The characteristic impedance is given by: Eq. Impedance. 3 shall be used. Most of the dedicated TL tools doesn't have this option, only if you use a full EM simulator. The intermediary nodes that attach to the main network bus should be short in order NOT to disrupt the end-to-end characteristic impedance of the main network bus cable. Separation between Planes. PCB Impedance-Calculation: Help with the dimensioning of the impedances of your circuit board. ’. , inverting, from the drop-down menu. *Strictly, a loss quantity when expressed in dB should be positive. This sets the single-ended impedance AND differential impedance to the target value, even if there is no. There are a lot of online trace impedance calculators specifically designed for differential traces. The differential impedance will be a bit less than 2x the single-ended impedance. , equal in magnitude but opposite in polarity). This calculator is designed to calculate the characteristic impedance of an edge coupled microstrip. 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. Usually Microstrip separated from the ground by using dielectric substrate. Differential and common mode impedance measurement Figure 8 shows the variation of impedance versus frequency. Choose a calculator to tune your PCB design. Note: Units do not matter for this calculation as long as they are consistent. To find the impedance of a capacitor (Zc) and an inductor (Zl) in an AC circuit, you use the following formulas: For the capacitor: Zc = 1 / (2πfC) For the inductor: Zl = 2πfL Where f is the frequency, C is the capacitance, and L is the inductance. 25mm between the differential pair with a width of 0. Ask the board manufacturer for the values. This will also provide the time delay the cable provides for a signal and also the capacitance and inductance per. Reaction score. Differential microstrip impedance calculator is an online electronic tool to measure the opposition to alternating current. Learn the limitations and advantages of this tool and how to use a field solver for more accurate results. The tool implements numerical solutions of Maxwell’s equations to render accurate and consistent results. 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. Both LM and CM converge to zero as spacing goes to infinity. Impedance is the resistance that a transmission line presents to the flow of energy along the transmission line. Schelkunoff in 1934, is a very convenient parameter for the prediction and control of EMI coupling through cable shields. You can make your own spread sheet using formulas provided by Wedell's book "Transmission Line Design Handbook" page 196, 197, 198 (attached)Impedance Calculators. T. Figure 25. First, calculate the amplifier input impedance R. The differential impedance is measured across the two lines with the pair driven differentially. 6 inches - this is very easy to do; If you can access a differential impedance calculator, then use. 3 V, etc. Hello! I'm trying to calculate the trace width for a differential pair (carrying MIPI CSI-2 signals), using JLC's impedance calculator. Units do not matter as long as they are consistent. 864. For example, if you wish to input "25000000", just type "25M" instead. s + JX s (1) The “+“ implies “in series”. H. The impedance of each trace of the differential pair references to ground. The characteristic impedance of a cable and its connec-tors should match the impedance of the transmitter and receiver. The equipment can be an impedance meter such as the E4991A from Agilent technology, working up to 3 GHz. 4 Comments. Calculate Coplanar impedance for given PCB parameters. Near 50 ohms, many of these approximations are within a few percent accurate. Activity points. It has been correlated many times. For differential pairs of microstrips, the formulas involved are much more complicated because coupling between the traces in the differential pair must be included in the impedance calculation. The advantage a 2D field solver wields over an approximation is the flexibility to consider almost any arbitrary cross-section geometry. Coplanar Waveguide Calculator. 75 inches without impedance control; Keep the differential pair length mismatch within 0. PCB Impedance and Capacitance Calculator: Asymmetric Stripline. ZO *. 1. Multiple Differential Pairs Insert multiple Differential Pair definitions per layer. Or the odd mode impedance is half of the differential impedance. The USB 2. Figure 4. Since it is always important that the trace impedance remain constant over the entire length, this means that the coupling must remain constant over the entire length. The trace width (0. The impedance of the surface microstrip shown is 50 ohms, so the impedance of the broadside coupled differential pair will be twice 50. 