2. However, the main advantage of the current configuration is that it offers high input impedance. Impedance. The guides says spacing under 0. Reference layer. (Differential Impedance, Ohms) = Note: 1oz = 1. The user only needs to enter the attenuation in decibels (dB) and the given characteristic impedance of the transmission lines to be matched. 3mm, as shown below: This increases the impedance slightly to 91. Let’s start new project together!. The calculator is set up to handle an asymmetric arrangement, where traces are not centrally located in the PCB layer stack. The top surface and the sides of the traces are exposed to air and are referenced to a power or ground plane. Differential Impedance of. mm mils. This calculator helps you calculate the characteristic impedance of a coaxial cable given its dimensions. Track and gap widths are of concern in controlled impedance, but unless your speaker and mic connections (I assume audio frequencies) are going a long way you should not have any particular requirement for controlled impedance. Separation between Planes. The greater the mismatch. Vias (plated holes) are used to route electrical signals among different layers of a multilayer. STEVE: High speed systems today face many simultaneous impedance requirements that must be achieved in the same stack-up – differential impedances at 100 ohms for ethernet, USB at 90 ohms, PCIe at 85 ohms, and a myriad of single-ended DDR requirements. Parallel Wire Impedance, Inductance, Capacitance and Propagation Delay Calculator Results. The differential impedance depends upon the D/H ratio. To improve calculation speeds, impedance profiles are calculated in separate threads (when available). Equation 9 For high gain, the CMRR of the 2-op amp instru mentation amplifier is similar to the CMRR of the differential amplifier. SET2SEIL. Fig. Supports Anritsu ShockLine™ MS46524B and VectorStar™ MS4647B. Visit to learn more about our other electrical engineering tools and resources. Radio Frequency Engineering Calculator. 2) S/T > 5. 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. Z LC is the LC circuit impedance in ohms (Ω),. For this calculation, the units of d and h can be ignored as long as they have the same unit. Differential impedance at low frequency represents the sum of inductance resistances of the filter. Z diff equals Ra||(2Rb) (p model) or 2R 1 (T model). To find what a given trace width will have a single ended or differential impedance, we can use a built in calculator from Allegro. T represents trace thickness. Assuming both are accurate enough. Relative Dielectric Constant ( εr ): Track Width (S): mm. While this calculator will provide a baseline, any final design considerations should be made towards loss, dispersion. Thinking about the problem as the impedance the differential signal sees (the thing we care about) compared to the impedance the common mode signal sees (the part of the signal we typically try to eliminate in a difference amplifier) is probably more appropriate for most practical signals, and gives us a nice simple expression. 378tA. A typical 4 Layer Stackup looks as follows. If you don't have access to a differential impedance calculator, or you don't know how to calculate it, then just keep trace lengths less than 0. 8. EEWeb offers a free online broadside coupled stripline impedance calculator. The bottom line is, study carefully, and take PCB trace impedance equations with a proper dose of. Now you're done with your resistors! The values should be R₁ = 76. There are a lot of online trace impedance calculators specifically designed for differential traces. Select the dimension unit of the stripline: mm mils. You can use all the calculators you want if you don't know the exact board make up they're using; they should be able to help you. 6 inches - this is very easy to do; If you can access a differential impedance calculator, then use. Check here: UltraCAD's new Differential Impedance calculator will calculate single-ended and differential impedance for the common configurations we all deal with: Microstrip. and thats the first time iam doing ethernet things. 5mm, T=0. The impedance of a differential via pair cannot be treated with a single via impedance calculator unless the vias are spaced far apart. The characteristic impedance (Z o) of a microstrip transmission line can be calculated using the active calculator or the equations at the bottom of the page. ZL =. But for extreme geometries and for many differential pair structures, these approximations can. The differential gain ADM of an amplifier with a differential output is defined as: # ½ Æ 8 È ½ 8 ½ Æ where VOD is the differential output voltage. Eurocircuits’ PCB DEFINED IMPEDANCE pool is a fast turnaround solution for PCB’s with a specific impedance requirements for certain tracks. 