[SOLVED] Chassis ground, dc ground, Logic ground, signal ground , have different resistances

[ParkerMike]

I have noticed these is a resistance difference between logic ground and another logic ground , some are 10 ohms and others are 27 ohms

Same with the resistance difference between a DC ground to another DC ground

The Buss wire or rail wire is connected across , so why would it have a resistance different on the same Rail or buss?

If one DC ground is 10ohms and another DC ground is 27 ohms, is there a Voltage difference or potential difference?

from 5ohms to 100ohms does this really change the potential differences of the grounding even on the same rail or buss?

 

[FoxyRick]

Yes, there certainly is a potential difference if there is a resistance between two 'ground' points, and any current is flowing through that ground connection. This is one of the common issues in designing stuff and fault finding.

Any conductor will have some resistance, including a ground line. Your values sound very high though. Are you sure you are measuring just a ground connection to the ground rail, not going through some component?

 

[D.A.(Tony)Stewart]

Sometimes ground noise will interfere with Ohmmeter DMM pulse measurements.

Naturally if you expect any current flow on those grounds the difference will be I*R in DC potential. There is also a stray inductance per meter, which will add impedance with frequency.

Litz wire uses parallel magnet wire for lowest inductance connections. Braid wire is ok when reduced skin effect is all that is needed.

But don't rely on high impedance grounds to suppress magnetic fields very well as the IR drop may induce a voltage from inductive coupling. For high impedance electric fields, it makes little difference, as long as there is a local cap for filtering.

Consider where all your noise sources are, sensitive inputs and ESR of any pertinent part.

 

[ParkerMike]

Are you sure you are measuring just a ground connection to the ground rail, not going through some component?

Each rail or buss has components tied to the rail or buss ground wire

The Signal ground DC buss rail
The Logic ground DC buss rail
The Audio DC ground buss rail
The Servo motor ground DC buss rail

All these grounds get tied together to a Chassis ground

I don't understand also, is that the circuit board under test has a chassis ground wire that goes to the test fixture on jack plug#1, and jack plug#2 has a chassis ground also but it's bonded with a chassis copper bar that bonds the chassis grounds together.

It's connecting the chassis ground of the test fixture to the chassis ground of the circuit under test

If you disconnect the Chassis bar so jack#1 and jack#2 is not bonded together , only the test fixture chassis ground goes to the circuit board under test

I don't understand why these is a chassis ground that is bi-directional with the test fixture and the circuit under test that needs two chassis grounds that are on two separate jacks plugs

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What I mean is that the chassis ground from the Test fixture goes to the circuit board under tests jack plug#1 and goes through the chassis copper bar to jack plug#2 which goes back to the test fixture.

I don't understand this type of "Loop" of the chassis ground used like this

Do you see the loop theory? and why it's used like this?

 

[chuckey]

How about if the loop is closed, the test fixture "knows" there is a board plugged in?
Frank

 

[ParkerMike]

cable#1 from the test fixture has the test fixtures Chassis ground
Cable#2 from the test fixture has the Board under tests chassis ground

The board under tests chassis ground is connected to the cable#2

I don't understand why the test fixtures chassis ground is connected to the cable#1 and Cable#2 is connected to the chassis ground of the board under test

They are bonded together inside the board under test with a copper metal bar to connect both the chassis grounds together

There is test procedures where you short out the +30 volts an output of the test fixture to the chassis ground of the test fixture. You measure with the DVM meter from the chassis ground of the board under test to various test points on the board under test.

See how the chassis grounds are separated but get looped inside the board under tests chassis but you can short out outputs from the test fixtures to the test fixtures chassis ground and use the DVM meter to measure from the board under tests chassis ground to various nodes or test points

What kind of chassis ground loop is this called?