BNC connector and ground

[s.mosfet]

The BNC have an outer connector (the shield). How is this connector connected to the ground? Just with the plug itself, or I need to connect it in some other ways?

thank you

[s.mosfet]

please! It's a very simple question...

[c4757p]

It's not a simple question! More details!

[s.mosfet]

The coaxial cable is composed by:
1- the inner wire (the one that carries the signal)
2- a metal shielding

http://en.wikipedia.org/wiki/Coaxial_cable

1- How is the metal shielding usually connected to the ground?
2- Is it usually connected to the ground?

[Monkeh]

The coaxial cable is composed by:
1- the inner wire (the one that carries the signal)
2- a metal shielding

http://en.wikipedia.org/wiki/Coaxial_cable

Y'know, I think he knows what a coaxial cable is. And what a BNC is.

[s.mosfet]

OK, but I would like to know if the outer shielding is connected to the ground when I plug it into my Instruments (voltage generator for example) or not.

[s.mosfet]

Sorry if I get confused between the BNC and the Coaxial cable, in the first post.

I would like to know if the outer shielding metal in che coaxial cable is connected to the plug, or to something connected to the ground.

Thanks.

[tszaboo]

You look at the instrument. The front panel. There is a ground signal, then it is connected to ground. If there is "oh no, watch out, this is actually not connected to ground, danger, danger, apply maximum 12Vrms to it" then it is not connected to ground. If there is nothing, well, you can measure it with a multimeter. But I suggest to just throw it out because of bad design.

[peter.mitchell]

The outer braid of the coaxial cable gets sorta crimped into the housing of the BNC connector, and the entire of the BNC connector is grounded, aside from the dielectric material and the signal conductor. The connector housing on the test equipment is also completely grounded, so, upon connecting the two BNCs the housing of the cable connector attaches to the housing of the instrument connector, connecting the grounds, there is not other terminal inside the connector, it is the housing itself that is the terminal.

I think this is the answer you wanted.

[KJDS]

The correct answer is "sometimes."

The true answer is Read the Manual, but no-one ever does unless it's vital. It's also possible to measure it with a meter but that could mislead in the event of a broken internal ground.

Some equipments, notably those used on ships, in television studios deliberately don't ground the outer.

[s.mosfet]

In my case is a keithley source meter (2400)
If it doesn't put the outer shield to the ground, what should I do?
Can I cut the cable, and solder a cocodrile connected to the outer shield? And then I can use it to connect it to the ground (that I guess is somewhere on the keithley?)

[KJDS]

In my case is a keithley source meter (2400)
If it doesn't put the outer shield to the ground, what should I do?
Can I cut the cable, and solder a cocodrile connected to the outer shield? And then I can use it to connect it to the ground (that I guess is somewhere on the keithley?)

Of course you can. Whether you should is dependent on how you like your ground loops to be arranged and whether there are likely to be connections between areas with differing earths.

[edavid]

In my case is a keithley source meter (2400)

OK, Keithley provides excellent manuals.  What does the 2400 manual say on the subject?

If it doesn't put the outer shield to the ground, what should I do?
Can I cut the cable, and solder a cocodrile connected to the outer shield? And then I can use it to connect it to the ground (that I guess is somewhere on the keithley?)

It depends on the rest of your measurement setup, but most people never have a need to cut their cables and add ground clips to them.  The fact that you are asking makes people think that you are in over your head.  Maybe there is someone local who can help you out, that would be a lot more practical than trying to explain everything about grounding in a forum post...

[robrenz]

The  Keithley 2400 does not have BNC connectors at all, only banana jacks.

[s.mosfet]

Ok, banana jack. But can I use a coaxial cable so that the inner conductor is connected to the banana jack and the outer is connected to the ground....?

[cyr]

I'm sure you *can*, but isn't the interesting question is if you *should*?

What are you trying to accomplish? Do you need shielded cables for some sensitive measurement? Aren't there any recommendations in the manual?

[robrenz]

Make sure you understand the difference of measurement common terminal and a chassis ground terminal.

Using a bnc to banana adapter on each end of a bnc coax cable works fine for reasonable currents. The little tab on the adapter represents the outer shield and should go to the common terminal and the other will go to whichever jack is appropriate for the measurement you are doing.

[s.mosfet]

I need to do current measurements (nA) with a probe station.

[popamp]

I just checked the Keithley 2400 in my lab at work and the black (neg) connector is NOT connected to earth.

There is a chassis earth connection on the back of the meter if you need to use it.

[s.mosfet]

Is there any company selling the coaxial cable with banana jack?

[s.mosfet]

As I mentioned, I need to do nA or uA current measurement (DC).
Do you think the connection of the external shielding of the coaxial cable is important for this kind of measurements?

[alm]

I suggest you read the Keithley 'Low Level Measurements Handbook', available for free from the Keithley website. It discusses all the relevant issues including type of cable, shielding, grounding and guarding.

[s.mosfet]

thank you, but it is not so useful. They explains some circuits to improve the sensitivity. I don't want to do that.

I want to know if the coaxial cable is deleterious for my measurements or if it can help, and how to connect it. That's it.

[tszaboo]

As I mentioned, I need to do nA or uA current measurement (DC).
Do you think the connection of the external shielding of the coaxial cable is important for this kind of measurements?

I don't see a reason to use coax cable for this. Coax is made for AC, or where the AC noise is not acceptable. In fact since in coax, the conductors are close to each other, they leak more, so they generate more error. Just use banana. If you really-really want to use a coax cable just use an adapter like above.

[robrenz]

Saying the Keithley handbook is "not so useful" says you don't want to take the time to learn. If you need to use a source meter properly you would do well to digest that book completely. I am not an EE so I know it is not an easy read. I had to read some sections many times before I grasped some of the concepts.


A generic coax cable with bnc connectors is two conductors, the inner solid or stranded conductor and the outer shield which is connected to the bnc shell.  They work fine with the 2 pin adapters I showed in a earlier post here. Using a bnc cable with a single pin banana adapter will not be better than a standard banana cable. Unless you had special bnc to banana adapters that allowed you to connect all the outer shields together and to ground or measurement common.  Coax bnc cables could actually be worse in higher current applications from the generally small inner conductor which is often not copper.

That's it!