L R ≈ 5mm (0. The auto-router will add a trace to match the longest trace as shown in Figure 5. 1. The tool implements numerical solutions of Maxwell’s equations to render accurate and consistent results. Differential pairs are defined on the schematic by placing a Differential Pair directive (Place » Directive) on each of the nets in the pair. Take account, the internal used formulas sometimes don't equal on Conductor Impedance tab and on Differential pairs tab (e. Eurocircuits’ Impedance calculator uses real values as defined in the supplier’s data sheets. You can define various routing constraints such as length matching for differential pair, controlled impedance, and crosstalk prevention. Using the KiCad Impedance Calculator. 2 Answers. This is more than the to times trace width which is recommended (also read as close as possibly). Using the KiCad Impedance Calculator. Using the built-in Simbeor ® impedance calculator, the PCB editor's Layer Stack Manager calculates the trace width needed to deliver the specified impedance. This coupling lowers the differential impedance between the traces. Added inches and um to conversion data. 0 interface. The following are typical line widths and spacings for the more common impedance values when using ½ OZ copper: 50 Ohm Single Ended: 0. Impedance Calculators. You want to select “Microstrip Zdiff”. 10. By running a mating ref trace through the crosshatch you emulate a solid plane and can use a standard impedance. The PCB Impedance Calculator in Altium Designer. For differential pairs, there is also a spacing value given. This routing width rule targets a class of DRAM nets. 5) and 1. This 8W rule also applies to ground planes on the same layer. 5 mil wide traces with about 6 mils of. I am designing a flexible PCB to carry two types of differential signals. In the first case, assuming edge coupled traces on (or in) a PWB, each trace couples much more strongly to the ground plane than it does to. The Impedance Calculator is a tool which allows a quick pre-layout estimation of several kinds of line impedances between different layers. While the length and impedance are both important, 1mm of length differential will not affect your system's performance in any way, even for usb-2. The trace width (0. See moreCalculate the impedance of PCB traces quickly and easilly using DigiKey's Trace Impedance Calculator. PCB Traces. Also, I tried the differential tools in KiCad, and I matched both pairs to the same length (otherwise, one track is about 6 mm longer). To calculate geometry for 90OHMs Differential pairs routed on 2 layer PCB (Differential impedance of Microstrips) you need to know: width of the traces, space (gap) between the traces, height of dielectric above return plane, trace thickness and relative permittivity of the dielectric. There are four different types of impedance used in characterizing differential trace impedances. They will give very similar answers. eu. PCB Impedance and Capacitance Calculator: Asymmetric Stripline. There are some advantages to using a microstrip transmission line over other alternatives. trace width: 0. The required values are trace thickness, substrate heights, trace width, and subtrate dielectric in the calculator above and press the "calculate" button. For differential signals on the top layer, I have 100 ohm impedance for trace width 5 mil and gap 5. The differential micro strip calculator requires some parameters. Let’s move onto another example. Going to route the following signals: 100R differential pair (Zo = 50R) 90R differential pair (Zo = 45R) Planning to use the stack up JLC7628, with a 4 layer PCB, in total 1. arrow-right Download and install the Analog Engineer's Calculator. (5mil prepreg) But in your case if using a 10 mil dielectric; Impedance is almost constant ratiometric to the conductor geometry ratios if <2:1 ratios for planar and < 3:1 for differential coplanar. I tried to calculate differential impedance in three independent calculators and calculate manually from the formula. Best-in-Class PCB Fabrication, Assembly, & Components. 8 substrates of various thicknesses. It is the ratio of voltage and current amplitudes of a single wave propagating along the line in the absence of reflections from other directions. As most designers know, the impedance control in FR4 PCBs and PTFE PCBs are easy since the surface are almost absolutely flat, and there are many impedance calculating tools for choice. For differential vias, stitching vias can affect the impedance, but the impedance is much more sensitive to antipad sizes and layer arrangement/Dk value on each layer. Contents. BGA land calculator based on the IPC-7351A specification. Some differential impedance requirement for differential pairs, where the signals must be routed together Just as an example, consider a 2-layer board on standard thickness 62 mil core (Dk = 4. The receiver, which is on the motherboard, is a standard LVDS receiver, with 100Ω termination resistors. 24 mm. Keep the spacing between the pair consistent. Grounded Differential Coplanar Wave Guide. trace width: 0. 