3) to calculate trace width and. Sierra Circuits’ Via Impedance Calculator uses the physical dimensions of a via to calculate its capacitance, inductance, and impedance. View All Tools The edge coupled differential microstrip transmission line is a common technique for routing differential traces. Speedstack Si – Insertion loss & impedance PCB stackup design including Si9000e. To maximize signal integrity, proper routing techniques for differential signals are important for high-speed designs. For coupled surface microstrip, Wadell[1] gives 7 pages of equations to evaluate the impedance. 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. Differential Impedance Calculator - Stripline. We recommend the software from Polar Instruments if you want to get exact impedance calculation. Impedance Calculation 5. Instead, field solvers normally focus on impedance and calculating the current distribution around the trace. ; j represents the imaginary unit (√(-1)). The following formulas are used for the calculation: where . Even mode impedance is less clear to me. The minimum width for a stripline that is encased by metal on the edges is 5 times the line width, in order for the impedance to calculate with the "normal" closed form equations. Excite the other end of the trace with the function generator. on layer 3 from the top, between power and ground plane. For microstrip traces, a solid ground plane is needed under the signal traces. This is an iterative process that repeatedly adjusts the via dimensions and runs EM simulations until achieving the desired impedance and bandwidth. Altium Designer makes it easy to calculate geometry when you need to calculate impedance for differential pairs. 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. Figure 1. Click on Setup -> Constraints > Electrical Constraints Set - > Differential Pair Value - > Calculator >. Check the CAN bus specifications, examine the maximum allowed hysteresis, and verify the signal-swing when loaded with the additional 120 ohm load will still produce a signal swing at least 50% higher than the maximum hysteresis. For this calculation, the units of d and h can be ignored as long as they have the same unit. The IPC-2141 trace Impedance calculator will help make initial design easier by allowing the user to input basic parameters and get a. The impedance is based on the cross-sectional geometry of the trace. Z = V I = Z0= 1 vC V 1 vC LV = vC Features of the impedance: looks like a resistor dependant on intrinsic properties only is an intrinsic property independent of length. Now as the driven signal propagates down the transmission line an impedance mismatch is still encountered at the far end of the cable (receiver inputs). Then calculate R T for impedance matching and the value of the R2 for differential balance. Example. Impedance examples 4 layers 1. mm mils. Pasternack PCB Microstrip Calculator is an online calculation tool that lets you compute a microstrip's height/width ratio, impedance and relative dielectric constant for a microstrip transmission line. Saturn to Allegro differ more than 10%, which would result in huge trace width and spacing. For this calculation, the units of d and h can be ignored as long as they have the same unit. Here are some tips for measuring differential impedance. Single ended. I used some online calculator to calculate parameters of trace so it has 50 Ohm impedance. The INA134 has 25kohm resistors for the network and the datasheet specifies 50kohm for both common-mode and differential input resistances. For an embedded (internal) microstrip, the formulas provided in paragraph 4. Using a differential pair to avoid common mode noise is common in audio circuits. Z diff = 2* Z o (1 - 0. Parallel Wire Impedance (Zo) = Ohms. L is the inductance in henries (H),. Enter the values of trace width (w), trace thickness (t), trace dielectric thickness (h), overall dielectric thickness (h1), and the dielectric constant (er). FWIW, the calculator I've used in the past is called Polar SI8000 which apparently has been superceded by Speedstack PCB. where Z 0,surf can be computed as specified in Eq. 6mm spacing with a trace width of 0. A PCB impedance calculator uses field solvers to accurately approximate impedance values. The impedance calculator in the Allegro stack-up editor uses a static 2D field solver by default (bem2d). We’ll go over how differential pairs can hel. Is this a good practice for Ethernet? This is a capture of the PCB right now: This is the schematic that I am using. Tech Consultant Zach Peterson is talking Signal Integrity again today with a deep dive into Differential Impedance. In addition, the termination resistor value used should take into account the input impedance of the amplifier circuit. Such a circuit will deliver an exact cutoff frequency of. Differential signalling is a method for electrically transmitting information using two complementary signals. 