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42 The PCB Design Magazine • September 2016 can be done with small-signal excitation or large-signal excitation. As an example, in Figure 2 we show the output impedance magnitude measured with fixed input and output voltages, with DC load current varied from 0A to 2A. The photo in Figure 3 shows the measurement setup in my basement lab. The setup has the LM20143 evaluation board connected to a small AC-DC adjustable power supply, serving as the input source. On the lower left there is a small home-made elec- tronic load circuit, which can draw an adjust- able constant current. The voltage, proportion- al to the DC current is shown on the handheld digital multimeter. Two cables connect to the vector network analyzer, not shown on this photo. When we look at the data on Figure 2, we have to answer the question: Is this close to what we expected? What would be warning signs that something is wrong with our mea- surement or the evaluation board may not meet our expectations? We can start with items that we know. In case of evaluation boards, we get a schematics and BOM as well, reproduced in Figure 4 [3] . The schematics and BOM show that on the evaluation board the main inductor has 1.2 uH inductance and 17 mOhm DC resistance. There is a single capacitor on the output, a 1210-size 47uF 6.3V X5R ceramic capacitor. With or with- out this knowledge about the component val- ues, we can take the measured output imped- ance and fit to it a simple model. The plot in Figure 5 shows the result of a very quick curve fitting we can do in a spreadsheet in seconds. The measured data is the blue trace, using the data at 1A DC load current from Figure 2. The red line, almost completely behind the blue trace, is the result of a simple three-capacitor model, where the third capacitor is fit around the impedance minimum at 0.5 MHz. The val- ues to get this match come at as C 3 = 38uF, R 3 = 3.5mOhm, L 3 = 3nH. Are these values reason- able? The 38uF capacitance is 80% of the nomi- EVALUATING EVALUATION BOARDS Figure 2: Output impedance as a function of frequency and DC load current. Figure 3: Test setup with input power supply and electronic load. Figure 4a: Schematics of the LM20143 evaluation board used in reference 3.