01) and dielectric constant (3. The range of accepted values is 20 to 90 Ω for single-ended and 50 to 150 Ω for differential signals. You can interpret the name 'RLC circuit' to mean. Vias (plated holes) are used to route electrical signals among different layers of a multilayer layer PCB. 50 Ohms. Reflection Coefficient Calculator. Relative Dielectric Constant ( εr ): Track Width. 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. I agree. In that case there is no reflection. For better impedance control, use PCB design software you can trust. Mutual inductance and capacitance exist between the two traces in a differential pair. So move to your left, use Omni's Cable impedance calculator and enjoy. This will leave the rest of the signals which are rather non-critical like I2C, power good signals, etc. Tightly coupled pairs (within a pair) are quite common and this has the advantage of having a somewhat higher single ended impedance on a per track basis within a differential pair which is often easier to implement. A wire stripline is constructed with a round conductor suspended between two ground planes. Incidentally, the trace thickness parameter does have a significant effect. I found that a W of 0. A coaxial cable is designed with 50-Ω or 75-Ω characteristic impedance. Differential Trace Microstrip Impedance Calculator; Differential Trace Stripline Impedance Calculator; The standard geometries that are supported by these trace impedance calculators are shown in the graphic below. RF Amplifier PAE Calculator. The coaxial cable, along with the balanced two-wire, is the most common type of transmission line used in RF communications. Once you know the characteristic impedance, the differential. Figure 8. There are 3 types of single-ended models: single-ended non-coplanar, coplanar single. The impedance calculator in the Allegro stack-up editor uses a static 2D field solver by default (bem2d). Figure 5. 2 Loss of CPW lines; 3. The characteristic impedance of your microstrips is determined by the trace width for a given layer stackup. A 3-D EM simulator is used to design a differential via with the desired impedance and bandwidth. Coplanar Waveguide Calculator. Our impedance calculator features two main types of impedance models, single-ended and differential, within each trace structure. **Note: Like our stripline impedance calculator, all of our RF calculators allow SI prefix input. A twisted-pair cable is simply two wires that are twisted together so as to reduce radiated EMI (electromagneticinterference) and mitigate the effects of received EMI. ALGEBRAIC EQUATIONS Single Track For the stripline of Figure 2 with a symmetrically centredThe concept of Shield Transfer Impedance (Zt), introduced by S. 15 mm spacing. Differential Impedance Differential Impedance: the impedance the difference signal sees ( ) ( ) 2 2( ) Z 0 small I V I V diff Z diff one one = = ≈ − Differential impedance decreases as coupling increases +1v -1v I one x I two How will the capacitance matrix elements be affected by spacing? C 12 C 11 C 22 Eric Bogatin 2000 Slide -18. The calculator will process the values and display the calculated Differential Impedance (Zd) in Ohms in the designated input field. Controlled differential impedance starts with characteristic impedance. (1+ρ) (1-ρ) With most of today's TDR-capable instruments, such as the Tektronix sampling oscilloscope, TDR measurements can be displayed with units of volts, ohms, or ρ (rho) on the vertical magnitude scale. has two length-matched 45 Ohm lines. ALTIUM DESIGNER For. Relative Dielectric Constant ( εr ): Track Width: mm. The Smith Chart achieves this by superimposing a grid on S11 that maps load impedance. When dealing with differential pairs, thi. Suppose we have a system with the following parameters: R= 30 Ω;; L = 10 mH; and; C = 100 μF. Power Divider Calculator. Typical values for most common differential signal types are 90 ohms differential, 100 ohms differential or 120 ohms differential. C is. T Match Impedance Calculator. Enter values for R1 and R2 to calculate attenuator loss and impedance. 1 to 2. View All Tools The edge coupled differential microstrip transmission line is a common technique for routing differential traces. All About Circuits is the largest online electrical engineering communities in the world with over 700K engineers, who collaborate every day to innovate, design, and create. Calculate Coplanar impedance for given PCB parameters. Use several of them and compare. Turning over to the digital logic side of things, our favorite way to spew ones and zeros is to calculate the difference between two matching lines rather than trying to read a single line that is subject to momentary instances of noise as a voltage spikes across the landscape. However, due to manufacturing techniques and tolerances, the two tracks may have different cross-sections — the unbalanced track case. The trace width required to deliver a specific impedance is calculated as part of the impedance profile, configured in the Impedance tab of the Layer Stack Manager. Differential impedance is the double of odd mode. , equal in magnitude but opposite in polarity). 