Before I asked a question about the film capacitor, as shown in the figure below, what does the stripe on one end of the capacitor mean?
These are all non-polar capacitors, so this stripe is not a polarity marker. One reader has to answer correctly, which represents the pole that is wound around the outer layer when the capacitor is wound. I have found that few engineers now know what the stripes on one end of the capacitor represent, and the different effects of stripe and stripe ends. Even if you never use these types of capacitors, knowing this will make your PCB look different. Let us discuss this topic this time.
The conductor of the outer layer of the film capacitor shields the inner conductor. In a simple low-pass RC circuit, as shown in Figure 1a, the stripe side of the capacitor is grounded to shield electromagnetic coupling and electromagnetic interference.
For the Qualcomm RC circuit, as shown in Figure 1b, there is no ground at both ends of the capacitor. But overall, the front-end drive has a low-impedance characteristic, which is not susceptible to induced noise. Therefore, the striped end should be connected to the low impedance side.
Now let's take a look at the integration circuit. As shown in Figure 2a, the integrating capacitance of the integrating circuit is driven by a low-impedance op amp, which is not susceptible to external interference. In this circuit, the inverting input is clearly a sensitive node, so the stripped end should be connected to the output side of the op amp.
The layout of the circuit shown in Figure 2b needs to be considered more. The result is different when the connection order of C1 and R1 is different. The same is true for R2 and C2. In theory, the connections in different orders will not be different, and the results in the SPICE simulation will be the same, but the small volumes R1 and R2 can be placed close to the inverting input pins. This reduces the parasitic capacitance of the area where the antenna effect occurs and the sensitive area (this parasitic capacitance affects system stability). A large-volume film capacitor C2 is connected across the output of the op amp and the inverting input, and the stripe end is connected to the output of the op amp.
First, the layout needs to consider analog circuits that are sensitive to interference, some of which are potential. Second, careful layout and adjustment of the device's ports may improve circuit performance. Adjusting the device's ports here is not just a matter of adjusting the capacitive stripe port and the stripeless port. There may be other large devices in your system that will absorb noise and radiated noise. When you realize this, you can have the guidance to adjust and improve your PCB layout.
A striped capacitor is a hint, and it suggests that we still have a lot of knowledge about board grounding, signal loops, device selection and layout. Specific measures to help us optimize performance are available in many data sheets.
CCTV Baluns,Video Balun Transceiver Transmitter,Passive Video Balun,HD CCTV Video Balun
Chinasky Electronics Co., Ltd. , https://www.cctv-products.com