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OPA858: Feedback capacitor

Part Number: OPA858

Hi,

My customer consists of feedback capacitors in series to reduce capacitance as below.

Is this constitution allowed?

Best Regards,

Kuramochi

  • Morning Kuramochi, 

    Certainly that is a good way to get lower feedback C

    1. Be careful about parasitic C  to an AC ground at the midpoint, 

    2.The feedback R itself will have a parasitic C, typical surface mount R's are about 0.18pF

    3. If you need lower, there are specialized R's for very high frequency needs - look for those used in network analyzers. 

  • Hi Kuramochi,

    from a theoretical point-of-view this is possible, of course. But I wouldn't do that with a decompensated 5.5GHz OPAmp. I would keep the PCB area covered by the feedback components as small as ever possible. With two caps in series you not only increase the stray capacitance but also double the unwanted series inductance of feedback cap.

    You should also keep in mind that the optimum value of this 0.5pF cap will depend on the actual layout. So, the best results might be obtained with a slightly lower or higher capacitance.

    By the way, these 0.5pF caps are available at a very low price:

    https://www.mouser.de/Passive-Components/Capacitors/Ceramic-Capacitors/_/N-5g8m?P=1z0vqiwZ1yxboka&Ns=Pricing|0

    Kai