1 Answer. In general, it is double the odd-mode impedance, which is the value we care about for differential signaling, as it is used in high-speed PCB design. The iCD Stackup Planner offers unprecedented simulation speed, ease of use and industry leading impedance accuracy at an affordable. Figure 25 shows a differential pair using the microstrip layout. 6mm) That should give the traces around 90 Ohms differential impedance according to the PDF, but when I try to input it to my impedance calculator (Saturn PCB toolkit V7. As long as the loop gain is very high, the output impedance, ZOUT, in this circuit is approximatelyI used the differential impedance calculator in the x-section dialog and also the allegro impedance workflow a few times. Differential pair into PCB calculator. To avoid microstrip modes, have h >> (w+2g) and extend component side ground away from the trace by > (w+2g). Based. Added option to select between default asymmetrical stripline formula and the IPC and Wadell formulas. 1. The T match circuit gets its name because the circuit topology looks like the letter “T”. 7. This calculator finds both odd and even transmission line impedance. High speed digital designers should. 8 substrates of various thicknesses. With t = 35 um (typical for copper with plating on a pcb), Z odd drops to 44 ohms, according to LineCalc. 50 - 55 ohms). It seems I could largely do without any impedance matching considerations, given what the trace lengths are. 3) to calculate trace width and. Inputs Trace Thickness Substrate Height Trace Width Trace Spacing Substrate Dielectric Outputs. The impedance is based on the cross-sectional geometry of the trace. width|space|width. Single and differential coplanar structures are supported by the Simbeor impedance calculator. Multiply the result by the square root of the ratio of (1 + (2 * h / w)) to (1 – (2 * h / w)). 2. Relative dielectric constant (er) mm. The two most common impedances. For now let us focus on the single ended impedance. beammy. Overview. Asymmetric Stripline Impedance Calculator. 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). Note that Zdiff (differential impedance) is equal to 2 * Zodd, so a Zodd of 45 Ω gives us a Zdiff of 90 Ω. Overview. Using this calculator, you can find the resonant frequency, which means that you can disregard the reactive impedance (reactance) and only pay attention to the resistive impedance (resistance). To use this tool, enter the values for. So, I am trying to figure out how to use Kicad’s PCB Calculator for “Coupled Microstrip Lines”:A Grounded Differential Coplanar Waveguide (GDCPW) is a transmission line structure used in RF and microwave circuits. I found that a W of 0. 25mm between the differential pair with a width of 0. Class creation for controlled impedance routing using Altium Designer. Figure 4. ZO *. ; We can use each of these parameters separately in each equation to find the resonant frequency, the Q-factor, and the damping ratio. In the limit of a very long transmission line (such as when the line length is many multiples of the wavelength), then the tanh function eventually converges to 1. 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. Description. Dielectric Thickness: mm. The differential via design shown in Figure 2 was built and tested. Dual Stripline. The twisted pair helps create a more. Try TNT or TxLine. Or the odd mode impedance is half of the differential impedance. Microstrip. Then enter the values of the impedances as complex. 9 Ω, R₂ = 3,062 Ω, and R₃ = 50. ; G is the conductance component of the admittance. As discussed previously, the lengths of the two lines in the pair must be the same length. 3 LVDS Traces • As shown in Figure 1, traces should be 100-Ω(±5%) differential impedance of differential microstrip or differential stripline. Learn how to use a differential impedance calculator to design your PCB for high-speed digital data transfer. Typical values for most common differential signal types are 90 ohms differential, 100 ohms differential or 120 ohms differential. Saturn to Allegro differ more than 10%, which would result in huge trace width and spacing. Hi, I am working on my board Stack-up to tune Single/Differential impedances of my board in Allegro PCB 16. Using a power plane as signal reference. [m] width of the trace. Edge Coupled Stripline Impedance Calculator. The High speed protocol that is being designed for determines what the single and differential trace Impedance the traces need to meet as well as the tolerance for the impedance (e. differential amplifier is accomplished in four steps. One of the features of this type of microstrip is the coupling between lines. To see how this works, consider the diagram below. RF Transformer Calculator. Differential Pair Impedance. 