90 Ohm Differential Pair: 0. While, when it comes to flexible and. too. The CAN bus shall be split-terminated with 120Ohm, 60Ohm for each line. Differential Microstrip Impedance From Zo. 0175mm of Copper (1/2 oz), gives me a best estimate of 9mil trace, 4mil space for a 90R differential microstrip impedance. Several Web sites such as the examples listed below contain additional reference information. It shows the minimum trace-to-ground spacing required to. g. Some calculators will allow you to calculate trace impedance in a number of geometries, e. It was very hard to achieve a differential impedance of 100R with such a thin dielectric, so it was suggested that I used a hatched ground. This calculator finds both odd and even transmission line impedance. 120E (Differential pairs): CAN; Note: Actual impedance during manufacturing can vary by 5%/10% due to micro-level trace width changes (due to temperature variations) during the manufacturing process. Using the KiCad Impedance Calculator. ZO *. Differential pairs are very simple: they are composed of two traces, routed side-by-side, and that carry equal magnitude and opposite polarity signals on each trace. For instance, I want a 100 Ω controlled impedance pair, and have used "differential" before which seemed to work fine. This sheet has the FTDI chip which converts the input serial signal into a USB differential signal. Incidentally, analog signal integrity may be a bigger problem than digital in this design. 0 and CAN-FD might be slow enough to get away without it (and I'm honestly not sure, the performance will depend on the specific configuration), but if I were reviewing a layout with diff pairs without a ground reference I would be very unhappy with it. The other yellow rectangle is the USB connector. Value of Zo calculated by impedance calculator is not relevant here. If you read many PCB design guidelines, particularly on parallel protocols and differential pair routing, you’ll see many mentions of trace length matching. In differential pairs, each trace in the pair carries the same magnitude, but opposite polarity. Impedance; Signal Loss; S Parameter; Crosstalk; Geometry Information . The impedance calculator in the Allegro stack-up editor uses a static 2D field solver by default (bem2d). One of the features of this type of microstrip is the coupling between lines. The closer you place the pairs together, the differential impedance gets smaller because LM and CM get larger. Hence calculating impedance can be pretty tedious. H eff = H 1 + H 2 2 H e f f = H 1 + H 2 2. Is there a best practice or any documents that goes into details or even a online calculator would be great. Use an online differential stripline impedance calculator Calculating Differential vs. This sets the single-ended impedance AND differential impedance to the target value, even if there is no. Check design with the online gerber viewer, Easy and quick PCB Price Calculator from JLCPCBWhen you are creating your schematic, you must use one of the following suffixes when naming your differential pair nets: +/- (e. If you have placed Differential Pair directives on nets in the schematic, the default project options settings will result in the differential pairs members being created on the PCB. 254mm. Rule setup for differential pair (100 ohms) routing using Altium Designer. 159 OS X 10_15_7 EasyEDA. Remember, the lower the impedance the greater the current drain, the higher the. frequency capabilities. The differential impedance will be a bit less than 2x the single-ended impedance. Therefore i tried using this paper to calculate a characteristic impedance of 60Ohm for a coplanar strip attaching the CAN-Transceiver to a Twisted Pair cable, the CAN bus medium. To have designs be robust from PCB manufacturing errors and defects design the traces impedance be as close to the recommended value. 1. 8. Remember, the lower the impedance the greater the current drain, the higher the. The ICD Stackup Planner calculates characteristic impedance plus edge coupled and broadside coupled differential impedance. 15 mm spacing. It is recommended that DP link PCB traces maintain 50 Ω±15 % single-ended and 100 Ω±20 % differential impedance to maintain signal integrity. This rule maintains the desired signal impedance. On a 1. 3. This transmission line calculator. The PCB is the JLC7628 Stackup:. The main takeaway for me: To get to around 80 ohms, I should not pull ground on the signal. 5. It consists of a pair of center conductors separated by ground planes on the same substrate. The signal is 264MHz. 36 mm substrate, has an impedance to ground, unbalanced, of about 49 ohms. Differential pair PCB design basics, covering differential signalling benefits, references, impedance control, inter- and intra-pair matching, and terminatio. 5; w, t, s, h > 0;Made in Europe. When you calculate differential impedance you still need to have a ground reference. D+ and D-) _P/_N (e. To calculate the impedance, I use Saturn PCB impedance calculator software, Inc - PCB Toolkit V8. Dielectric constant = 3. Differential Stripline Impedance Calculator. Other calculators are like a black box; you have no idea which formulas they are using and no way to check the accuracy of these calculations. 