Single-ended Vs Differential PCB Impedance

byee-diary • • 2 min read
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When designing printed circuit boards (PCBs) for high-speed digital circuits, controlling the impedance of signal traces is essential to ensure optimal performance. Impedance mismatch can result in signal reflections, data errors, and power loss. Two important types of PCB impedance are single-ended impedance and differential impedance. These are critical for ensuring signal integrity, especially in high-frequency applications like high-speed data transmission, RF circuits, and PCB designs.

In this article, we’ll explore the differences between single-ended and differential PCB impedance, provide a step-by-step example calculation, and offer helpful resources such as PCB impedance calculators to simplify your design process. If you want to dive deeper into the subject, you can check out the PCB Impedance Calculator Online Free, which can assist in determining these values for your PCB designs.

What is Single-Ended Impedance?

Single-ended impedance refers to the impedance of a signal trace relative to a reference ground or power plane. It’s the most commonly used impedance type in many circuit designs where a single signal is transmitted over a trace.

PCB Signal Propagation Delay Calculator

Formula for Single-Ended Impedance:

To calculate single-ended impedance Z0Z_0 for a microstrip trace (one signal trace above a ground plane), we use the following formula:Z0=87ϵr⋅ln⁡(5.98⋅W0.8⋅H+T)

Where:

  • Z0 = Single-ended impedance (in ohms)
  • ϵr\epsilon_r = Dielectric constant of the PCB material
  • WW = Trace width (in mm)
  • HH = Distance from the trace to the ground plane (in mm)
  • TT = Trace thickness (in mm)

You can use an online PCB Impedance Calculator to simplify these calculations, ensuring your impedance values are accurate for your design.

What is Differential Impedance?

Differential impedance is used for signals carried by two complementary signal traces, typically in differential pairs, such as in high-speed data buses like USB, HDMI, and Ethernet. Differential impedance is typically higher than single-ended impedance and ensures better noise immunity and faster data rates. PCB Differential Pair Calculator - Trace Width, Spacing, and Dielectric Parameters

Formula for Differential Impedance:

Differential impedance ZdiffZ_{diff} for microstrip traces can be calculated using:

Zdiff=2⋅Z0⋅(1−0.48ϵr)Z_{diff} = 2 \cdot Z_0 \cdot \left( 1 - \frac{0.48}{\sqrt{\epsilon_r}} \

Where:

  • ZdiffZ_{diff} = Differential impedance (in ohms)
  • Z0Z_0 = Single-ended impedance (calculated above)

To explore how differential pair impedance is related to signal integrity, check out the PCB Differential Pair Impedance guide, which goes into detail on optimizing differential pairs for high-speed circuits.

Example Calculation with typical values

Let’s calculate single-ended and differential impedances using the following values:

  • Dielectric Constant ϵr\epsilon_r: 4.5
  • Trace Width WW: 0.2 mm
  • Trace Thickness T: 1.4 oz/ft² (converted to mm)
  • Distance to Ground Plane H: 0.2 mm

Converting Trace Thickness from oz/ft² to mm

PCB trace thickness is typically given in oz/ft². To convert this to millimeters, we use the conversion factor 0.0348 mm per oz/ft²0.0348 \, \text{mm per oz/ft²}.

For 1.4 oz/ft², we get:

T=1.4⋅0.0348=0.04872 mmT = 1.4 \cdot 0.0348 = 0.04872 \, \text{mm}

Calculating Single-Ended Impedance

Now, let’s calculate the single-ended impedance using the formula:

Z0=874.5⋅ln⁡(5.98⋅0.20.8⋅0.2+0.04872)

Breaking this down:

  • First, calculate the denominator of the logarithmic term:
0.8⋅0.2+0.04872=0.16+0.04872=0.20872 mm0.8 \cdot 0.2 + 0.04872 = 0.16 + 0.04872 = 0.20872 \, \text{mm}
  • Next, calculate the fraction inside the logarithm:
5.98⋅0.20.20872=1.1960.20872≈5.73\frac{5.98 \cdot 0.2}{0.20872} = \frac{1.196}{0.20872} \approx 5.73
  • Then, calculate the natural logarithm:
ln⁡(5.73)≈1.744\ln(5.73) \approx 1.744
  • Finally, calculate the single-ended impedance:
Z0=872.12132⋅1.744≈41.01⋅1.744=71.52 ΩZ_0 = \frac{87}{2.12132} \cdot 1.744 \approx 41.01 \cdot 1.744 = 71.52 \, \Omega

So, the single-ended impedance is approximately 71.52 ohms.

 

Calculating Differential Impedance

Now, let's calculate the differential impedance using the formula:

Zdiff=2⋅Z0⋅(1−0.484.5)

 

Substitute the value of Z0Z_0 from the previous calculation (71.52 Ω) and ϵr=4.5\epsilon_r = 4.5:

  • First, calculate 0.484.5\frac{0.48}{\sqrt{4.5}}
  • Next, subtract this from 1:
1−0.2262=0.77381 - 0.2262 = 0.7738
  • Finally, calculate the differential impedance:
Zdiff=2⋅71.52⋅0.7738≈2⋅71.52⋅0.7738=110.68 ΩZ_{diff} = 2 \cdot 71.52 \cdot 0.7738 \approx 2 \cdot 71.52 \cdot 0.7738 = 110.68 \, \Omega

Thus, the differential impedance is approximately 110.68 ohms.

Conclusion

In this article, we’ve covered the key differences between single-ended and differential impedance and shown you how to calculate both using practical examples. If you're designing high-speed circuits, understanding these impedance values is crucial for minimizing signal integrity issues.

For further details, such as in-depth examples, explanations, and access to calculators, you can visit the following resources:

These tools and guides will help you with designing PCB traces that meet your impedance requirements, ensuring better signal integrity for high-speed digital circuits.

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