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. Spacing and width value pairs that will give a differential impedance of 100 Ohms on Dk = 4. Asymmetric Stripline Impedance Calculator. The required inputs are the Dk value for the dielectric constant of the PCB substrate, and the. Both gives me only 20 Ohm differential impedance. Some field solvers can also calculate the current distributions inside conductors. This will leave the rest of the signals which are rather non-critical like I2C, power good signals, etc. The iCD Stackup Planner offers unprecedented simulation speed, ease of use and industry leading impedance accuracy at an affordable. 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. Jul 8, 2014 at 21:15. For example, the circuit in Figure 6. An example of this calculation using the Si8000m Quick Solver is shown below in figure 4. SAR Calculator. D = Separation between the differential traces. this type of termination. For example, if you wish to input. Simons, "Coplanar Waveguide Circuits, Components, and Systems", Wiley. As a compromise, at least for surface microstrip configurations, the value of the dielectric constant can be increased by approximately 14% in differential impedance calculators[2] to average the effect of the layers. If I calculate it for embedded edge coupled differential pair, it is still only 39. g. However, convention and some literature quote return loss as a negative value akin to an S (1,1) measurement on a Network. I agree. The closer you place the pairs together, the differential impedance gets smaller because LM and CM get larger. W represents trace width. This calculator finds both odd and even transmission line impedance. By inputting the known values and leaving one variable blank, you can quickly calculate the missing parameter. Modeling approximation can be used to understand the impedance of the edge couple differential stripline transmission line. This calculator finds both odd and even transmission line impedance. 2 Loss of CPW lines; 3. An FDA with equal-value resistors in each output to provide differential output termination is shown in Figure 1. 01) and dielectric constant (3. f is the frequency in hertz (Hz), . 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. I tried to use the tool “PCB calculator” to determine the size of mi USB tracks. GDCPW is known for its balanced configuration, which helps reduce common-mode noise and interference in high-frequency signal. When considering differential traces, add the space between traces (s) to this list. Differential impedance: 90 ohms (tolerance: +/- 15%) On a typical 1. This is 1. Press the Analyze button to calculate the electrical parameters. Suppose we have a system with the following parameters: R= 30 Ω;; L = 10 mH; and; C = 100 μF. Z 0 = ( Z 0o * Z 0e )^0. 1. Wavelength calculator ; Calculates the wavelength of a signal using Ereff. Fig. Where: Y is the electrical admittance. Reflection Coefficient Calculator. Differential impedance is defined as impedance between the two lines when the line pair is driven differentially. Acknowledgements: The source for formulas used in this calculator (except where otherwise noted) is the Design Guide for Electronic Packaging Utilizing High-Speed Techniques (4th Working Draft, IPC-2251, February 2001. However, due to manufacturing techniques and tolerances, the two tracks may have different cross-sections — the unbalanced track case. Board Setup. The IPC-2141 trace Impedance calculator will help make initial design easier by allowing the user to input basic parameters and get a calculated impedance according to the IPC-2141 standard. For instance, I want a 100 Ω controlled impedance pair, and have used "differential" before which seemed to work fine. Embedded Microstrip Impedance Calculator. Supports Anritsu ShockLine™ MS46524B and VectorStar™ MS4647B. 1,393. The differential impedance of MIPI tracks should be 100 Ohms, not 90 (USB requires 90). 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. To find what a given trace width will have a single ended or differential impedance, we can use a built in calculator from Allegro. KiCad 5. There are many calculators available online, as well as built into your PCB design software. 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. At higher frequencies, the output impedance of the output amplifier can change, resulting in a small imbalance. ; Z 0 is the characteristic impedance of free space (approximately 377 ohms). 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. 