EMC Question of the Week: March 14, 2022

Capacitors in series with a differential trace pair on a circuit board

Two 0.1-μF capacitors are placed in series with a high-speed differential trace pair on a circuit board to block low-frequency noise. Which of the following would generally be the best choice for these capacitors?

  1. C0G ceramic
  2. X7R ceramic
  3. Z5U ceramic 
  4. tantalum


The best answer is “a.” The C0G ceramic capacitors are Class 1 ceramics with a capacitance that is relatively independent of temperature and applied voltage. X7R and Z5U ceramic capacitors are both Class 2 ceramics. Their capacitance is a relatively strong function of the applied voltage. The non-linear behavior of Class 2 ceramics can introduce unexpected signal distortion and imbalance.  

Keeping high-speed differential traces balanced is generally a high priority. Class 1 ceramics with a low tolerance and small package size generally exhibit the same impedance over a wide range of frequencies. 

Class 2 ceramics are best suited for applications where there is a significant DC bias relative to the signal and/or where the precise value of the capacitance is not critical. High-frequency printed circuit board decoupling is a prime example of an application well-suited to Class 2 ceramics. 

Tantalum capacitors are polarized capacitors and have a high ESR compared to ceramic capacitors. They also generally have a higher tolerance. Like other electrolytic capacitors, they are well-suited to low-frequency applications with a DC-bias such as low-frequency printed circuit board decoupling.     

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