What do the stripes on a film capacitor represent

It represents the outermost terminal of the capacitor during winding. I've noticed that few engineers know what the stripes on one end of a capacitor represent, or the different effects of swapping the striped and unstriped ends. Even if you never use these types of capacitors, understanding this will make a difference in your PCB designs. Let's discuss this topic.

The outer conductor of a film capacitor shields the inner conductor. In a simple low-pass R-C circuit, as shown in Figure 1a, the striped side of the capacitor is grounded, thus shielding against electromagnetic coupling and electromagnetic interference.

For the Qualcomm R-C circuit, as shown in Figure 1b, neither end of the capacitor is grounded. However, overall, the front-end driver exhibits low impedance characteristics, making it less susceptible to induced noise. Therefore, the striped end should be connected to the low-impedance side. Now let's look at the integrator circuit. As shown in Figure 2a, the integrating capacitor in the integrator circuit is driven by a low-impedance op-amp, and this connection is less susceptible to external interference. In this circuit, the inverting input is clearly a sensitive node, so the striped end should be connected to the op-amp's output side.

The circuit layout shown in Figure 2b requires more consideration. The connection order of C1 and R1 will affect the results. The same applies to R2 and C2. Theoretically, different connection orders will not make a difference, and the SPICE simulation results will be the same. However, the smaller R1 and R2 can be placed close to the inverting input pin. This reduces the area generating antenna effects and the parasitic capacitance in the sensitive area (which affects system stability). The larger thin-film capacitor C2 is connected between the op-amp output and the inverting input, with the striped end connected to the op-amp output.

First, layout considerations should focus on interference-sensitive analog circuits, some of which may be potential sources of interference. Second, careful layout and port adjustments can improve circuit performance. Port adjustments here go beyond simply swapping striped and unstriped capacitor ports. Your system may also contain other bulky components that absorb and radiate noise. Once you understand this, you can make informed adjustments and improvements to your PCB layout.

    The striped capacitor is a reminder that we still have much to learn about circuit board grounding, signal loops, component selection, and layout. Many datasheets provide specific measures to help us optimize performance.