254mm. *The structure may propagate in 3 modes: coplanar waveguide, coupled slot lines and microstrip. [2] Calculate the top layer signal end impedance: Two tools are giving Zo=48 and Zo=47. Use an online differential microstrip impedance calculator; Use an online differential stripline impedance calculator; Calculating Differential vs. Matching the differential trace length and keeping the distances properly, this calculator is used to calculate the characteristic impedance of an edge coupled microstrip. They will give very similar answers. 5mil clearance on a 63 mil board (1. 6 ohms and Z even = 110 ohms for your geometry, very close to your result. , inverting, from the drop-down menu. Eurocircuits’ Impedance calculator uses real values as defined in the supplier’s data sheets. For instance, I want a 100 Ω controlled impedance pair, and have used "differential" before which seemed to work fine. The closer you place the pairs together, the differential impedance gets smaller because LM and CM get larger. W W is the PCB trace width, in thousands of an inch (mil); A. IPC 2141 Trace Impedance Calculator. Their hardware design guideline said the traces between the USB and the termination resistors should have a differential impedance of 90 ohms. Asymmetric Stripline. The typical common impedance of an unshielded twisted pair will vary from around 100 Ω to 200 Ω. 6 ohms and Z even = 110 ohms for your geometry, very close to your result. SEDZ = R. 0 differential insertion loss test to 43. B = separation between reference planes. Facebook. Learn the limitations and advantages of this tool and how to use a field solver for more. The odd mode impedance is the impedance of one transmission line when the pair is driven differentially. ϵ r is the substrate dielectric. If you're trying to get your own pcb design project , you should use this tool. Differential impedance, Z diff, is the impedance between the two conductors. 5 (FR-4) ! even for Conductor/Plane layers. It supports both balanced and unbalanced lines. Some of them include Microstrip, Stripline and Coplanar Waveguide. Thus a "differential pair" any long distance apart will have an impedance of 98 ohms, without any coupling between the lines, i. Track Thickness: mm. Excite the other end of the trace with the function generator. This impedance profile shows how differential impedance can stay flat over frequency while common-mode impedance can vary. The track widths should. then the differential impedance will be less sensitive to over-etching or under-etching during pcb fabrication. f c = 1 / (2π × 3. Differential Impedance The impedance measured between the two lines when they are driven with opposite polarity signals. Zo (diff) is calculated to be 244 ohms, so dividing by two yields a value of 122 ohms for the semi-coplanar structure. Figure 4. dBm to Watt converter Stripline Impedance calculator Microstrip line impedance Antenna G/T Noise temp. Ask the board manufacturer for the values. Suggested Solution: for 100 Ohm Differential, use half the board thickness and make the tracks equal to the board thickness. Differential impedance values are normally specified for impedance matching of differential pairs, rather than odd mode impedance. ’. For budgetting reasons I'm tied to a stock pooling layer stackup, so I went for OSHpark's 4-layer stackup (35um copper - 0. The overall single-ended to differential gain (GAIN) must take into account the input attenuation of the R S and R T resistive divider and the effect of adding R2. You can use all the calculators you want if you don't know the exact board make up they're using; they should be able to help you. PCB samples, finding that predicted accuracy varies according to target impedance. Usually Microstrip separated from the ground by using dielectric substrate. Impedance Control. Eurocircuits also calculates the correct Differential Impedance values using the correct formula and not a ‘Rule of Thumb’ method. The Smith Chart achieves this by superimposing a grid on S11 that maps load impedance. Trace Thickness. Common-mode impedance occurs with the pair driven in parallel from a common-source. Locally hatched ground plane to increase differential impedance. This slope resistance, in parallel with the collector resistor (Rc) dictates what the output resistance of the amplifier is. Figure 81 shows the topology. t. Sierra Circuits’ Via Impedance Calculator uses the physical dimensions of a via to calculate its capacitance, inductance, and impedance. Edit: also, think ahead if you are going to use ground pours around the traces. Step 1 Calculate singled ended impedance for each: Use "impedance + trace space" from top drop down box: Set Z=50R, click single ened, gives a result of track. Microstrip is a single trace on the top layer, with no ground close to it on the same layer. Differential impedance is the ratio of voltage to current on a pair of transmission lines when driven in the differential. 