Guard Ring

Thread Starter

shibin_varghese

Joined Jan 14, 2019
73
I have heard about Guard Rings many times, and I know they are supposed to avoid currents in places where there shouldn't be any currents, but I never found a good text to read more about it or how to implement a guard ring in my own PCB

Can someone describe them properly or please recommend some material to further reading?
 

TeeKay6

Joined Apr 20, 2019
572
I have heard about Guard Rings many times, and I know they are supposed to avoid currents in places where there shouldn't be any currents, but I never found a good text to read more about it or how to implement a guard ring in my own PCB

Can someone describe them properly or please recommend some material to further reading?
One good and easy place to start would be to search AAC for "guard ring." Many other people have asked and answered questions about guard rings.

As to basic implementation, a guard ring is a PCB trace that surrounds (but does not touch) a specific node (=a junction of component leads & traces) that is sensitive to unwanted currents that may flow across the surface of the PCB from other nearby traces. Thus, if the surface path from a sensitive amplifier input pin is close to a power supply pin, an intervening trace connected (most often) to ground will limit the leakage current from the power supply pin (and connected traces) into the amplifier input pin (and its connected traces), as the leakage current will then flow to ground. There are more complicated situations where ground is not the best place to connect the guard ring; worry about them when you know more. A guard ring can also help to reduce capacitive coupling effects. For example, when the amplifier input pin is near a trace/pin carrying an oscillator signal or fast switching signal; the guard ring can reduce the stray, unwanted, capacitance between the switching signal and the amplifier input signal. Note that a guard ring essentially operates in the plane of the PCB, protecting against interfering signals also in the plane of the PCB. The guard ring helps, but less so, to reduce leakage within the thickness of the PCB. The guard ring offers no help against interference originating above or below the plane of the PCB. In summary, a guard ring is an intervening trace between two other traces that might interfere with each other, primarily due to current leakage from one trace to another.
 
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Thread Starter

shibin_varghese

Joined Jan 14, 2019
73
One good and easy place to start would be to search AAC for "guard ring." Many other people have asked and answered questions about guard rings.

As to basic implementation, a guard ring is a PCB trace that surrounds (but does not touch) a specific node (=a junction of component leads & traces) that is sensitive to unwanted currents that may flow across the surface of the PCB from other nearby traces. Thus, if the surface path from a sensitive amplifier input pin is close to a power supply pin, an intervening trace connected (most often) to ground will limit the leakage current from the power supply pin (and connected traces) into the amplifier input pin (and its connected traces), as the leakage current will then flow to ground. There are more complicated situations where ground is not the best place to connect the guard ring; worry about them when you know more. A guard ring can also help to reduce capacitive coupling effects. For example, when the amplifier input pin is near a trace/pin carrying an oscillator signal or fast switching signal; the guard ring can reduce the stray, unwanted, capacitance between the switching signal and the amplifier input signal. Note that a guard ring essentially operates in the plane of the PCB, protecting against interfering signals also in the plane of the PCB. The guard ring helps, but less so, to reduce leakage within the thickness of the PCB. The guard ring offers little help against interference originating above or below the plane of the PCB. In summary, a guard ring is an intervening trace between two other traces that might interfere with each other, primarily due to current leakage from one trace to another.

How can we implement it in a PCB?.
We all draw plane in the top and bottom layer on the PCB right?, so I think we need to separate the top or bottom plane with the guard plane
Still, I don't know how to implement or what are the design constraints with the Guard Ring while drawing a PCB
 

TeeKay6

Joined Apr 20, 2019
572
How can we implement it in a PCB?.
We all draw plane in the top and bottom layer on the PCB right?, so I think we need to separate the top or bottom plane with the guard plane
Still, I don't know how to implement or what are the design constraints with the Guard Ring while drawing a PCB
No, the guard ring should be placed on the layer containing both the to-be-protected node and the interfering node. As I noted, the protection exists only at the surface of the PCB, so putting protection on a different side from the layer being protected does not work. If the protected node includes through-hole components, then a guard ring should be placed on all layers having electrical connection to those components. A guard ring on a layer that does not have both a to-be-protected node and an interfering node is of no value. A guard ring is a fence; you place it to keep certain things separated. If you want to keep a stray dog out of your area, you place a fence around your area; it would do you no good to put a fence around some other area (or under the ground, or suspended in air).

As I suggested, search AAC for "guard ring" and read the articles/threads that are returned in the search. Some of them will be irrelevant and some will be exactly what you are looking for. A Web search for "what is a guard ring?" gives many useful results!
 