006” space to be 100 Ohms differential” please make sure that the specified geometry is present on ALL the signal layers. For context, as I understand it, the Ethernet pairs between the PHY and magnetics need to be impedance controlled to 100ohm and they need to be routed (ideally) in parallel, and the trace space and trace width are important variables in the impedance control. CSI-2 interface data pairs require a 100 ohm differential impedance. Perhaps the most common type of transmission line is the coax. This is not usually a routing rule a designer might consider unless they are doing something very intentional with RF design, or they are routing differential pairs that require a specific impedance. Critical Length. The differential impedance will be a bit less than 2x the single-ended impedance. Ethernet cables can be unshielded so obviously just two wires without a ground plane can work. Edit: also, think ahead if you are going to use ground pours around the traces. Note: Units do not matter for this calculation as long as they are consistent. Added ability to include Er effective to the wavelength calculator. Si TRC – Track Resistance Calculator calculates DC. I am routing differential pairs in Altium 20 and using controlled impedance profiles. By controlling the PCB impedance, unexpected damages or errors can be limited to some extent. The odd mode impedance is the impedance of one transmission line when the pair is driven differentially. Saturn PCB Design Toolkit Differential Pair Impedance Calculator. 1<S/H<3. 4mils, w = 12. Note: Use the following guidelines when using two differential pairs: D = Distance between two differential pair signals; W = Width of a trace in a differential pair; S = Distance between the trace in a differential pair; and H = Dielectric height above the group plane. For now. Note that the built-in impedance calculator does not account for the affect of vias, it assumes lossless transference from one signal layer to the next. 8. 5 to 4. I tried to use the tool “PCB calculator” to determine the size of mi USB tracks. (Differential Impedance, Ohms) = Note: 1oz = 1. e. common impedance: 19. The calculation looks correct, but the tracks are too wide. A cheap unshielded laminated polyester FPC cable with 0. I am using microstrips on external layers only. Because the traces are together on the same layer (plane) I'm not sure how "coplanar". If you have copper fill on other layers, you can tie them to. 0 which can operate from 12 megahertz to around 240 megahertz. If trace width is increased for the metal losses reduction, the single end impedance will be reduces. If you require a controlled impedance PCB, you should discuss. It seems I could largely do without any impedance matching considerations, given what the trace lengths are. different differential pairs must have at least 30 mils of separation between the pairs. The grounded differential coplanar wave guide is the differential version of the grounded coplanar wave guide and is used in high-speed digital systems that require maximum noise immunity. Dimensional units: mm mils. This will give a 50ohm impedance from each trace to ground. 25mm, a trace width of 0. Altium Designer makes it easy to calculate geometry when you need to calculate impedance for differential pairs. There are plenty Impedance calculators available. With today’s modern PCBs,. For more information see "Interactively Routing with Controlled Impedances on a PCB in Altium Designer: Routing Width Design Rule" in the official documentation. A PCB impedance table will provide the trace width required to have a target impedance value on a standard stackup. $egingroup$ Note that it is possible to get 'free' impedance test coupons (the PCB fabricator will charge you for them). g. 6mm pcb thickness ) with 8 mil trace width, 8 mil trace spacing , 1. H 2 H 2 = subtrate height 2. NOTE: The formulas used in the program are valid for 0. #PCB #KiCad #HowtoIn this video, we show you how to create a deferential pair in KiCad. This calculator is designed to calculate the characteristic impedance of an edge-coupled microstrip. External Copper Thickness: Copper weight for the top and bottom layers. The vias on these differential pairs have their own impedance, which can create signal integrity problems on long interconnects. We’ll go over how differential pairs can hel. PCB Impedance and Capacitance Calculator: Asymmetric Differential Stripline. With PCBWay impedance calculator, you can calculate the approximate impedance of your PCB / high frequency circuit board. . 22 mm; trace distance: 0. This routing width rule targets a class of DRAM nets. Grounded Differential Coplanar Wave Guide. This is representative of a 50 Ω microstrip on the top layer of a 4-layer PCB. So, strive to keep your traces short and far apart in high-speed design. FWIW, the calculator I've used in the past is called Polar SI8000 which apparently has been superceded by Speedstack PCB. However, in odd mode the centre of the resistor will always be at 0v. . 