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" True " Earth-Ground Problem

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superluminal

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Hi all

I've already searched the forum for issue like that. I found a lot of information & a lot of posts about grounding. But I think this is quit different.

I have a problem in the REAL GROUND ( the third terminal or the earth ground , as you may call, that is digged underground) terminal in our lab. I can detect ~ 1mV on it & actually seems it can not provide a calm reference & make it very difficult to get a real accurate results for characterization & RF instruments , for example.

Can anyone tell me how to compensate its effect ?

Thanks in advance
 

flatulent

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earth ground

This connection to real dirt is for safety purposes only. By having the outer metal boxes of all equipment connected to this point a human putting their hands on two different boxes will not have any significant voltage across their body and therefore little current will flow. It also serves as a conducting path for line faults where the power delivering wires inside a box get connected to other things due to a part failure. The current flows to ground and not to other items. This is the case in power systems where one line (neutral ) is also connected to the earth. In EC countries this is not always the case which is why the test procedure for CE marks require the mains common mode voltage be up to 1500 V or so above the earth voltge.

RF measurements should be done with respect to a local ground which can be formed by a sheet conductor on a lab bench. All parts of the measurement system and device to be measured are on top of this and relatively close to each other. In a coaxial measurement system, the outer conductors of all coaxial cables will be directly connected together by being connected to the ground plane of the circuit being measured or by the outer box of the system being measured.
 

superluminal

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Thanks for this information

The issue is not about safety only ;but, some of the measurement instruments actually connect their local common point to the ground of the chassis ( the earth) & may not have the ability to compensate the variation of the ground ( which MUST be a stable 0 V ) & we do not have the ability to change this ( or we may made it but with a degradation in instrument capabilities - for example ; we have an HP instrument with 4 channels ,we can make it by allocating a channel for ground compesation ( a ground unit connection)which results in 25% waste of the avaliable just for grounding :( )

So, the instrument local ground point will not be clean. This is a strong requirement for DC charcterization of MOSFET , for example, & will actually affect the meaurement results ( this is already checked !).

The problem with RF & transient time meaurements is that it sometimes coupled with a DC meaurement , so providing a common ground for both is essential.

A small note : the work is on a small range of voltage variation ( mV => uV & mA=> fA) & this ranges require a very accurate ground.

Thanks in advance
 

ted

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As flatulent points out, you have to use a local ground reference plane, and everything must be grounded to it. Also the safety ground must be connected to it, in exactly one point. I would try to install all the power outlets on that plane in one group, with preferably only one feed from the fuse box. That is then the point where the incoming safety ground is connected to the plane, and the plane is then connected to the safety ground pins of the outlets, to essentially form a one-point-ground for all instruments. This should reduce the ground voltages greatly.

On the other hand, with uV/fA signal levels, you have to consider many other sources of disturbing stray signals as well: Thermoelectric voltages when joining wires/components of different metals, induced low and high frequency signals on the cables (not least the power cords of the instruments, and secondary ground loops formed when connecting multiple instruments to the device-under-test).

The nature of problem depends quite much on the frequency band you measure, for instance thermoelectrics voltages are DC, while mains leakage currents and magnetic inductions are 50 or 60 Hz (+ often plenty harmonics!), depending where on earth you are (where is that quantum well, anyway? :) ). The local AM or FM stations, cellphones etc. may also add their signals to the mess, if your bandwith is high enough. Sometimes even a component or bad connection is forming an unintended rectifier/detector for those RF signals, and transposing them to lower frequencies, like audio from an AM transmitter.

You should carefully study the nature of disturbances in your lab, and then try to figure out where they come from, and how. Then it is easier to medicate the problem. But, once more, a good ground reference point (and -plane) is always necessary--use of several wall outlets and messy extension cords is a sure way to trouble.

Good luck,
Ted
 

S

sick_man

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to get a perfect earth plane

lay squares wire "cage" wire under a plot of ground 144ft by 144 ft square made of iron 5 - 10 mm iron that has been galvinised

this gives you a perfect earth you can feed four buildings from

you take a tap from a corner

bury at least 1.44 meters below ground layed by points too each mag pole position and not in sand

good soild types are clay and red earthy sandy and moist

youll have 0 mv

the problem you will have is from cable leackage

i recomend disconnecting the earth from the supply cable

and instead run the whole earth from the grid and
run the cable earth too the ground via just a spike

this gives a good return and little earth leackage

a star point earth of your equipment will also solve localised machine distortions and effects
 

flatulent

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classical screen room

With your signal levels, you may want to use a classical screen room with everything inside it. I once used a metal waste paper basket for a shield and powered some of the equipment with batteries.

You may be stuck with your use of one input for compensating for the environment. At your levels, the leakage currents from the mains EMI filters in each box will produce stray currents.
 

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