Q: How to control impedance on PCB traces: single-ended and differential pairs?
Answer
Impedance depends on trace width (W) copper thickness (T) substrate height (H) and dielectric constant (Er). For microstrip: Z0 approximates (87/sqrt(Er+1.41)) x ln(5.98H/(0.8W+T)). Use a PCB impedance calculator (Polar SI9000) and specify target impedance in your fab notes. Common targets: USB 3.0 (90 Ohm differential) USB 2.0 (45 Ohm differential) Ethernet (100 Ohm differential) HDMI (95 Ohm differential). Always include stack-up specification in your Gerber readme. For high-speed designs request impedance-controlled PCB from your manufacturer and include test coupons on the panel.
Filed under: FAQ