Guide

Impedance Control: The Importance of Impedance Control in PCB

Impedance Control in PCB

Data signals and high-speed signals are prone to noise and attenuation during transmission, hence it is crucial to maintain signal integrity throughout the trace of the signal. One such factor is the impedance of the trace. The problem with impedance-related issues is that they cause unexpected and sometimes unrecognizable problems which are further complicated by the placement of nearby components which makes the diagnosis more difficult, even when the problem is addressed it requires changes in design which is both expensive and time-consuming.

What is Impedance Control?

Impedance by definition, is the obstruction of alternating current due to the combined effect of reactance and it obstructs every voltage-changing signal, be it a digital (for example square wave) or an analog signal (for example sine wave). The formula for impedance is 𝑍²=𝑅²+(𝑋ₗ-𝑋𝑐) ², where Z is the inductance R is the resistance, 𝑋ₗ is the inductive reactance and 𝑋𝑐 is capacitive inductance. Further from the above equation, impedance is directly proportional to resistance which is in turn directly proportional to the length of the conductor and inversely proportional to the cross-section of a conductor (in PCBs we take the width of the trace).

So impedance control means a certain impedance value is achieved by controlling the copper trace width, space and reference plane layers. It is used to transmit high-speed signals in printed circuit boards and is widely used in high-speed and high-frequency applications.

Factors Affecting the Impedance of a Trace in a PCB

For a normal trace (single-ended), the above formula gives the expected impedance as seen in the picture below which is of an impedance calculator tool. Such tools are also available on the websites of PCB vendors.

Fig1. Calculation of net Impedance(Zo) of a single end trace.

Fig1. Calculation of net Impedance(Zo) of a single end trace.

As shown in Fig 1. there are not many parameters affecting single trace impedance the formula 𝑍²=𝑅²+(𝑋ₗ-𝑋𝑐) ² mostly gives the required impedance. Propagation delay(Tpd) and the frequency of the signal also contribute to the impedance but are mostly negligible under 100 MHz. It is after Gigahertz signals that the frequency starts affecting impedance in an appreciable value. Further material of the substrate also contributes to the impedance of a trace.

It is the differential pairs where things start getting tricky with impedance, as in differential pairs a new concept of differential impedance is introduced.

Fig 2. Calculation of differential Impedance(Zo) of differential pair traces.

Fig 2. Calculation of differential Impedance(Zo) of differential pair traces.

As seen in Fig 2. the calculation for impedance now has too many variables, majorly conductor width, conductor spacing, and coupled length which must also be equal for each trace in the differential pair. Almost all of the differential pairs utilize impedance matching for signal integrity. Other reasons for affecting impedance are, the addition of inductors (like ferrite beads) at any end of the trace and improper use of vias.

What Happens When Impedance is not Controlled?

First of all things, we must know where controlled impedance is required. High-frequency signals, data signals, and RF signals require impedance matching. One might notice the following problems if the impedance is not properly controlled for the above-mentioned signals.

  1. Increase in Bit Error Rate (BER). High-frequency signals such as those used for rising-edge applications cannot lose even a few microseconds of data, attenuation in such signals might also result in the rejection of the whole data packet sent by the receiver.
  2.  Some protocols such as Ethernet, RS-485, SATA, HDMI, etc. specifically use fixed impedance which is to be maintained for example, ethernet requires 100Ω and RS-485 requires 120Ω differential impedance. Their transceivers are designed to send signals in specified impedance to improve signal integrity. In some cases, if the transceiver is not compatible with the specified impedance, then it might lead to unreliable connections due to packet collisions and loss of synchronization.
  3. In the case of differential pairs if the impedance of either of the traces is mismatched, then the trace may suffer from crosstalk making each trace of the differential pair a noise source for the other.
  4. When there is an impedance change in the circuit or when the uniformity of the impedance in a circuit is compromised, then the possibility of signal reflection increases. Signal reflection is the return of a voltage-varying signal back to the source due to non-uniform impedance traces. Signal reflection is a critical problem in RF analog signals.
  5. In antenna circuitry, the impedance of the trace has to be precise with tolerances of the impedance of the trace connecting the transceiver else the antennas lose sensitivity and again possibility of signal reflection increases.
  6. Besides data signals and high power (high voltage and high current) AC signals, while not compulsory, it is recommended to calculate and use appropriate impedance throughout the track to reduce power attenuation within the conductor. For circuits utilizing AC-DC or AC-AC power transmission, impedance control becomes more important.

 Communication Protocols Using Fixed Impedance

Some protocols require fixed differential impedance for them to function properly. Here are a few examples: –

  1. Ethernet – 100Ω
  2. USB 3. X (Universal Serial Bus) – 90Ω
  3. RS 485 – 120Ω
  4. PCIe Gen 4 (Peripheral Component Interconnect Express) -80Ω
  5. PCIe Gen 1 (Peripheral Component Interconnect Express) -100Ω
  6. HDMI (High-Definition Multimedia Interface) – 100Ω
  7. SATA (Serial Advanced Technology Attachment)- 100Ω

Please note that almost all of these protocols have 50 Ω single-ended impedance The tolerance for these impedance values decreases as we increase the frequency of the signal. For lower frequencies (till 1000Mhz) 10% tolerance is acceptable but it is still a good practice to keep the impedance tolerance to 5% during calculations, as it gives a margin to errors that are beyond control when the circuit is physically made. However, the use of modern fabrication tools has made manufacturing errors appreciably minimized today.

PCB Impedance Control

Conclusion

While impedance control is not required in circuits having low-speed signals, DC power, or very short connections which is mostly the case with basic circuits, it is very important to factor impedance control in High-Speed signals, RF signals, Media Protocols, and High-power circuits. Impedance control, being an important factor of signal integrity becomes even more important as we progress to even higher speeds.

The calculations and techniques for designing impedance control are now much easier thanks to modern CAD tools (like Altium), calculation software (like PCB toolkit), signal integrity check software (like Hyperlynx), and support from PCB vendors. Still one must be very cautious when working with impedance control, even after using all the tools and support, even a miss of a small detail makes debugging circuits very difficult, especially details of signal integrity and circuit impedance.

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