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 ############################################################## TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c ############################################################## To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body: To subscribe: SUBSCRIBE TECHNET <your full name> To unsubscribe: SIGNOFF TECHNET ############################################################## Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional information. For the technical support contact Dmitriy Sklyar at [log in to unmask] or 847-509-9700 ext.311 ##############################################################