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Thread Starter

shibin_varghese

Joined Jan 14, 2019
73
No, the guard ring should be placed on the layer containing both the to-be-protected node and the interfering node. As I noted, the protection exists only at the surface of the PCB, so putting protection on a different side from the layer being protected does not work. If the protected node includes through-hole components, then a guard ring should be placed on all layers having electrical connection to those components. A guard ring on a layer that does not have both a to-be-protected node and an interfering node is of no value. A guard ring is a fence; you place it to keep certain things separated. If you want to keep a stray dog out of your area, you place a fence around your area; it would do you no good to put a fence around some other area (or under the ground, or suspended in air).

As I suggested, search AAC for "guard ring" and read the articles/threads that are returned in the search. Some of them will be irrelevant and some will be exactly what you are looking for. A Web search for "what is a guard ring?" gives many useful results!
I mean do we need to isolate the guard plane with the TOP or BOTTOM ground plane which is GROUND all the time or is it needed to ground our guard plane
 

TeeKay6

Joined Apr 20, 2019
572
I mean do we need to isolate the guard plane with the TOP or BOTTOM ground plane which is GROUND all the time or is it needed to ground our guard plane
We are discussing a guard RING, not a guard PLANE. The guard ring (that is NOT located in the ground plane) should be grounded by any convenient method: a trace connecting the ring to a nearby ground connection, a via connecting the ring to a ground pad or plane on another layer, a wire running from the ring to a convenient ground point, etc.

Have you performed any of the suggested searches and read any of the results?
 

Thread Starter

shibin_varghese

Joined Jan 14, 2019
73
We are discussing a guard RING, not a guard PLANE. The guard ring (that is NOT located in the ground plane) should be grounded by any convenient method: a trace connecting the ring to a nearby ground connection, a via connecting the ring to a ground pad or plane on another layer, a wire running from the ring to a convenient ground point, etc.

Have you performed any of the suggested searches and read any of the results?
Yes, I did. But all the explanations says what is guard ring and why is it used in a circuit.
none of them say how to implement it in a PCB.
Can you explain how to implement it in a circuit?
What is the difference between the guard ring and guard plane?
 

TeeKay6

Joined Apr 20, 2019
572
Yes, I did. But all the explanations says what is guard ring and why is it used in a circuit.
none of them say how to implement it in a PCB.
Can you explain how to implement it in a circuit?
What is the difference between the guard ring and guard plane?
@shibin_varghese
"Guard plane" is not a standard term and has no defined meaning. That is why I corrected your language in post#5. The correct term is "guard ring". When I did a Web search for "what is a guard ring?", one of the first search results was the link that @bertus provided above, that clearly shows a drawing of a guard ring.
 

carloc

Joined Oct 8, 2018
13
[....]The guard ring (that is NOT located in the ground plane) should be grounded by any convenient method[...]
I believe that the "concept" of guard ring is different from "ground it".

As far as I know it could be simplified in "Connect the guard to the same potential as the sensitive track but buffered in such a way not to load the sensitive track itself"
eeeeeeeeee.png
 

TeeKay6

Joined Apr 20, 2019
572
I believe that the "concept" of guard ring is different from "ground it".

As far as I know it could be simplified in "Connect the guard to the same potential as the sensitive track but buffered in such a way not to load the sensitive track itself"
View attachment 185186
@carloc
Certainly you are correct. However, the TS was having a very difficult time understanding even the simpler definition. While I understand "buffered in such a way not to load the sensitive track itself", that simple phrase covers much complexity, especially for someone inexperienced in the art; what do the terms guard, buffered, load, and sensitive mean to such a novice?
 

carloc

Joined Oct 8, 2018
13
@TeeKay6
I see your point, teaching novices is indeed a very hard task,

It is so difficult to make things clear enough removing (say postpone to a further stage) unnecessary details, but at the same time give a full insight "tuned" to the novice's current knowledge and electrical "toolbox".

What sometimes is even harder is avoiding to say something wrong while trying to simplify.
 

WBahn

Joined Mar 31, 2012
26,398
I have heard about Guard Rings many times, and I know they are supposed to avoid currents in places where there shouldn't be any currents, but I never found a good text to read more about it or how to implement a guard ring in my own PCB

Can someone describe them properly or please recommend some material to further reading?
Imagine that you have two traces that have two different signals on them:

signal 1: -----------------------------------------
signal 2: -----------------------------------------

These two signals form a capacitor and so if the voltage on signal 1 changes that change will be capacitively coupled to signals 2 (and vice-versa). This allows "crosstalk" between these two signals. If both signals are relatively large and strong, this is likely not an issue. But if one of them is large and strong and the other is small and weak (which is often the case with analog signals at the point at which they first enter an amplifier), then the strong signal will inject signals into the weak signal and cause significant disruption to it.

