## Stripline Differential Impedance Calculator

PCB stripline refers to a type of trace/transmission line that is routed in the inner layers of the PCB. Due to this, it is enclosed by a single material, usually the PCB substrate material. This is most commonly noticed on multilayer PCB designs, where the signal trace is equipped with ground planes both above and below it. Due to this arrangement, the high-frequency signal remains inside the PCB leading to fewer emissions and also provides shielding against incoming signals.

This calculator calculates the differential (ex : D+ & D- from USB) impedance between the positive and negative trace of the transmission line. Here the differential impedance is slightly less than twice the impedance of the single ended impedance. The closer the two traces, the smaller is the differential impedance. Now calculating the differential impedance is a two step process. First you need to calculate the single ended impedance Zo (Characteristic Impedance) then input the characteristic impedance, space between traces, and trace height to determine the impedance between two lines driven differentially.

**Understanding Stripline Differential Impedance:**

**Odd Impedance (Zodd)**: This is the impedance, measured with reference to the ground plane, when testing just one differential trace. Signals of opposing polarity must be used to drive the differential signals. The differential impedance, or Zdiff, has a value of half that of Zodd.

**Differential Impedance (Zdiff)**: The impedance between the two differential traces is as follows. Trace width, trace thickness, the separation between ground planes, and relative permittivity (DK) are some of the variables that affect it.

**Asymmetric Stripline Transmission Line**: When the distance between the trace and the planes varies from top to bottom, this kind of stripline is frequently seen in printed circuit boards. Since asymmetric striplines’ impedance is frequently present in designs, modeling it is advantageous.

**Differential Microstrip Impedance Calculator**: This kind of calculator is used to find differential microstrip line impedance. It takes into account variables like the substrate’s dielectric constant, trace width, and trace thickness.

**Differential Stripline Impedance Calculator**: The differential stripline lines’ impedance can be found using this calculator. It takes into account variables including relative permittivity (DK), trace width, trace thickness, and the separation between ground planes. An integrated layer stack editor in the Altium Designer software program can function as a potent differential stripline impedance calculator for both coplanar configurations and ordinary striplines.

**Etch Factor**: Etching results in a trapezoidal-shaped final trace rather than a rectangular one, which alters the impedance of the actual trace. This aspect is not taken into consideration by most internet calculators. **Propagation Delay**: Propagation delay values are either not provided by the majority of internet calculators or, when they are, they are estimated using a method that is known to be inaccurate.

To summarize, the Stripline Differential Impedance plays a crucial role in high-speed digital systems and is impacted by a number of parameters, including relative permittivity (DK), trace width, thickness, and distance between ground planes. For accurate design and simulation, it is imperative to employ exact calculators and take into account variables like the propagation delay and etch factor.

##### Step – 1: Calculate the Characteristic Impedance (more Info)

##### Step – 2: Calculate the differential Impedance

**Formula**

\[d = 2 * z * (1 – 0.347 * e^{(-2.9 * (\frac {S}{H}))})\]

**where** :

- d = Differential Impedance
- z = Characteristic Impedance
- s = Space between traces
- h = Height of trace

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