Max Harris wrote:
>
> Jeff,
>
> I agree with everything you have said, but my original question still
> stands. How do I connect to a node that is being guarded on every
> layer of the PCB without breaking the ring on at least one layer, or
> routing from standoffs, on an external layer, over the top of the
> ring?
>
Max,
Sorry if I wasn't clear enough. The guard ring and/or other
isolation techniques must include the rest of the path leading
up to the op-amp, all the way back to where it becomes high-
impedance. It had best be very short.
> Author: Jeff Seeger <jseeger*remove_to_reply*@appliedcad.com> at smtp_gway
> Date: 11/3/97 4:27 PM
>
> Max Harris wrote:
> >
> > Can anyone out there help me with the subject of using guard rings
> > around op amp inputs to minimize leakage at the high impedance nodes?
> > I will be using an FR-4 PCB, multilayer, PTH technology. From what
> > I've read, it is suggested that the guard rings be placed on every
> > layer.
>
> The nature of your question implies an analog world, where high
> impedance can mean in the meg-ohms - not the impedance range of the
> board itself. With an input _that_ sensitive, it is advisable to
> make full use of the fact that Farraday was right.
>
> For the guard ring to be fully functional, you want sheilding be-
> tween your input and _any_ other possible affector. This may in-
> clude putting a void in any plane layers, and if the op-amp is a
> through-hole part you should carry the guard to all layers.
>
> > This begs the question: "How do I then connect traces to the
> > node being guarded?". Also, any traces that do connect to the guarded
> > node and then run outside of the ring are now subject to the same
> > leakage that the original node was.
> >
> In such an impedance range, the source of your signal had better
> be close enough to not be an issue as far as including it in your
> guard ring. If there is a large value resistor in the circuit as
> it leads up to your input, this is the point where the impedance
> jumps, and is the point where the guard should extend to. If not,
> then it may be such a signal back to the last device, connector,
> or summing junction.
>
> Length in such a signal path is your enemy.
>
> Good luck,
--
Jeff Seeger Applied CAD Knowledge Inc
Chief Technical Officer Tyngsboro, MA 01879
jseeger "at" appliedcad "dot" com 978 649 9800
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