50 to 60 Ω is a common target impedance used in many designs, and for differential pairs, 90 Ω or 100 Ω differential impedance is common. MX6 Design recommendations about 100 Ohms for differential pair is quite reasonable for FlexCAN. There is only one method I am aware of that gives any level of accuracy for differential via impedance calculations, and it starts from a calculation of the propagation delay through a pair of differential vias. Learn more about working with design rules and constraints. Polar or rectangular grids only allow direct reading of S11. The differential pair impedance depends on the characteristic impedance and the spacing between each end of the differential pair. For complex impedance matching, the input impedance is important as this is the impedance seen by a. The default units for all given values, except the subtrate. Coplanar Waveguide With Ground Characteristic Impedance Calculator. Note: The results are only for approximation and rough. The crosshatch should be at 45 deg (as shown in the diagram) The trace alignment over the hatch is important; it should be placed over the crosshatch intersections. The same in true for bottom layer. Agilent's LineCalc tool calculates Z odd = 50. The differential pairs should be loosely coupled. Finally, all calculators provide theoretical impedance values as they cannot take in to account the variables. they normally use 7. 1 ohms; In addition to those calculation I found some other resources with some sample trace properties: Mikrocontroller. zip (170k) AP117. 50 - 55 ohms). I am trying to design a USB circuit, where the signaling is over a differential pair D+ and D- and these two lines are required by the spec to have 90 ohms of differential impedance. 4 mil trace thickness. The only unified PCB design package with an integrated trace length calculator and PCB trace length matching vs. With PCBWay impedance calculator, you can calculate the approximate impedance of your PCB / high frequency circuit board. Calculates the outer and inner layer diameters of a padstack given the drill size. 0 differential insertion loss test to 43. Single-ended impedance of 50 Ohm. We recommend the software from Polar Instruments if you want to get exact impedance calculation. Using the KiCad Impedance Calculator. 0037” line/0. There are 3 types of single-ended models: single-ended non-coplanar, coplanar single. g. At some point the amplitude will stay constant. ALTIUM DESIGNER. 1. 6193. net Forum. The geometry of the traces, the permittivity of the PCB material and the layers. The flat flex cable used with Raspberry Pi CSI-2 camera interface has 0. Added edge. Broadside coupled non-shielded differential pair. 965 mm. PCB Trace Impedance Calculator Calculates the characteristic impedance and per-unit-length parameters of typical printed circuit board trace geometries. ’ Go to the ‘Differential pair rules’ >> ‘New rule. The USB port (Pins USB DP and USB DM) must be routed as 90 ohm differential PCB traces. ; To. In a differential pairs, there exists four kinds of characteristic impedance: Single-ended impedance ($ ext{Z}_{ ext. As logic circuits developed, the need for faster interconnects increased, making impedance levels increasingly important. Differential Microstrip Impedance Calculator (Using Zo) Embedded Microstrip Impedance Calculator. Atlas Si for VNA – Insertion loss test system – Delta-L 4. Against the two rows select the. While commonly constructed using. 1. 4. And this leads to our third rule: The separation between the two traces (of the differential pair) must remain constant over the entire length. Our impedance calculator features two main types of impedance models, single-ended and differential, within each trace structure. beammy. An impedance mismatch will. This calculator is designed to calculate the characteristic impedance of an edge coupled stripline. Copper. Suggested typical numbers for LVDS PCB using microstrip are, t = 1. Uncoated Microstrip Differential Pair Impedance Calculator . PCB manufacturer generally verifies the impedances with impedance coupon (traces drown during design in a separate portion outside our. 331 1 2 5. I. 225m s = 2mm. This will create the differential pair. Coplanar Waveguide Analysis/Synthesis Calculator. 1<W/H<3. 1. A PCB impedance calculator uses field solvers to accurately approximate impedance values. Formula: Applicability: w ÷ t ≤ 0. Sorted by: 1. The total length of the signal path is about 1,330mils. For a Dk = 4. 0 mils and h = 8. The typical method for matching timing in a differential pair is to match the lengths of the two lines at the source of the interconnect, also known as phase matching. 