Guard Plane, used for what?

[stanleystan]

What are guard planes used for?

A Guard Plane is at the same voltage potential as the output of a circuit?

Or how do you use a Guard plane?

 

[betwixt]

They are usually used in very high impedance situations where leakage across a board surface or between layers has to be kept to absolute minimum. It works by ensuring there is no voltage difference between the measurement point and it's surroundings. No voltage means no current should flow and hence the impedance is infinite. Where the voltage feeding the guard plane comes from depends on the application, it may come from the output of a circuit if it's a unity gain buffer or it may come from a different unity gain buffer that only drives the guard plane. In some instances, for example in ICT, the guard voltage may be produced by the test equipment itself and chosen to closely match although not necessarily be exactly equal to that at the measurement point.

Brian.

 

[stanleystan]

They are usually used in very high impedance situations

Why don't you want a potential difference between the input of a high impedance circuit or component compared to the output potential difference?

I thought a high impedance input blocks leakage current or a high impedance input or circuit doesn't had any load to the circuit

Why would you want the guard plate to be the same potential as the high impedance circuit or input/output?

I thought high impedance doesn't add any load and blocks leakage current

 

[ted]

The problem is not what the high-impedance input does, but "everything else" nearby - leaking potentially some disturbing signal to the high-impedance (and therefore sensitive) input.

By having a guard "virtually" at the same potential than the sensitive high-impedance input, there will be absolutely minimized leakage current to that input. (Ohm's law: Voltage (difference) divided by resistance (leakage) is the unwanted current, minimized by keeping the voltage low over the leakage resistance/impedance).

 

[stanleystan]

I'm trying to understand this

1.) Lets just say there is leakage current going into the high impedance input or sensitive input, what will happen?

2.) Lets just say there is leakage current going into the high impedance input or sensitive input, what will having the leakage current at the same potential do? the guard plate just puts a voltage potential on the leakage current? so having a voltage potential on the leakage current does what to the high impedance input?

 

[ted]

If we first assume, that the potential leakage is not enormous, but just a significant fraction of the input current level, then it is easier to figure what happens.

Let's say for illustration that for instance our input works around a range of 1 nA. The a leakage impedance would be in this example 1 GigaOhm. In that situation, one volt difference on a circuit board trace on other side of the leakage path would produce that 1 nA current. That is same magnitude than the intended signal, and would essentially mess up the intended current level totally.

However, if we gave the input a guard, which tracks the input within, let's say 10 mV, the leakage current would be only 10 pA, and would not disturb at all so much.

 

[betwixt]

Where you are misunderstanding Stanleystan is that the leakage you are guarding aginst isn't in to the device itself, it's from surrounding tracks and wiring. You surround the connection to the device with the guard plane so it acts as a barrier to the leakage entering from outside. Using ground as the barrier may not be appropriate, it's easy to think of ground as being 'a clean zero voltage' but in reality it could carry noise and signals and of course the signal you are guarding may not be at zero volts itself. Using an equal voltage eliminates all risk of potential difference to the surroundings.

Brian.

 

[stanleystan]

Where you are misunderstanding Stanleystan is that the leakage you are guarding aginst isn't in to the device itself, it's from surrounding tracks and wiring.

I don't understand how a guard plane can lower the leakage current because it will be at the same voltage potential

if the leakage current is in the milliamps, a guard plane can lower the leakage current to picoamps, how so?

How do you know when your circuit or stage in a circuit needs a guard plane?

What are some common signs of when a circuit or stage needs a guard plane?

 

[ted]

First, the leakages on a circuit (a printed circuit board) are very small. not milliamps, but much smaller. The guarding usually may be required only, when the current levels are below microamps. That is an answer to your last question: When guarding might be necessary: When the input signal currents are very low. There is no "hard limit" as depending on application, environment, and other circumstances, value of "low" can vary quite much. On printed circuit boards we talk usually about nanoamperes or at most microamperes. On some other cases, fractions of picoamperes. Or in the other extreme, we might have many tens of microamperes or even more - but rarely.

The reasoning for guarding is quite straight forward:

You may have heard about Ohms law. That law defines the equation for leakage currents, (as well as for any other currents) as function of voltage and resistance (or impedance, if we consider AC and reactive components as well).

Let's keep it simple, and the leakage is just a resistance, value R.

The voltage between the sensitive input node and "something else" on other side of the leakage, has a voltage U, relative to the input we are observing.

Then the leakage current "I" would be = U/R. It is then obvious, that if the value of R (the leakage resistance) is constant, the current would be reduced easily by reducing voltage U.

The guard, by tracking closely the input voltage, would keep the voltage U very low, thus reducing the leakage current.

I don't understand how a guard plane can lower the leakage current because it will be at the same voltage potential

if the leakage current is in the milliamps, a guard plane can lower the leakage current to picoamps, how so?

How do you know when your circuit or stage in a circuit needs a guard plane?

What are some common signs of when a circuit or stage needs a guard plane?

 

[Audioguru]

A guard plane helps AC leakage too.
The guard plane (a ring) around a high impedance input with the same AC signal as the input will prevent capacitance coupling of interference from affecting the input.