How to Determine the Impedance of a Circuit
In electronics, understanding the Impedance of a Circuit board is crucial to designing and troubleshooting electrical systems. Impedance, the measure of opposition that a circuit presents to the current flow, can significantly impact a circuit’s overall performance and efficiency. Determining the Impedance of a Circuit board can be a complicated process, but it is essential for ensuring that the circuit operates correctly and efficiently.
In this blog, we will explore impedance fundamentals, the different impedance types, and the methods to calculate impedance. By the end of this blog, you will have a solid understanding of how to determine the impedance of a circuit and why it is essential for the design and operation of electronic systems.
What is the difference between resistance and impedance?
Resistance and impedance have several differences. The primary difference between these is that Resistance opposes AC and DC circuit flow. On the other hand, the Impedance of a Circuit opposes the AC. It does not have any significance in a DC circuit. Moreover, below are the critical differences between impedance and resistance:
| Resistance | Impedance |
| The value of resistance is unaffected by the frequency of supply. | The value of impedance is affected by the supply frequency. |
| You can add two or more resistances arithmetically. | You cannot add two or more impedance arithmetically. However, you can do it with the help of vectors. |
| The current flowing through a resistance is always in phase with the voltage passing through it. | A phase distinguishes between the current and the voltage and impedance across it. The magnitude of the phase difference is determined by the presence or absence of inductance and capacitance in the impedance. |
| There is no phase distinction between current and voltage. | If the impedance Z = R+jX, the phase difference between voltage and current is calculated as tan = (X/R). |
| Resistance is used to consume active power. | The resistive section of impedance uses active power, whereas the inductive section uses reactive power. However, a capacitor is a reactive power generator. |
| R donates it | Z donates it. |
Why is impedance matching essential in PCBs?
Board trace works like a transmission line with a specific impedance at each point at a high frequency. The PCB’s impedance matching process makes sure that it is constant along the entire trace. Changing impedance in a transmission line can cause signal reflection, compromising signal integrity. Controlled impedance also ensures that no signals are degraded as they propagate the trace.
Remember that the Printed Circuit Board trace’s function transfers signal power from the driver device to the receiving device. Power must be transmitted along the entire length of the trace. However, maximum signal power is only possible with matching impedances on the PCB.
Factors affecting controlled impedance
The trace impedance is defined by the board material’s trace dimension, dielectric constant, and dielectric thickness. It ranges from 25 to 125 ohms. Below are the factors affecting controlled impedance:
- The copper signal trace’s thickness, or T, and width, or W, on both the top and bottom
- The thickness of the core on either side of the copper trace
- Dielectric constant, or ER, of the core and prepreg material
- Distance from other copper materials.
You should not forget one thing: controlled impedance is essential for high-speed applications like telecommunication, signal processing, and more.
What determines the overall impedance of a circuit board?
The arrangement of the components is used to determine the overall impendence of a circuit. A capacitor, an indicator, and a resistor are the top three fundamental circuit elements. The resistor resists content flow, and the resistance value is independent of the supply frequency. On the other hand, inductors and capacitors provide reactance, which is the frequency of the function.
The capacitor should have a reactance inversely proportional to the frequency of the signal angular. The inductor must also provide reactance directly proportional to the frequency of the angular signal.
The impedance of a circuit also depends on the inner conductive plane and PCB substrate. The insulator, along with the interior layer, produces parasitic capacitance and inductance. These elements lead to cross-talk and affect the overall Impedance of a Circuit. However, below are a few factors on which the Impedance of a Circuit id depends:
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Transmission line impedance
The transmission line’s impedance is primarily determined by its characteristic impedance, which is the impedance of the transmission line in total isolation. Even and odd mode impedance is the other metric that can be used to quantify transmission line impedance.
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Via impedance
Vias and through the hole will have parasitic effects between neighbouring conductive elements. The inductance of a via will be determined by its aspect ratio in the order of nanometers. The impedance of vias is around 25 to 35 ohms. Hence, there is a significant distinction between vias and trace impedance.
When a via hole is situated in a conductive trace, noise coupling occurs and causes impedance discontinuities. This is because you must always avoid placing vias between the distinct pairs to reduce discontinuities.
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Power delivery network
The delivery power network will have to lower the power bus resistance that drives the current return path at lower frequencies. It also has captive impedance. The physical separation between rail, power, trace, and inner planes defines PDN impedance. Remember, the PDN impedance rises with the input frequency.
The impedance spectrum can be used to determine the bandwidth with the lowest power delivery network impedance. It should remain constant across the entire operational range. The placement of the ground plane is crucial to maintaining the power integrity of the board.
Ways to determine the impedance of a circuit
The Impedance of a Circuit is essential to calculate to improve the overall performance of the printed circuit board. Several ways can help you determine how to calculate impedance. Below are the top ways to calculate impedance:
Online calculator
Controlled impedance or trace parameters can be determined with the help of the online calculator. These tools can get you close enough to where minimal adjustment for your CMare is required for manufacturability.
Practical method
You can also calculate impedance by following the physical method. You must follow the formula below to find a circuit’s impedance-
- Er is the dielectric constant of the element.
- H is the height above ground.
- W is the trace width.
- T is the trace thickness.
Circuit simulation
Circuit simulation is the technique used to verify the functionality of the printed circuit board. Impedance calculation includes several PCB design software programs. The impedance design can be modified for various simulations. You can choose the best design for manufacturing after the simulation is carried out.
Conclusion
Determining the impedance of a circuit is a fundamental skill essential for anyone working with electronics. Impedance can impact an electrical system’s performance, efficiency, and even safety, so it is crucial to understand how to measure and calculate it accurately. By considering the various factors that contribute to impedance, including resistance, inductance, and capacitance, and using the appropriate formulas and techniques, you can confidently determine the impedance of a circuit. With this knowledge, you can design and troubleshoot electronic systems more effectively and ensure they operate safely and efficiently.
