Which decoupling capacitor to choose for electro optic circuits?

Question

Background:

My application is a typical electro optical design where analog electric pulse from a photo diode is digitalised, Pulse Width=10ns, pulse repetetion rate= 20uS

my anlog pulse to digital conversion is like below, with a gain of 1K through TIA and pulse inversion using limiting amplifier and digital conversion through high speed comparator

I-V conversion > limiting amplifier > High speed comparator

as i am working with a pulse of rise time 2ns, my BW of operation would be <200MHz

Problem Statement

when i have read the previous question and answers i have seen people suggested to go for a 0.1uF,0.01uF ceramic capacitor near to the power supply pins

i am clue less which capacitor to choose exactly for high frequency circuits which are working around 200MHz

to confuse me more i have seen a app note saying this idea of choosing 0.1uF is age old for high frequency circuits

when they say high frequencies do they mean really > 1Ghz ?

do i really have to worry ? or for my 200MHz i can choose the 0.1uF and 0.01uF in parallel proceed ?

Answer 1

You can look at the datasheets of 100 0402-sized ceramic 0.1 uF capacitors, and the impedance vs frequency curve will mostly all look like this:

The resonant frequency will be somewhere in the 20 - 50 MHz range.

If you look at a 0.01 uF capacitor, still 0402 sized, the capacitive part of the curve (the downward sloping part) will be raised up by one decade, but the inductive part of the curve (the upward sloping part) will be damn near the same as for the 0.1 uF part. There will be only a very small frequency range where the 0.01 uF part provides lower impedance than the 0.1 uF part.

You might think you can do better by trying an 0201 or 010005 size capacitor. And the datasheets will show a slightly higher resonant frequency. But remember that these curves neglect the series inductance added by the traces on the board where you mount the part. Realistically that will become the limiting series inductance as you go to smaller parts, you won't get much improvement in the actual resonant frequency of the part mounted in the circuit.

So what can you do?

Use a 0.1 uF 0402 sized capacitor, place one as near to each power supply pin on your IC as you can. And hope the IC designer has provided adequate on-chip bypass capacitance to deal with frequencies above 200 MHz.