75 mm track on a 0. Enter one of the three values and the calculator will output the other two. with the following layout parameters (width|space|width): Ohm. The greater the mismatch. Differential impedance, Z diff, is the impedance between the two conductors. Differential Impedance Calculator - Microstrip The differential impedance between the positive and negative trace of the transmission line is slightly less than twice the impedance of the single ended impedance. 3 Answers. Cohn, Seymour B. 75 mA corresponds to a CE range of 1V to 8V. How to verify performance. Altium Designer’s differential line impedance calculator will set up your impedance-controlled differential pair routing widths for you. ’. If a. This calculator finds both odd and even transmission line impedance. Signal layer. As seen from the formula when D decreases, while keeping B. 3. An example of a wire microstrip might most often be found in prototypes or reworked boards where a wire is used over the top of pcb or copper clad material. Z 0 Differential impedance (Ω): Calculate. Using an online differential impedance calculator, the height of the dielectric layer, h, is needed. The impedance of a differential via pair cannot be treated with a single via impedance calculator unless the vias are spaced far apart. JX. Signal 1. An example of this calculation using the Si8000m Quick Solver is shown below in figure 4. Telegram. Figure 3. 100 ohm differential pairs that are tightly coupled have a typical single ended impedance of around 65 ohms. Our impedance calculator features two main types of impedance models, single-ended and differential, within each trace structure. It consists of a pair of center conductors separated by ground planes on the same substrate. Parallel Wire Impedance, Inductance, Capacitance and Propagation Delay Calculator Results. where g m can be calculated as follows:The differential impedance depends upon the D/H ratio. 8. ; H. In stripline specifically, the trace width (w), the trace thickness (t), the distance between ground plates (b), and the relative permittivity (better known as DK). The calculator is available in the Electrical Domain in Constraint Manager, from theI'm designing a custom PCB around the Raspberry Pi RP2040 MCU. Propagation Delay (T pd) = ps/ cm. If I think correctly. This is the same reason we have to consider spacing between traces in differential pairs: the two conductors interact with each other to determine the odd-mode (and differential) impedance . Provide values for the four parameters H 1, H, T, W, and the relative permittivity of the dielectric. I used the KiCAD PCB calculator, asked the PCB manufacturer about the tangent loss (0. We can calculate the differential voltage gain as follows. The IPC-2141 trace Impedance calculator will help make initial design easier by allowing the user to input basic parameters and get a calculated impedance according to the IPC-2141 standard. Thus, the TDR method is use-ful for measuring Z 0 and changes in Z 0 of a transmission line. Select the dimension unit of the stripline: mm mils. The designer then has the option of selecting a seed. Atlas Si for VNA – Insertion loss test system – Delta-L 4. Let’s start new project together!. Two conductors can create a transmission line. Also does diff. For budgetting reasons I'm tied to a stock pooling layer stackup, so I went for OSHpark's 4-layer stackup (35um copper - 0. 9 Ohms). Actually the "odd impedance" is the correct impedance to look at for the USB requirement of 90 Ohms differential impedance. 5x would be best, but 2x is acceptable. A typical 4 Layer Stackup looks as follows. The required inputs are the Dk value for the dielectric constant of the PCB substrate, and the. Reference: R. Following is the list of useful converters and calculators. The equations quoted below are from Reference 1, and are offered here as a starting point for a design, subject to further analysis, testing and design verification. Most often, the only specified requirement of a differential pair is its differential impedance. The op-amp calculator will display the voltage gain of the op-amp, e. 1 to 2. A variant of coplanar waveguide is formed when a ground plane is provided on the opposite side of the dielectric, which is called finite ground-plane coplanar waveguide (FGCPW), or more simply, grounded coplanar waveguide (GCPW). The PCB Impedance Calculator in Altium Designer. 50. Set the attenuation to 40 dB in the second box of the calculator. This calculator assumes. This is to avoid unwanted data signal reflections. 6mm board, 100R microstrip differential impedance, they suggest 4/5 mil trace/space, which is a bit different than 9/4. 