On my circuit I am protecting a sensitive node with a guard op amp. However, since the op amp has limited bandwidth, at higher frequency there will be feedback through capacitance between the guard and the node.

I modeled this as the following: enter image description here

The input is a simple voltage divider , so the frequency response should be flat. However, the op amp (universal op amp in LTSPICE) has a GBW of 10MHz and there is a slight phase shift which feeds back through the capacitor.

In the real world, if I can work out its open loop gain poles/zeros, can I use this to model the circuit in the frequency domain? I want to determine the overall transfer function at the guarded node. Any pointers would be appreciated.

  • \$\begingroup\$ 10 pF seems mighty high as a guard/node capacitance? What bandwidth are you trying to keep the guard effectively operating at? \$\endgroup\$
    – Andy aka
    Commented Jan 30, 2018 at 17:06
  • \$\begingroup\$ Stray capacitance depends on track edge or surface area/gap ratio to node or track . Inductance depends on length/width ratio. Perhaps you want to equalize stray C and add R to balance RC feedback both on pins (+-) then it will be flat. \$\endgroup\$ Commented Jan 30, 2018 at 17:41

1 Answer 1


Just want to say, I ended up solving the transfer function through modeling the op amp (Aol). Thanks for the suggestions above but my PCB has already been designed so I can't reduce stray capcitance.

enter image description here


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