6 mm;. Such a microstrip is constructed with two traces referenced to the same reference plane with a dielectric material between them. This interactive controlled impedance routing greatly simplifies the task of designing a controlled impedance PCB. PCB manufacturer normally provides information about PCB stackup and track geometry. I checked out TNT, but couldn't seem to be able to do the same with that either. D+ and D-) _P/_N (e. 0 high-speed. Inductance – the impedance calculator uses the Impedance value to calculate the inductance per unit length. Difficulty in achieving 100 differential impedance accurately In order to have maximum coupling of the magnetic field lines, the space between two conductors of a differential pa ir should be kept to a minimum. Assuming both. 006” spacing. The etch length of the differential pair groups do not need to. For sample: 45-46 Ohm single end traces impedance can be used in diff. The test sample consists of a pair of differential traces at the top layer, followed by a differential via to the inner traces, then a second differential via connects to the BGA landing pads at the top layer again. 9. Complimenting these features are various differential pair routers, high speed design rules, and much, much more to help you get the job done. g. The S50 Impedance Profile is defining the routing widths, which will change according. Impedance. 6. Make sure you use an accurate tool like this. In the PCB panel, as you click on an entry in a list, a filter will be applied based on that entry. D_P and D_N) KiCad looks for these suffix pairs when using the differential pair routing tool during PCB layout. Spacing and width value pairs that will give a differential impedance of 100 Ohms on Dk = 4. Note that the y-axis is on a logarithmic scale for clarity. 8 mm to 0. Whenever there is an imbalance in a differential system, the fields no longer completely cancel and radiate in proportion. 08-07-2018 02:42 AM. Units ( mils ) Show PCB Dielectric Material. 50 - 55 ohms). 3) to calculate trace width and. To access the differential calculator, in the Primary Gap, Neck Gap, or +/- Tolerance cells, do the following steps: 4. H1: Height of the dielectric between the trace and the reference plane. Impedance and Differential Pairs in KiCad. The design approach of controlled impedance routing is a key ingredient of high speed PCB design, in which effective methods and tools must be adopted to ensure the intended high speed performance for your PCBs. Z 0 Differential impedance (Ω): Calculate. So if layer 1 and layer 3 are ground planes and you place an RF signal track on layer 2 it will need to be x mm wide for 50. The characteristic impedance is generally defined to. Here it is already defined in the schematic that this is D+ and D-. In order to maintain the impedance, I would like to use a power plane as reference beneath the signals, instead of the GND plane, as it provides better dimensions for routing (width and spacing) in the PCB toolkit calculator. If you control impedance, you place a tolerance in the fabrication notes. 01) and dielectric constant (3. Stripline Trace. Trace width and spacing depend on your PCB stackup. In the case where you are routing differential pairs, then the potential between the two pairs will always be opposing (i. Coax Impedance (Transmission Line) Calculator. 6 mm. By understanding the microstrip transmission line, designers can. High speed PCBs and signaling standards almost entirely use differential pairs with precise impedance. Impedance and PCB Stacking The trace impedance is affected by many factors such as the width of the trace, th e thickness of the copper, the PCB material, the PCB stacking specification, and the spacing between the differential pairs. Just as important as determining an appropriate trace width for a single-ended trace is determination of an appropriate spacing between two traces in a differential pair. A pair of lines can be modeled as follows: Differential pair line. 5mil (7628 prepreg) when tracks are inside. It could result 5 Ohm resistance. First of all, we need to clarify the concept of characteristic impedance. g. Home. You will need to know the thickness of the dielectric you will be using as well as the relative permittivity (dielectric constant). Differential pair length tuning. The Differential microstrip impedance calculator is used to compute the differential (Zd) and single-ended impedance (Zo) of an edge-coupled microstrip line just by entering the width, separation and thickness of the trace along with the dielectric thickness and constant of the coupled line. Based on Zo Equation Per: IPC-D-317A:. This calculator finds both odd and even transmission line impedance. When you have a USB you need to create differential pairs and KiCad ma. Twisted Pair Impedance. With the Multi-CB impedance calculator, you can calculate the approximate impedance dimensioning of your PCB / high frequency circuit board. Differential Pair Calculator The Differential Pair Calculator is used to perform what-if analysis to determine what values of Primary gap and width are needed to get specific differential and single line impedance. This calculator is designed to calculate the characteristic impedance of a microstrip - a flat conductor suspended over a ground plane with a dielectric between them (see diagram below). Let’s discuss the need for impedance. Antipads around through-hole vias are a point of contention in modern PCBs, and the debate around the use of these elements in a multilayer PCB is framed as a binary choice. differential impedance accurately In order to have maximum coupling