1 of the Microstrip Calculator using H2 in place of h, εr,eff can be computed as specified in Eq. Go to ‘ Design’ >> ‘Classes’ >> ‘Net classes. Edit: also, think ahead if you are going to use ground pours around the traces. Quartz Crystal Parameter Calculator. Ohm. The drawing below is a 3D electromagnetic model of stripline with perfect electrical conductors encasing all four sides along the z and y axes (created using Ansoft's. To calculate the external resistor values, start by calculating Z IN. Differential. 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. Series Equivalent Differential Impedance (SEDZ) Model Details The SEDZ format schematic is represented by Figure 55. D = Distance between two differential pair signals; W = Width of a trace in a. Even mode impedance for commonly-driven transmission lines . 1. , the structure is made up of two dielectric materials: air and the substrate material). Probe with the oscilloscope where the function generator feeds into the PCB. Newsletter. Yet I got a ZDiff CPW with ground capable calculator: Differential surface coplanar waveguide with ground calculations: *0. Grounded Differential Coplanar Wave Guide. 1 Answer. Just as important. Also, intuitively, from an examination of the field pattern, the even mode impedance Z 0e is likely to be high because the. If the return path of a single wire is freespace then the impedance is quite high. In case I reduce the width and the distance of tracks to 0. f is the frequency in hertz (Hz), . 378t} W = 1. Routing Diff Pairs with a Differential Line Impedance Calculator for Optimal Signal Transmission Twisted pair: \qquad \footnotesize \text {Impedance} = \frac {120 \times \ln (2s/d)} {\sqrt {\varepsilon_r}} Impedance = εr120 × ln(2s/d) As you can see, it is not overly complicated to calculate by hand, but it will definitely take longer than using our calculator. The guides says spacing under 0. The conductor and ground planes are separated with a dielectric. 50 Ohms. Depending on the configuration of the components you can construct a low pass or high pass filter. The characteristic impedance is generally defined to. Power. 1. 1. RF Amplifier PAE Calculator. Following is the formula for differential impedance for a Stripline. ; 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. The ground plane helps keep the EMI localized and the trace impedance continuous. Choose from different surface finishes,. Also note that your best bet may actually be VERY SHORT (few mm) traces, a thinner board or a 4 layer board :-) Thanks for your suggestions. If your structure has two microstrip feed lines, you can extract even and odd modes by creating weak coupling between main resonator and feed lines. The PCB internal/external trace resistance shall be calculated according to the following formula: R = (ρ * L / (T * W)) * (1 + α * (TAMB – 25 °C)) Where: R is the trace resistance [Ω] ρ is the resistivity parameter, whose value for copper is 1. The odd mode impedance is measured by calculating impedance across one line while a complementary signal drives the other line. Quarter Wave Transformer Impedance Calculator. The impedance value you calculate is the transmission line impedance the signal sees as it reflects off the mismatched load and travels on the line. It seems I could largely do without any impedance matching considerations, given what the trace lengths are. There are four types of impedance used in. Differential impedance changes with coupling, which changes with trace separation. Select the box with the geometry that you would like to calculate. I use FR-4, so H=1. Z represents the Impedance of the stripline. *The structure may propagate in 3 modes: coplanar waveguide, coupled slot lines and microstrip. Inputs Trace Thickness Substrate Height Trace Width Trace Spacing Substrate Dielectric Outputs. 75 mm track on a 0. IMPEDANCE CALCULATIONS • Hyperlynx Linsym- Does whole cross section at once, allows mixed materials. Differential Trace Routing. Overview. Differential Microstrip Impedance From Zo. Dividing the differential impedance. Doing this results in a gain change. 8. Even mode impedance is defined as impedance of a single transmission line when the two lines in a pair are driven with a common mode signal (the same amplitude and the same polarity). Can calculate skin effect. Then enter the values of the impedances as complex. Common-mode impedance occurs with the pair driven in parallel from a common-source. W W is the PCB trace width, in thousands of an inch (mil); A. There are four different types of impedance used in characterizing differential trace impedances. If there is an insulator around the wire then this. USB 2 e. Re: Calculating Differential Impedance for USB Differential Pair. The calculator shown below uses Wadell’s equations for differential impedance, which can be found in the seminal textbook Transmission Line Design Handbook. Coplanar Wave Guides 5. Modeling approximation can be used to understand the impedance of. Grounded Differential Coplanar Wave Guide. It is important todetermine the characteristic impedance of a twisted-pair cable because this impedance should match the impedance. • Polar Instruments Si6000c- Calculates single transmission line at a time. 