# How to calculate the differential impedance for a stackup design

The design is basically a copy from a reference design. I used a few different tools to calculate the trace impedance with different results. While the manufacturer uses Si9000, the result does not match with what the other tools get.

The questions are:

• Which of the results below is the accurate?
• If the reference design is not that accurate in impedance control, how to evaluate the effect of the mismatch on the actual PCB it produces?

The questions are mostly on the inner layer differential impedance item [4] below.

## Details:

In the list below, [1] is the reference design stackup information; [2] and [3] are the processes to get the result close enough to the reference design target so as to know the tool is valid; and [4] is the calculation in question because it is either 5% higher than the target or 10% lower.

• [1] The reference stackup information is read from an original .brd file by an "Allegro Free Physical Viewer 17.2". The screenshot is as shown in the picture q1-pro-pcb.png . As also noted in the picture, the subsequent calculations are on the: [2] top-layer single-end impedance; [3] top-layer differential impedance; [4] inner-layer differential impedance.

• [2] Calculate the top layer signal end impedance: Two tools are giving Zo=48 and Zo=47. Assuming both are accurate enough.

• [2.1] Using the "Saturn PCB Toolkit V7.04", set W=3.5, H=2, F=500MHz, T=1.05, Er=4.5, the result is Zo=48 Ohms. This is close enough to the 50 Ohms target. It gives an Effective Er=3.12.
• [2.2] Using the online Montaro Impedance Calculator, choose Microstrip Zo. Set w=3.5, t=1.05, h=2, Er=3.12, the result is Zo=46.3. Change to t=1.0, the result is Zo=46.9.
• [3] Calculate the top layer differential impedance:

• [3.1] Using Montaro, choose Microstrip Zdiff. Set w=3, d=6.5, t=1, h=2, er=3.12. The result is Zd=100.6.
• [4] Calculate the inner layer differential impedance:

• [4.1] Using Montaro, choose Asymmetric Zo. Set w=3, t=1, h=3, h1=10, er=3.12, the result is Zo=57.143. Then choose Zdiff from Zo, set Zo=57.143, d=7, h=14, the result is Zd=104.984.
• [4.2] Using the "Polar Si9000 PCB Transmission Line Field Solver v7.1.0", set H1=10, H2=4, Er1=Er2=4.5, S1=7, W1=W2=3, T1=1, the result Zdiff=90.02. See the picture q42-polar-si9000.png .