48 e-0. Z0 = (60 / √εr) * ln ( (8h) / (w + s)) Where εr is the dielectric constant of the substrate material, h is the substrate height, w is the trace width, and s is the spacing between traces. Correct track impedance is the first point usually made in various online sources I've found on the subject, so I tried to get the microstrip impedance calculator to get as close the desired 100 Ohm. This calculator is designed to help calculate the value of the resistors R1 and R2 to be used in T-pad attenuator. 5 Ohms higher than the formula method. INPUT DATA. The impedance of each trace of the differential pair references to ground. Pi Attenuator Calculator. R is the resistance in ohms (Ω),. Note: The results are only for approximation and rough estimation, the final values and the. By understanding the microstrip transmission line, designers can. p = (Zi - Zo) / (Zi + Zo) Let us consider the case Zi = Zo. 75 inches without impedance control; Keep the differential pair length mismatch within 0. 1. Power Added Efficiency (PAE) Calculator. Sierra Circuits’ Via Impedance Calculator uses the physical dimensions of a via to calculate its capacitance, inductance, and impedance. Following is the list of useful converters and calculators. Calculating the Differential impedance is a two-step process, first Calculate the single ended impedance Zo and then use this value to calculate the differential impedance. In addition, the equivalent circuit parameters have been extracted by using an E4991B Keysight Impedance analyzer. 4 Differential Line and Coplanar Strip; 3. Agilent's LineCalc tool calculates Z odd = 50. 347 e -2. 5. ; j represents the imaginary unit (√(-1)). In case I reduce the width and the distance of tracks to 0. This tool calculates the matching network necessary to terminate a line of the specified characteristic impedence (Z o) in a specific complex load impedence (R L + jX L) at a specified frequency. For a single-ended differential amplifier, the gain is. The only requirement is the required attenuation in decibels (dB) and the impedance in ohms. 8. Z diff = 2* Z o (1 - 0. The Calculator Calculates the values of the Characteristic Impedance, Capacitance per unit length and propogation speed. Embedded microstrips are commonly crafted using printed circuit boards, although other. 77 mm gave me a differential impedance very close to 90 Ω. ; Both the conductance (G) and susceptance (B) components of admittance are real numbers, and the complex term jB accounts for the phase relationship between them. Loss is calculated as a combination of dielectric loss and conductor loss at the analyzed/synthesized length. 8. The desired behavior of the differential amplifier is to amplify the differential mode voltage and attenuate the common mode voltage. g. JLCPCB Impedance Calculator. 6mm 4 layer PCB, 1oz outer copper, 0. For example, a single 0. This calculator assumes. 0 specs requires a differential impedance of 90 Ohms and a common mode impedance of 30 Ohms (with some tolerance). Please provide a rating, it takes seconds and helps us to keep this resource free for all to use. Tpd: propagation delay. Classes for 50 and 100-ohm traces. To achieve this, you may have to put small sections of trace tuning into the shorter line to equalize them. g. Now I'd like to get into this topic and the first task would be to create a ~ 10cm board to board interconnect flex PCB with 90 ohm (USB) and 100 ohm (LVDS) controlled impedance traces on it. 25 Dimensional units: mm milsloop of the amplifier, which tends to drive the impedance at the output terminals, VO– and VO+, to zero, depending on the loop gain. Z0 = (L/C)^0. But, you usually want some decent coupling between the traces, so a good single-ended impedance target for each trace is between. 2. The vias for these high-speed traces will be backdrilled to be a total length of 23 mil, keeping the via stubs no longer than roughly 1/20th of a wavelength. In fact, striplines often make components such as band-pass filter's more accurately, with more symmetric roll-off and no lower cutoff frequency. *Strictly, a loss quantity when expressed in dB should be positive. 75 mm track on a 0. mils: Distance between planes (B) mils: Relative Dielectric constant (Er) Differential Trace Impedance: ohms: References. 1 mil or so instead of 0. Print - Advertisement - This calculator helps you compute the characteristic impedance of an asymmetric stripline Inputs. Using the Impedance Calculator one can calculate the eigen modes as characteristic line impedance and the characteristic propagation constant gamma of available signals in a given cable segment. Differential Stripline Impedance Calculator. The Si8000m adds enhanced modelling to predict the finished impedance of multiple dielectric PCB builds and also takes into account the local variations in dielectric constant on close spaced differential structures. These equations are. Finally, all calculators provide theoretical impedance values as they cannot take in to account the variables. 