Guard Traces...  - Page 1
Forum section: electronics / projects

author: Smokey
author_rank: Super Contributor
timestamp: August 23, 2012, 05:39:39 pm
content: What is your take on guard traces (Traces that run in-between noisy signals traces and sensitive traces)? Do you use them generally, only in special cases, never? Horror stories about incorrect termination of guard traces that ended up causing more problems than they would have solved?Here is an interesting article from Altera where they took some guard trace measurements. They came up with some interesting data.http://www.altera.com/technology/signal/columns/2006/2006-10-oct_1.html

author: AndyC_772
author_rank: Super Contributor
timestamp: August 23, 2012, 06:52:17 pm
content: I've heard this too - the the guard trace itself adds little real value, it's the physical separation between the two coupled traces that delivers most of the isolation. The rule seems to be just to keep noisy stuff away from sensitive stuff.

author: jahonen
author_rank: Super Contributor
timestamp: August 23, 2012, 07:30:20 pm
content: Howard Johnson also recommends that just increasing the separation between aggressor and victim is the simplest option, fiddling with guard traces in digital system is probably not worth it. Also, a guard trace is not going to work for something like a differential pair, since each trace of the pair has different distance from the guard trace, thus shielding effect is different and there will be a crosstalk seen as differential signal.Regards,Janne

author: Neilm
author_rank: Super Contributor
timestamp: August 23, 2012, 07:40:52 pm
content: Generally if I add guard traces it is because I am worried about the surface leakage on the PCB causing offsets in my readings. Connecting a guard trace around the sensitive signals stops any leakage getting in.Neil

author: Mechatrommer
author_rank: Super Contributor
timestamp: August 24, 2012, 02:49:59 am
content: as i understand from reading and "blind implemented it", guard trace only usefull for sensitive, ie high impedance signal/sensor. and terminating it to the low impedance output of the same signal, usually after buffer opamp.

author: PA4TIM
author_rank: Super Contributor
timestamp: August 24, 2012, 06:20:49 am
content: There are several reasons and ways of guarding. Decreasing leakage, decreasing paracitic capacitance, decreasing electro magnetic coupling.I use it often when buiding meaurement gear ( things like volt references, femto Farad meter ect)There is a passis way. Traces that just are there to seperate, i have some 60's electrometers that do that. Often 5 of 6 traces next to each other.Some times at gnd level and what I most use is active. A sample of the same signal surrounds senitive high impedance pointsBut I'm only involved in analog design, not digitsl.Fred

author: jahonen
author_rank: Super Contributor
timestamp: August 24, 2012, 06:34:34 am
content: Yes, I forgot to mention that the guard traces make much more sense on low-level analog stuff, like in measuring at fA and pA levels, where surface leakage becomes an issue.Regards,Janne

author: PA4TIM
author_rank: Super Contributor
timestamp: August 24, 2012, 08:31:40 am
content: Yep everything is an issue there. When i made my fF meter i had two digits drifting ( plus/min 30fF)Then i cleaned the inide with ipa that reduced drift to about 15 fF , then I guarded the input connector and low wire ( bnc with guardsignal on coax outside) and the last improovement was cleaning the surface around the connectors. Then it was stable upto around 5 fF.If you open a Fluke calibrator, Fluke 720 or HP3400 multimeter there are warning signes not to touch certain boards and parts. In the 332 some boards are ultrasonic cleaned.On the KV devider i only got 10E10 resistance at 1000V. After cleaning the whole inside using tri and ipa the resistance raised to 10E12 like it should.Yesterday I calibrated a 7,5 digit meter, leakage current was to high and Rin to low. Cleaning the alog circuit reduced leakage to 6E-11 and calculated Rin to 33Gohm in the 0.1V range.Only problem from doing this sort of things is you must be carefull to not become nuts ( to late for me)

author: nctnico
author_rank: Super Contributor
timestamp: August 25, 2012, 09:28:23 pm
content: When routing digital and analog signals close to each other is makes much sense to add a guard trace connected to ground. I often do that in mixed signal designs.

author: Smokey
author_rank: Super Contributor
timestamp: August 31, 2017, 08:46:13 am
content: bump for stories/opinions about guard traces...

author: Andreas
author_rank: Super Contributor
timestamp: August 31, 2017, 08:57:32 am
content: Hello,In a digital design I would use them generally for clock lines and sensitive (edge triggered) signals like Interrupt ..If you use them you have to contact the ground plane at least all 0.5 to 1 inch by a via to have a good working shield against cross talk.With best regardsAndreas

author: Rerouter
author_rank: Super Contributor
timestamp: August 31, 2017, 09:23:36 am
content: Talk about flogging a dead horse, Interesting to see by the op (5!) years later.For digital stuff, pull up an online crosstalk calculator, make sure your within margins, if not you increase your gap.For low level, or sensitive nets, its not uncommon for me to locally surround it with a seperate ground on any relevant layers, e.g. for a crystal oscillator i will seperate it from the ground pour by a keepout zone, having a guard ring of a ground trace around the crystal nets, terminating in to the devices oscillator ground pin, and only tying it to plane there, to keep as much possible noise out as reasonable, Generally also removing the ground plane from underneath it and doing the same on the reverse, so any currents in the plane cannot capacitivly couple in. but that is me being a little overkillNever played with active guards, so haven't had anything go wrong there, I have yet to need to measure less than 100's of nano amps, which is still within the realm of not caring about board leakage.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 31, 2017, 09:58:20 am
content: I've traced out a hokey perimeter around a crystal, from time to time. But for no particular reason. I haven't done any low noise, high precision, low jitter, etc. sorts of things yet.The comment about distance is key: on a 4-layer board, the distance from trace to inner ground plane is about as small as the distance between traces. Edge-to-edge coupling is pretty poor to begin with, and it's that much weaker with the shielding effect from the nearby plane.Which BTW is a good lesson about diff pairs: they aren't. To a more significant extent, they are two single traces, which are routed through the same locations as much as possible, so as to pick up the same interference, which is then subtracted at the receiver. The lesson is this: each trace is a transmission line, not so much the pair together. You cannot simply ignore normal or common mode behavior! (Fortunately, most transmitters handle this -- it's not well documented, but their common mode output impedance is modest, which both sets Vcm and helps dampen standing waves.)Tim

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: August 31, 2017, 02:04:54 pm
content: Quote from: T3sl4co1l on August 31, 2017, 09:58:20 amI do, my boss insists on what he calls "moats" around crystals. I've put in the keepouts in the footprint so I don't forget.

author: CopperCone
author_rank: Super Contributor
timestamp: August 31, 2017, 08:30:40 pm
content: a guard trace adds some tiny amount of capacitance to the circuit too like a distributed capacitance plane.Its almost nothing though. If traces were tall, it would be more significant (like PCB bus bars).some cihps demand a guard trace around a crystal (i.e. dspic 33)

author: bson
author_rank: Unknown
timestamp: September 03, 2017, 12:40:46 am
content: They add capacitance between the guard trace and adjacent traces, but there's almost no capacitance between the guard trace and the guarded signal, because they carry the same signal and any difference is the minuscule stuff protected against. Hence the guard trace is sacrificed for the benefit of the signal.