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February 2015 • The PCB Design Magazine 13 EFFECTIvE CHARACTERISTIC IMPEDAnCE continues feature ohms. The effective impedance (effective Z0) is explained in the LineSim user guide as a value calculated by the field solver to indicate how much the capacitance of the load IC decreases the impedance the driver sees. 2 As shown later, the effective Z0 calculation is highly dependent on several key parameters. To understand the effective Z0 parameter, the IBIS model of the receiver is investigated. For those who aren't familiar with the IBIS for- mat; IBIS is used to show the buffer's behavior without compromising the manufactures pro- prietary information. An IBIS model describes the transistor pull-up/pull-down curves, the ESD diode clamp curves, the drivers' transition char- acteristics, pin parasitic information, and pack- age model parameters among others. Figure 3 is an excerpt from the receiver's IBIS model show- ing the package parameters for the IC. 4 Figure 4 shows both the pin RLC parameters and the val- ue C_comp under the appropriate buffer model. Capacitive Effects on Effective Characteristic Impedance Most interesting are the package/pin para- sitic capacitance and the parameter C_comp in the receiver model. It is important to note the pin RLC parameters for this model were added for demonstration purposes and do not truly represent the buffer characteristics. In this case, the addition of the package capacitance (0.605pF), the pin capacitance (2pF), and input capacitance (0.5pF) make up the total input load capacitance. According to the wizard, it is this capacitive load that ef- fectively lowers the characteristic impedance that the driver sees down to 46.5 ohms. In this case the driver impedance (52.7 ohms) is already higher than the characteristic imped- ance, so no additional termination is recom- mended. For demonstration purposes, the pin ca- pacitance is changed to something significantly higher (10pF for this example). Running the wizard for this case indicates that the effective Z0 is now 39.4 ohms, as shown in Figure 5. Now the pin capacitance is set to 0pF to ana- lyze C_comp's contribution to the results. After running the wizard on this scenario, the results show the effective Z0 to be 43.9 ohms, indicat- ing that C_comp also has an impact on the im- pedance the driver sees. Figure 4: Pin rlC parameters for pin 40 (rGBA0) and the values for C_comp. The pin rlC data was added to the model for demonstration purposes. C_comp is the input capacitance for the buffer when specified as an input and the output capacitance when specified as an output. Figure 3: receivers rlC package parameters for the IC. IBIs includes the minimum, typical, and maxi- mum values for all parameters.