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. The Impedance Calculator is a tool which allows a quick pre-layout estimation of several kinds of line impedances between different layers. Use several of them and compare. characteristic impedance, signal speed, crosstalk and differential impedance. 3 Answers. Modeling approximation can be used to understand the impedance of. Disc Air Capacitor Calculator; Impedance Calculator : XL, XC; Inductors, Transformers. This is an iterative process that repeatedly adjusts the via dimensions and runs EM simulations until achieving the desired impedance and bandwidth. Development Kits. with the following layout parameters (width|space|width): Ohm. 5 Summary; Microstrip is the most popular medium for circuit design at frequencies ranging from several hundred megahertz to tens of gigahertz. Use a Numerical Field Solver for Impedance and Return Path Calculations. While this calculator will provide a baseline, any final design considerations should be made towards loss, dispersion, copper roughness, phase shift, etc. There are a number of pcb transmission line typologies. For example, a single 0. Enter the relative permitivity, width of the trace, the ground plane spacing, and the substrate thickness to calculate the characteristic impedance. That means each trace would be a 50-ohm trace, if there were no coupling between the traces. USB 2 e. There are four types of impedance used in. We get a lot of questions about trace impedance and how to calculate the right trace size to hit a specific impedance in a manufacturable PCB. Just as important. The Twisted Pair Impedance Calculator is able to compute the characteristic impedance Z 0(twisted pair) based on the dimensions of the twisted-pair cable. How to Use the Calculator. 75 mA corresponds to a CE range of 1V to 8V. e. The rules shown below are a rough approximation, and you should always check the approximation using an impedance calculator or field solver. The efficiency of transmission of the embedded microstrip greatly depends on its characteristic impedance. The desired behavior of the differential amplifier is to amplify the differential mode voltage and attenuate the common mode voltage. Eurocircuits also calculates the correct Differential Impedance values using the correct formula and not a ‘Rule of Thumb’ method. Advertise with us;. may be positive (inductive) orFor instance, when IB I B = 40 uA, I estimate that a collector current range of 8 mA to 8. The characteristic impedance is generally defined to. differential trace impedance for USB (90 Ohms) on 2-layer FR4 board. φ = 0° if 1/2πfC = 2πfL and R = 0. H 1 is the subtrate h1. In the case where you are routing differential pairs, then the potential between the two pairs will always be opposing (i. 5. High-quality circuit boards as prototype and series at reasonable prices. 8 Ω. When driven in differential mode by two identical amplifiers, this impedance change will be the same for both lines. The edge couple differential microstrip transmission line is a common technique for routing differential traces. which is the characteristic impedance of the system. An example of this calculation using the Si8000m Quick Solver is shown below in figure 4. GDCPW is known for its balanced configuration, which helps reduce common-mode noise and interference in high-frequency signal. The stripline impedance calculator provided below is useful for gaining an initial estimate of trace impedance for striplines. Single and differential coplanar structures; Multiple adjacent dielectric layers, with different dielectric properties. We’ll go over how differential pairs can hel. In the ZS box, type a value of 75 Ω. Design Parameters. 1. determining the value of the differential impedance particularly for surfacemicrostrip differential tracks. 5 and I took 3. Differential signaling in flex circuits follows the design method of surface microstrip transmission lines. The bottom line is: check with your fabricator when planning a 4-layer PCB stackup with 50 Ohm impedance. 1 of the Microstrip Calculator using H2 in place of h, εr,eff can be computed as specified in Eq. 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. 5. Select "Unequal impedances" in the "Circuit type" box. The calculator will process the values and display the. An embedded microstrip is a microstrip transmission line or flat conductor separated from the ground place via a dielectric material. Remember that you can't. Still, as you correctly analyzed it is much better to think of the individual lines in a differential pair as two length-matched single-ended lines, both with the half impedance. For microstrip traces, a solid ground plane is needed under the signal traces. Z0 = (60 / √εr) * ln ( (8h) / (w + s)) Where εr is the dielectric constant of the substrate material, h is the substrate height, w is the trace width, and s is the spacing between traces. eu.