Impedance Matching for USB Interfaces in PCBs
Key Takeaways
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The differential characteristic impedance of the USB 2.0 interface must equal 90Ω ± 15 % tolerance.
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Geometrical parameters such as trace width, height above the reference plane, distance between traces, and trace length can be adjusted to achieve the desired standard impedance matching for USB interfaces.
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The impact of impedance mismatching for USB interfaces includes signal reflections, signal loss, crosstalk, and noise.
Modern electronics utilize USB interfaces for high-speed data transfer
Impedance matching for USBs is essential to obtain the desired performance from peripheral devices in consumer, industrial, and automotive applications. The USB interface is based on differential pair signaling, and standard values for the differential characteristic impedance are subjective to the USB generation. We will discuss impedance matching for the USB 2.0 interface in this article.
The Effect of Impedance Mismatches in USB Interfaces
Generally, USB interfaces allow signal or data transfer. Impedance matching for USB interfaces in PCBs is a significant factor controlling signal degradation. If the impedance standards are not met while routing the traces of the USB interface on the PCB, the receiving end signal is distorted. Impedance mismatches in USB interfaces can have negative effects on signal integrity. Signal degradation issues can result in output errors, which can be costly.
Negative Effects of an Impedance Mismatch in a USB Interface |
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Signal reflections |
Consider a USB interface on a board transferring signals from a microcontroller to a peripheral device. The impedance mismatch in the USB interface produces reflections towards the microcontroller and results in degraded performance. |
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Signal Loss |
From the source to the receiver, the signal may be subjected to heating loss due to impedance mismatch. Signal loss in the form of heat makes signal recovery difficult at the receiving end. |
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Crosstalk |
Data corruption resulting from crosstalk in impedance-mismatched USB interfaces should not be ignored, as signals in differential pairs are not confined to their respective paths. |
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Noise |
Impedance mismatches can cause signal reflections and crosstalk, reducing the signal-to-noise ratio. The signal might deviate greatly from the desired signal at the receiving end. |
Standard Impedance Matching for USB Interfaces in PCBs
Schematic of USB 2.0 interface
USB 2.0 utilizes the differential pair of signals D+ and D- along with the VBUS and GND, as shown in the figure below. There are two pairs of differential traces in USB 3. X generation. The table below gives the standard differential characteristic impedance values for USB 2.0 and USB 3.0.
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Characteristic Impedance |
USB 2.0/USB 3. X |
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Single-ended characteristic impedance |
45Ω ± 15 % |
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Differential characteristic impedance |
90Ω ± 15 % |
Standard impedance values of USB 2.0 and USB 3. X interface
The operating frequency of USB 2.0 can be categorized as given in the table below.
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Frequency Scale |
Frequency |
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Low-speed |
750 kHz |
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Full-speed |
6 MHz |
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High-speed |
240 MHz |
Frequency scale of USB 2.0 interface
Even if the frequency scale differs, the impedance values for all USB 2.0 speeds are the same.
Single-Ended and Differential Characteristic Impedance Influences Impedance Matching for USB Interfaces
A USB 2.0 interface differential pair can be routed on external or internal PCB layers. Let's consider a USB interface differential pair routed on the external PCB layer, as shown in the figure below.
Schematic of USB 2.0 interface traces
In this case, the single-ended characteristic impedance and differential characteristic impedance can be given by the following micro-strip impedance equations.
Single-ended characteristic impedance equation
Differential characteristic impedance equation
Simplified differential characteristic impedance equation
Note that Z0 is the single-ended characteristic impedance, Zd is the differential characteristic impedance, d is the distance between the D+ and D- trace, w is the width of trace, t is the thickness of trace, h is the thickness of the dielectric, and ∈r is the relative dielectric constant.
It is clear from the equations that the single-ended characteristic impedance of the transmission line influences the differential characteristic impedance. According to USB standards, the single-ended characteristic impedance must be within 45Ω ± 15 % and is mostly dependent on the trace width, height, and dielectric constant of the PCB material.
As per the standards, equations (2) and (3) must equal 90Ω ± 15 % tolerance. The ratio d/h is very important in the design of the USB interface, as the length of the differential traces (D+ and D-) and the distance between them control the differential characteristic impedance value.
Coupling Between Differential Pairs and Impedance Matching in USBs
The distance between the D+ and D- traces of the USB interface affects coupling. As the distance between the traces decreases, the coupling between them grows and vice versa. The higher the coupling, the lower the differential characteristic impedance. The geometrical parameters such as trace width, height above the reference plane, the distance between the traces, and trace length are adjusted to achieve the desired standard impedance matching for USB interfaces.
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