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. There is also a dielectric material placed above the conductor. (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. A A is the cross section area, in mils 2; and. Single-Ended Impedance. Loss is calculated as a combination of dielectric loss and conductor loss at the analyzed/synthesized length. Enter the values of input resistance (1 kΩ) and feedback resistance (10 kΩ). Eurocircuits’ PCB DEFINED IMPEDANCE pool is a fast turnaround solution for PCB’s with a specific impedance requirements for certain tracks. For example, if you wish to input "25000000", just type "25M" instead. Z diff = 2* Z o (1 - 0. The trace impedance (single-ended) in an individual trace is normally calculated by ignoring any neighboring traces, so no coupling to other conductors in the design will be. But for extreme geometries and for many differential pair structures, these approximations can. 4mils/35um of copper thickness. 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. On any given layer, the impedance of any one pair will be very close to the impedance of any other pair, so this dummy pair is representative of other pairs on that layer. Capacitance (C) = pF/ cm. Learn the limitations and advantages of this tool and how to use a field solver for more. 1 Effective Permittivity and Characteristic Impedance of CPW. This assumes ~0 trace thickness. 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). Note that Zdiff (differential impedance) is equal to 2 * Zodd, so a Zodd of 45 Ω gives us a Zdiff of 90 Ω. , 10. Moreover, this tool can also compute the signal propagation delay in inches per nanosecond (ns/in), the inductance per unit length in nano-Henrys per inch (nH/in), and. We care about maintaining the same differential impedance for the same reason we. 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. For now let us focus on the single ended impedance. Si TRC – Track Resistance Calculator calculates DC. Use several of them and compare. You could assist/improve the noise immunity by heavily loading the. The stripline impedance calculator provided below is useful for gaining an initial estimate of trace impedance for striplines. 12 equal to a constant voltage. A differential cable is usually constructed with a UTP, STP, STQ or twin-axial configuration and achieves 100-Ω differential impedance. A 3-D EM simulator is used to design a differential via with the desired impedance and bandwidth. 7 Keysight ADS schematic used to convert from 4-port single-ended to 2-port mixed-mode S-parameters using equations or 4-port Balun components. Differential impedance, Z diff, is the impedance between the two conductors. Figure 4: Common and differential impedance of ACMS-Q3225E-201-Tcalculate the pcb impedabce and capacitance of asymmetric stripline. Another example of using large distances to separate out unwanted components might be to calculate the impedance of a. **Note: Like our microstrip line impedance calculator, all of our RF calculators allow SI prefix input. Enter the values of Z0 and Zc into their respective input fields. Most often, the only specified requirement of a differential pair is its differential impedance. 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. ; Er is the relative permittivity (dielectric constant) of the substrate material. 6. Impedance captures the real. **Note: Like our stripline impedance calculator, all of our RF. I have already some experience on single ended controlled impedance but this is my first. 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. This calculator helps calculate the values of the output voltage and the inverting and non-inverting gains of an operational amplifier. It uses the lan9512 reference schematic. 0 specs requires a differential impedance of 90 Ohms and a common mode impedance of 30 Ohms (with some tolerance). Use the same trace widths throughout the length of the trace. Keep the spacing between the pair consistent. g. The differential impedance depends upon the D/H ratio. The characteristic impedance is given by: Eq. 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. In the special situation for which there is no coupling to ground, Differential impedance equals the Characteristic. 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. For example, if u = 1 and ε r = 10, then from the figure, Z 0 ε e = 126 Ω. 1.