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I saw these lines in a data sheet:

"To reduce unwanted capacitance, TI recommends cutting out the power and ground traces underneath the signal input and output pins. Otherwise, ground and power planes must be unbroken elsewhere on the board. When configuring the amplifier as a TIA, if the required feedback capacitor is under 0.15 pF, consider using two series resistors, each of half the value of a single resistor in the feedback loop to minimize the parasitic capacitance from the resistor."

  1. I know the ground plane cut-out is to reduce capacitance, but I don't really understand why. Also, why these ties to ground planes must be unbroken elsewhere?
  2. Why do two resistors have lower capacitance? Is it because capacitance in series has a lower total capacitance?
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  1. Two conductors with isolation in between is a capacitor. Therefore, any conductive structure such as tracks and solder pads over a ground plane with PCB in between is a capacitor, and therefore, removing the ground plane under the component solder pad or traces reduces the area of the capacitor so there is less capacitance. And as high speed return currents flow via least impedance, it flows right under the signal traces, so you need unbroken ground plane so that the return currents go back via intended routes instead some odd far-away route which increases loop area of current flow.

  2. Exactly. If one resistor has 1pF capacitance between terminals, then two resistors in series has only half a pF of capacitance. But on the other hand, the component pads again form capacitors with the ground plane, which is why ground plane cutouts are needed.

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