Now imagine that we put a third signal between the first two

signal 1: -----------------------------------------
guard: --------------------------------------------
signal 2: -----------------------------------------

Now signal 1 and signal 2 are capacitively coupled to the guard signal, but the coupling between signal 1 and signal 2 is significantly reduced. As long as the guard signal is driven by a strong, constant voltage source the variations in either signal 1 or signal 2 will be coupled into the guard but the coupling between them will be much less than before (there will still be some coupling, but greatly reduced).

So a guard ring is really nothing more than a trace inserted between two signals that you want to minimize the coupling between. It is called a "ring" because, ideally, it would completely surround the signal that you are concerned about. But this is not always practical, so you do the best you can. More often than not the guards are routed separately to the analog ground in such a way that those traces never carry any current because you want them to be firmly at a constant voltage. But sometimes what is more important is that what you really want is for there to be no differential voltage between the guard trace and the sensitive node and sometimes the analog ground is not the best choice for that -- some signals are referred to the power supply voltage and so you want the guard tied to that same power rail so that any noise in the power supply will appear as a common mode signal between the guard and the sensitive node, thus resulting in little, if any, capacitive current injection into the sensitive node. This is a more sophisticated view of the situation, so for now just tie your guards to ground and you will likely see a significant improvement, just not as good an improvement as you could if you had picked the proper signal to tie your guard to.
 

bloguetronica

Joined Apr 27, 2007
1,424
Hi,

You can see the attached pdfs for a guard ring example. The schematic diagram shows how a guard ring should be connected (ideally, it should be at the common mode potential of the sensitive input traces). The board layout shows its placement. Note that the guard ring should have no solder mask on top of it, in order to be effective.

Kind regards, Samuel Lourenço
 

Attachments

WBahn

Joined Mar 31, 2012
26,398
Hi,

You can see the attached pdfs for a guard ring example. The schematic diagram shows how a guard ring should be connected (ideally, it should be at the common mode potential of the sensitive input traces). The board layout shows its placement. Note that the guard ring should have no solder mask on top of it, in order to be effective.

Kind regards, Samuel Lourenço
What is the problem with having a solder mask on top of the guard ring?
 

bloguetronica

Joined Apr 27, 2007
1,424
What is the problem with having a solder mask on top of the guard ring?
Well, it kind of defeats the purpose of the guard ring. The guard ring is there to isolate an area from noise. To be effective, it has to be made of bare copper, in order to absorb charge. Think of it as an anti-static strap of sorts.

Kind regards, Samuel Lourenço
 

WBahn

Joined Mar 31, 2012
26,398
Well, it kind of defeats the purpose of the guard ring. The guard ring is there to isolate an area from noise. To be effective, it has to be made of bare copper, in order to absorb charge. Think of it as an anti-static strap of sorts.

Kind regards, Samuel Lourenço
That implies that this charge being absorbed is made of up free electrons moving about in the air next to he board. I've never seen anything that indicates that this is a significant source of noise in most electronic circuits.

It also goes counter to the common use of guard rings in integrated circuitry where leaving it as bare metal simply isn't an option.
 

nsaspook

Joined Aug 27, 2009
7,882
That implies that this charge being absorbed is made of up free electrons moving about in the air next to he board. I've never seen anything that indicates that this is a significant source of noise in most electronic circuits.

It also goes counter to the common use of guard rings in integrated circuitry where leaving it as bare metal simply isn't an option.
The bare metal is mainly for PCB contaminates interacting with atmospheric moisture/contaminates with sensitive circuits. Keeping it bare acts like a getter to attract possible contaminates away from active traces under the mask (not always a perfect seal). Soldermask is also not to be considered an insulator or a stable dielectric.

The operation in an IC is mainly for its electrostatic shielding properties, repelling and/or attracting unwanted charge.
 
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TeeKay6

Joined Apr 20, 2019
572
It's bare metal is mainly for PCB contaminates interacting with atmospheric moisture/contaminates with sensitive circuits. Keeping it bare acts like a getter to attract possible contaminates away from active traces under the mask (not always a perfect seal). Soldermask is also not to be considered an insulator.

The operation in an IC is mainly for electrostatic shielding repelling or attracting unwanted charge.
@bloguetronica @nsaspook
I should be very, very interested to see an authoritative source for the claims you make respecting the use of solder mask with guard rings. It would help your case considerably if you can cite any studies or reputable, well-known sources for such claims. (I suggest that a thread with title such as "Guard rings: Use solder mask or not?" be started for any further responses.)
 
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