of the magnetic field lines, the space between two conductors of a differential pair should be kept to a minimum. We can summarize that, theoretically, in differential pairs, there are two return paths: the adjacent trace (trace 2) running parallel to trace 1 and the ground plane. The two points at the beginning of the pair might be weakly coupled, but adding or removing some copper at the source end allows you to match pair lengths while ensuring common. Question #1: How to determine the required spacing between length-matched signals or differential pairs and the adjacent layers? If you are length matching that implies that the interface is probably fast enough to require controlled impedance. Differential pairs are commonly found in high-speed data communication systems, such as those used in computer interfaces (e. 13 mm; substrate thickness 1. D+ and D-) _P/_N (e. To access the differential calculator, in the Primary Gap, Neck Gap, or +/- Tolerance cells, do the following steps: 4. The required values are trace thickness, substrate heights, trace width, and subtrate dielectric in the calculator above and press the "calculate" button. The resistance value chosen for the two resistors will be referred to as R D (for d rain resistance). Using the KiCad Impedance Calculator. • As shown in Figure 1, traces should be 100-Ω(±5%) differential impedance of differential microstrip or differential stripline. To get the correct parameters, we have to use the specifications from our PCB house and the provided calculator. High speed digital designers should. (to the next pair and to the ground plane. PCB Routing Requirements for USB 2. Once the option is active, click on the shortest trace and move the mouse. 5mil clearance on a 63 mil board (1. best to use a stackup calculator to provide quick ‘what if’ analysis of the possibilities. You will need to know the thickness of the dielectric you will be using as well as the relative permittivity (dielectric constant). g. Grounded Differential Coplanar Wave Guide. Learn how to configure, define, and use your design’s impedance requirements for single-sided and differential on each layer, with Altium Designer’s Layer St. 8 core of FR4 material, we would have a propagation delay of approximately 150 ps/inch, or approximately 6 inches/ns. The trace width (0. The accuracy of a controlled impedance trace on a PCB is usually critical to the correct performance of the PCB. Thus a "differential pair" any long distance apart will have an impedance of 98 ohms, without any coupling between the lines, i. Microstrip Impedance Calculator. png . When routing differential signals across common PCB materials, each trace of the pair will experience different dielectric constants and corresponding signal velocities due to the differences in static permittivity (Ɛr) of theThe impedance of a differential via pair cannot be treated with a single via impedance calculator unless the vias are spaced far apart. But practically (especially in a dense design like your example), they also interact with surrounding conductors, and the coplanar ground, or ground beneath a differential microstrip will affect the characteristic impedance. The microstrip is a very simple yet useful way to create a transmission line with a PCB. Differential pairs are becoming common place in high speed design reducing. A coplanar differential raises the diff impedance while lowering Zo with adjacent grounds so it needs a thinner height of dielectric. Herndon's (Apple) paper from DesignCon 2003 "Noise Allocation for Near-end Crosstalk and for Uncertainty in Characteristic Impedance". 0 and 0. T T = trace thickness. In terms of how these calculators work, the impedance of a transmission line in a PCB can be calculated in four ways: Use the R, L, C, G parameters from the Telegrapher’s equations to calculate the impedance of the transmission line. Figure 5. 6mm thick. LVDS PCB layout guidelines aim to ensure optimal signal integrity and minimize noise interference by considering factors like impedance control, differential pair routing, grounding, etc. 6mm thick, 1. TheIt does not work. 1. When you need to do trace length matching in PCB design, your goal is to minimize timing differences between traces in a length-matching differential pair in a serial protocol, multiple traces. We will assume here that both of the lines of the pair are identical and uniform. I believe the 100-ohm differential equates to 50-ohm single ended because you can think of the two 100-ohm impedances as being in parallel resulting in an effective 50-ohm single-ended equivalence. To automatically select all traces that belong to a specific impedance profile, you can select Nets in the the PCB Panel and click the appropriate net class. 5mm pitch has about 100 ohm differential impedance in the configuration where a differential data pair wires are between ground. The idea here is to determine the spacing required for a given width with the goal of hitting a specific differential impedance value. 16 mil. PCB Design-Aid: Layout examples for impedances with defined layer buildup (single ended, differential pairs).