Optimizing Precision Photodiode Sensor Circuit Design-keep-out around the trace calculation

Thread Starter

hoyyoth

Joined Mar 21, 2020
196
Dear Team,


I am going through this document from Analog devices to learn more about different guarding methods.(https://www.analog.com/en/technical...ecision-photodiode-sensor-circuit-design.html)

In "DC Considerations" paragraph-3 you can see the statement given below.

"One way to minimize this external leakage path is to increase the resistance between the trace carrying the photodiode current and any other traces. This can be as simple as adding a large routing keep-out around the trace to increase the distance to other traces"

My question is how to calculate the required keep-out around the trace.

May I know is there any math or thumb rule to calculate the keep-out around the trace.

Regards
HARi
 

DickCappels

Joined Aug 21, 2008
8,710
This routing may be the removal of PCB material with a routing bit. I have used that to keep leakage current from a high voltage circuit from getting into the lower voltage circuits. There will be very little leakage current through air and your voltage will be too low to worry about arcing so calculating the width of the routing is not really needed. Just use a width that is convenient to manufacture. It will undoubtedly be the width of an available routing bit unless you will be into very high volume in which case the slots can be punched.
 

PadMasterson

Joined Jan 19, 2021
36
What DickCappels said is a very common thing to do for power supplies and the like, open an old wall wart for a USB power supply and you will see exactly what he's talking about. Those cuts are typically about .015" to .025" wide or so and routed on boards that are in the .031" thickness range and done in very large production quantities, can you say included USB power supply? :) There can also be an extra bit of cost when you get your board into fabrication if it's low quantities when you use such a small router bit if you really want to go that way. If not, space is your friend when it comes to preventing creepage/leakage currents from getting into your circuits. Put as much space as you can afford between the line(s) and you should be OK. That said, Soldermasks can be an issue and in some cases it's removed in those areas but that comes with the issue of cleanliness. Contamination on the board after soldering and cleaning can leave residues that will cause problems also. No clean fluxes can be the worst offenders so I would stay clear of them. If you are building these boards yourself, use plenty of cleaning solvent to rinse the boards and dry them in addition to selecting conformal coatings if you use one as they can be an issue also. There are several IPC documents on cleaning and related information you might look up and if you're laying out the board yourself, try for a minimum of 3X the trace width for spacing or more. If you have to be close to another trace, minimize the exposure length. Hope this helps some... Good luck. Oh and regarding Guard rings. Use them wisely, if you can afford the space for a guard ring, you can use that same space as distance from the circuit you're protecting. Space is your friend, remember. The idea is that if there is leakage current, the ring is supposed to keep it contained or prevent it from getting to the sensitive lines but the ohms per distance on the surface of the board are greater than a copper trace, why put the trace there to help the current cross to the other side? A guard ring needs to be grounded in multiple places and those vias take up room, room that could be in the form of open area/space... I'm not saying Guard rings don't have a place, they just need to be in the right place... :)
 
Last edited:
Top