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54 DESIGN007 MAGAZINE I JULY 2021 Multiple Impedance Considerations Some PCBs require multiple impedance val- ues on the same signal layers. It is critical that your PCB fabricator can create impedance coupons to reflect the appropriate model for each impedance requirement. However, test- ing multiple impedance values on a given sig- nal layer will cause the coupon to be wider than normal. ese wider coupons take up addi- tional valuable panel real estate, and because of this, designate one target impedance value to be tested per layer whenever possible. ere are two types of impedance classifica- tions that are generally specified: single-ended and differential. Single-Ended Impedance Single-ended impedance is established by the interaction of a single trace and its refer- ence plane(s). ere are four basic impedance classifications: 1. Microstrip: A trace on an outer layer with a single reference plane below it. 2. Embedded microstrip: A microstrip line that has a dielectric over the top of it; sol- der mask will change a microstrip into an embedded microstrip line. 3. Stripline: A trace on an internal layer that has a reference plane above and below it. 4. Dual stripline or offset stripline: A strip- line which is offset between the two refer- ence planes; it generally is used when two adjacent signal layers are routed orthogonally and have reference planes outside of them. Differential Impedance Differential impedance is established by the interaction of two traces and their reference plane(s). ere are five basic impedance clas- sifications: 1. Edge coupled microstrip: Comprised of two adjacent traces on an outer layer with a single reference plane below it. 2. Edge coupled embedded microstrip: An edge coupled microstrip line that has a dielectric over the top of it; solder mask will change a microstrip into an embedded microstrip line. 3. Edge coupled stripline: A configuration with two adjacent traces on an internal layer, which is centered between a refer- ence plane above and below it. 4. Edge coupled dual stripline or offset stripline: An edge-coupled stripline which is offset between the two reference planes; it is generally used when two adjacent signal layers are routed orthogonally and have reference planes outside of them. 5. Broadside coupled stripline: A configu- ration with the two differential lines on adjacent layers directly one above the other; these are offset striplines centered between their two reference planes. Controlled Impedance Design Guidelines • Standard impedance tolerance: ±10% – Tighter tolerances are available but need special care Figure 1: PCBs that contain controlled impedance lines require specific constructions and tighter manufacturing process controls.