This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TDP158RSBEVM: Input AC coupling capacitor size considerations

Part Number: TDP158RSBEVM
Other Parts Discussed in Thread: TDP158


The TDP158 evaluation board has size 201 coupling capacitors on the input. However, I have a need to use size 402 capacitors. How might using  size 402 capacitors affect performance of the input video signal? What considerations must be taken into account for selecting the best 402 coupling capacitor? 



  • Cristino

    The impact of DC blocking capacitor comes from its mounting pad on PCB and the impact is on the signal integrity. DC blocking capacitors are common sources of impedance discontinuities in high-speed serial channels. A narrow trace width and narrow trace spacing are used to construct a typical 100-Ω differential transmission line pair. However, as these narrow trace pairs are routed into the surface mount pads of a DC blocking capacitor, the sudden widening of the controlled impedance traces as they join with the capacitor pads cause an abrupt impedance discontinuity. The effect of this discontinuity appears as excess parasitic capacitance because the surface mount pads of the DC blocking capacitors act as a parallel plate with the reference plane underneath. The size of the capacitor is directly proportional to the size of the pad, and in turn, the parasitic capacitance. So bigger the size of the capacitor, the larger the parasitic capacitance.

    There are papers that go into more detail on the impact of DC blocking capacitor size on the signal integrity and method to mitigating the impact such as cutting a void underneath the pad to minimize the parasitic capacitance, and therefore the impedance discontinuity. Please look up these papers online if you are interested.



  • David,

    Thanks for pointing me in the right direction.

    I found this article, among many, for others to reference.