Issue link: https://iconnect007.uberflip.com/i/1256432
42 DESIGN007 MAGAZINE I JUNE 2020 a noisy unfiltered rail, we can watch on an os- cilloscope the output of the filter producing a sine-wave looking ripple. Also, when we look at the impedance pre- sented by the filter to the load (Figure 4), it has a significant peak at the same frequency where the transfer function had its peak. Whether this impedance peak poses a risk and potentially might create a problem heavily depends on the nature of the load. If the load is "quiet" in the frequency range of the peak, the peak in the filter's output impedance creates no problem. For instance, if the load is an oscil- lator with no digital logic (say a synthesizer) in the same package, this impedance peak would create no issue. But the peaking of the transfer function is still a risk of getting noise on the os- cillator supply pin, which will create a higher- than-expected jitter on the output. If we conclude that the peaking in the trans- fer function and/or in the output impedance is a risk we want to eliminate, the solution is to add sufficient damping to the circuit. We can do it in multiple ways. We can simply add a larger capacitor with higher ESR on the out- put, like an electrolytic or tantalum capacitor, or we can add a discrete resistor in series to the 22uF ceramic capacitor. We can also try the add a parallel resistor across the inductive element. Finally, for filters that need to handle only very little DC current, we can add a se- ries resistor. If we want to totally eliminate the peak- ing in this particular filter without modify- ing the series path, we need to increase the total capacitance, so adding a separate lossy capacitor would be the simple way to go. Al- though in LTSpice, ESR and ESL of a capaci- tor can be added as attributes of the capaci- tor symbol, and for the sake of clarity, those elements are shown in the following figures as separate components. Figure 5 shows the SPICE deck and the result for the transfer func- tion when we add a 220 µF tantalum capacitor with 0.1 Ohm ESR. On the left, the schematics in LTSPICE has a voltage source connected to the input. With flat unity source voltage, the output gives us directly the transfer function. The plot on the right shows the magnitude of the transfer function with a solid line and the phase with a dashed line. Figure 6 shows the SPICE deck and result for the output impedance. To simulate the imped- Figure 4: Impedance of the filter looking back into its output. Figure 5: Voltage transfer function with a lossy capacitor added.