, we will not detail how it functions to improve the circuit. Instead, we will discuss its most ideal location on the circuit board to be the most effective.

As every manufacturer has their custom circuit board, they can't specify one place for the guard ring. So to identify their proper placement, we must see what components to wrap with the guard ring.

For the guard ring to be effective, you must encircle the inputs of the circuit. This barrier minimizes the current leakage from the input devices. Once you tie one end to a low impedance node, you must be careful before connecting the second one. Before you can link the second end of the guard ring, you must consider whether you want inverting configurations or noninverting configurations.

In the case of inverting configurations, you must tie the second end to the positive input's potential. On the other hand, if you prefer noninverting arrangements, then you can connect the second end to the negative input's potential. Depending on the choice that you make with regards to the configuration, your result can vary significantly.

The thermocouple effect and the placement of the guard ring are two of the most common mistakes that people make when creating their printed circuit board. Once you deal with these two issues, you can easily create the perfect circuit board that gives accurate readings down to the nanovolt.

Now a circuit board can function without the addition of a guard ring as many different appliances do, but you won't get accurate readings. The core purpose of the layout of the guard ring is to minimize the current leakage, which wouldn't affect higher reading, but lower lessons that go down to a milliampere or a nanovolt. It makes it a necessary component in machines where you need incredibly accurate readings, and the margin for error is meager.

 

3、PCB Guard Trace

 

Now that you have a circuit ready with a guard ring in pace to avoid current leakage and have taken all of the precautions to reduce thermal-induced EMF, with regards to creating a foolproof circuit, there is just one thing missing, and that is the Guard trace. The guard trace is one of the most critical parts of the course as it reduces crosstalk within the circuit.

To understand how a guard trace functions and what makes it so important, it is necessary for you to understand the concept of crosstalk and capacitive coupling.

The two phenomena – crosstalk and capacitive coupling - are the heart and soul of the guard trace and are the reason for its making. To better understand how both of these work, we will start with capacitive coupling, and work our way up to crosstalk and then on how the guard trace functions.

 

Image 4: PCB Guard Rings

 

3.1 The Upward Cycle

 

Capacitive coupling is when energies transfer within remote networks or an electrical system through the displacement of current. In an AC circuit, capacitive coupling prevents DC from passing from one course into the other.

Capacitive coupling can have either a random effect or an intended effect on your result. That lays the groundwork for crosstalk, a phenomenon where unintentional conductive coupling occurs and affects the circuit. Crosstalk is a widespread problem between audio electronics, structured cabling, and integrated circuit design.

 

3.2 Guard Trace Purpose

 

The purpose of the guard trace is straightforward; it prevents crosstalk from occurring or reduces its effects. Each circuit has two or more guard traces at each side of a parallel signal making it very useful in blocking crosstalk from other courses. Guard traces are mostly used in analog circuits, and not many in digital circuits; even small and simplistic boards have multiple guard traces scattered throughout them.

Guard traces consist of three different components that help it reduce the crosstalk in a circuit. The first component in a guard trace is the aggressor. The aggressor carries a signal in one direction and current in the other; this creates EMI.

This EMI will affect the other trace on the circuit board, which is called the victim trace. Since the victim trace is further away from the aggressor trace, the EMI is significantly lower. However, to also lower this EMI, you will need to add the third and final trace, which is known as the guard trace. It is connected to the VCC and is in between the aggressor and the victim.

In the absence of a guard trace, especially in low-frequency audio, crosstalk within the circuit can increase by a magnitude. It shows that a guard trace under the specific circumstances is significant.

 

4、PCB guard ring EMI

 

EMI is one of the hardest disturbances to measure and resolve, as nearly every component connected to the power source has its interference no matter how small it may be. However, just because it is small does not mean that if you can neglect its presence entirely.

On a small scale, an EMI can cause your circuit to slow down or give inaccurate results; on a larger scale; the EMI can prevent your course from functioning. It makes it a significant factor to consider when creating your printed circuit board.

Each component has its own thermal and electromagnetic interference. This interference can be as low as 50 kHz, which is not worrying at all, or it can be as high as 50 MHz, which can disrupt or even shut down your circuit. So, to properly counter and deal with the issue of EMI, you must look at all the components above.

To better help you understand the importance of a reduced EMI and guard trace, let's look back at crosstalk. Crosstalk is one of the most significant problems that can occur within your circuit, as it produces the highest electromagnetic interference. So, to properly combat this interference, you will need guard traces. As you are aware, guard traces eliminate crosstalk; it substantially reduces EMI created by crosstalk.

 

5、Conclusion

 

The guard ring is one of the most critical components in a low voltage circuit. It's copper wiring that encircles the course is quite advantageous to the overall transient voltage circuit. With the copper wires surrounding the inputs of the circuit, the guard ring lowers the risk of current leakage. It does so by passing a voltage within the cables that are equal to that of the input. It gives a similar potential difference within the circuit and significantly reduces leakage current.

However, a guard ring's effectiveness depends much on where you put it. Encircling the inputs of a circuit is the single best place to add a guard ring, as it can help eliminate current leakage more effectively.

The two most significant problems a person can face with a circuit are current leakage and EMI. Depending on the type you are trying to create, EMI can significantly affect your trajectory. Whether the interference is small or large, it is always best to take protective measures against it, and the best ones are guard traces. With the help of the guard trace and the aggressor and victim trace, you can nullify the electromagnetic interference within the circuit.

With all of the above information in mind, guard rings are among the most valuable parts of a low voltage circuit. Its ability to reduce significantly current leakage makes it very valuable when measuring shallow units of current or voltage.

Guard rings are only as useful as the layout of a Printed Circuit Board, making it very important that you find a company that can print PCBs to your liking. Outsourcing your PCB printing to accommodate your guard ring placement adequately this is where we come into play. If you want to know more about us and how we can help your business prosper, you can contact us.