USB Termination & Signal Integrity Calculator
Calculate USB bus termination resistor values, differential impedance, cable propagation delay, signal rise time, and eye opening for USB 2.0 and USB 3.0
Formula
How It Works
This calculator determines USB differential termination resistor values for signal integrity at High-Speed (480 Mbps), SuperSpeed (5 Gbps), and SuperSpeed+ (10-20 Gbps) data rates. PCB designers and hardware engineers use it to meet USB-IF compliance requirements. Per the USB 2.0 specification Section 7.1.2 (Universal Serial Bus Specification Revision 2.0, USB Implementers Forum) and IEC 62680-1-2 (Universal Serial Bus interfaces for data and power — Part 1-2: Common components — USB Power Delivery specification), High-Speed mode requires 90 Ohm (+/-15%) differential impedance with 45 Ohm termination to ground at each data line. USB 3.2 (SuperSpeed) specifies 85 Ohm differential impedance per USB 3.2 specification Section 6.5.2. Without proper termination, signal reflections at impedance discontinuities cause eye diagram closure - USB-IF testing rejects devices with eye height below 150 mV (HS) or 100 mV (SS). A 10 cm unterminated USB 3.0 trace exhibits 30% eye closure at 5 Gbps, while proper 42.5 Ohm termination reduces this to under 5%. The USB receiver includes on-die termination (ODT) of 45 Ohm to ground, but external series resistors (typically 22-33 Ohm) are required for impedance matching at the connector interface.
Worked Example
A USB 3.2 Gen 2 (10 Gbps) hub design requires termination calculation for a 4-layer PCB with 8 cm SuperSpeed traces. Per USB 3.2 specification Section 6.5.2.1: Target differential impedance = 85 Ohm (+/-15%). Trace geometry (1 oz copper, 5 mil trace, 5 mil space on 8 mil FR4): Calculated Z_diff = 90 Ohm. Series termination resistors: R_series = (90 - 85) / 2 = 2.5 Ohm per line. Round to nearest standard value: 2.7 Ohm (E96 series). With TUSB1002 redriver (on-die termination 42.5 Ohm): Total termination = 42.5 Ohm to ground per line, matching USB 3.2 requirement. Eye diagram simulation shows 180 mV eye height at 10 Gbps, exceeding the 100 mV minimum with 80% margin.
Practical Tips
- ✓Per USB-IF PCB design guidelines, place 22-33 Ohm series termination resistors within 500 mils (12.7 mm) of the USB connector to absorb connector reflections
- ✓For USB 2.0 High-Speed, maintain trace impedance within 90 Ohm +/-10% (81-99 Ohm) to pass USB-IF compliance testing eye mask
- ✓Use 0402 or smaller SMD resistors for SuperSpeed termination to minimize parasitic inductance below 0.5 nH (critical above 5 Gbps)
Common Mistakes
- ✗Omitting series termination resistors entirely, relying only on on-die termination - this causes 15-25% eye closure at High-Speed and compliance failure
- ✗Using 0805 package resistors for SuperSpeed termination - the 1-2 nH parasitic inductance causes 10 dB insertion loss at 10 Gbps
- ✗Routing USB 3.x TX and RX pairs with mismatched lengths - 5 mm mismatch creates 50 ps skew, exceeding the 30 ps USB 3.2 specification limit
Frequently Asked Questions
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