Do you understand the stripes on one end of the capacitor?

Regarding the film capacitor, as shown in the figure below, what do the stripes on one end of the capacitor represent?

Striped capacitor

These are non-polarised capacitors, so this stripe is not a polar mark. It represents the pole wound around the outer layer when the capacitor is wound. Few engineers know what the stripes on one end of the capacitor represent, nor do they know the different effects of the interchange between the striped and non-stripe ends. Even if you never use such capacitors, understanding these contents will make your PCB design different. Let us discuss this topic this time.

The outer conductor of the 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, thereby shielding electromagnetic coupling and electromagnetic interference.

For the high-pass R-C circuit, as shown in Figure 1b, both ends of the capacitor are not grounded. But in general, the front-end drive has a low impedance characteristic, which will not be easily affected by induced noise. Therefore, the striped side of the capacitor should be connected to the low impedance side.

Now let's take a look at the integration circuit. As shown in Figure 2a, the integration capacitor of the integration circuit is driven by a low-impedance op amp, and this connection is not easily affected by external interference. In this circuit, the inverting input is obviously a sensitive node, so the striped side of the capacitor should be connected to the output side of the op amp.

The layout of the circuit shown in Figure 2b requires more consideration. If the connection order of C1 and R1 is different, the result is also different. The same is true for R2 and C2. Theoretically, the connection in different orders will not be different, and the result of the SPICE simulation is the same, but the small size of R1 and R2 can be placed close to the reverse input pin. This can reduce the parasitic capacitance of the antenna effect area and the sensitive area (this parasitic capacitance will affect the system stability). The bulky film capacitor C2 is connected between the output of the op amp and the inverting input, and the striped side of the capacitor is connected to the output of the op amp.

Striped capacitor is a reminder. It reminds us that we still need to know a lot about circuit board grounding, signal return, device selection and layout. Many data sheets provide specific measures to help us optimise performance.