BNC connector and ground - Page 1
Forum section: electronics / beginners

author: s.mosfet
author_rank: Contributor
timestamp: December 12, 2013, 02:07:10 pm
content: 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

author: s.mosfet
author_rank: Contributor
timestamp: December 12, 2013, 02:49:17 pm
content: please! It's a very simple question...

author: c4757p
author_rank: Super Contributor
timestamp: December 12, 2013, 02:51:53 pm
content: It's not a simple question! More details!

author: s.mosfet
author_rank: Contributor
timestamp: December 12, 2013, 02:57:43 pm
content: The coaxial cable is composed by:1- the inner wire (the one that carries the signal)2- a metal shieldinghttp://en.wikipedia.org/wiki/Coaxial_cable1- How is the metal shielding usually connected to the ground?2- Is it usually connected to the ground?

author: Monkeh
author_rank: Super Contributor
timestamp: December 12, 2013, 02:58:48 pm
content: Quote from: s.mosfet on December 12, 2013, 02:57:43 pmY'know, I think he knows what a coaxial cable is. And what a BNC is.

author: s.mosfet
author_rank: Contributor
timestamp: December 12, 2013, 03:15:08 pm
content: 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.

author: s.mosfet
author_rank: Contributor
timestamp: December 12, 2013, 03:26:27 pm
content: 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.

author: tszaboo
author_rank: Super Contributor
timestamp: December 12, 2013, 04:48:37 pm
content: 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.

author: peter.mitchell
author_rank: Super Contributor
timestamp: December 12, 2013, 05:02:20 pm
content: 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.

author: KJDS
author_rank: Super Contributor
timestamp: December 12, 2013, 05:11:11 pm
content: 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.

author: s.mosfet
author_rank: Contributor
timestamp: December 12, 2013, 07:14:59 pm
content: 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?)

author: KJDS
author_rank: Super Contributor
timestamp: December 12, 2013, 07:42:31 pm
content: Quote from: s.mosfet on December 12, 2013, 07:14:59 pmOf 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.

author: edavid
author_rank: Super Contributor
timestamp: December 12, 2013, 08:01:06 pm
content: Quote from: s.mosfet on December 12, 2013, 07:14:59 pmOK, Keithley provides excellent manuals. What does the 2400 manual say on the subject?QuoteIt 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...

author: robrenz
author_rank: Super Contributor
timestamp: December 12, 2013, 08:19:21 pm
content: The Keithley 2400 does not have BNC connectors at all, only banana jacks.

author: s.mosfet
author_rank: Contributor
timestamp: December 12, 2013, 08:42:54 pm
content: 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....?

author: cyr
author_rank: Frequent Contributor
timestamp: December 12, 2013, 08:51:58 pm
content: 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?

author: robrenz
author_rank: Super Contributor
timestamp: December 12, 2013, 08:53:38 pm
content: 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.

author: s.mosfet
author_rank: Contributor
timestamp: December 12, 2013, 09:09:48 pm
content: I need to do current measurements (nA) with a probe station.

author: popamp
author_rank: Newbie
timestamp: December 12, 2013, 10:35:40 pm
content: 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.

author: s.mosfet
author_rank: Contributor
timestamp: December 12, 2013, 11:19:29 pm
content: Is there any company selling the coaxial cable with banana jack?

author: s.mosfet
author_rank: Contributor
timestamp: December 13, 2013, 09:41:08 am
content: 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?

author: Unknown
author_rank: Unknown
timestamp: December 13, 2013, 09:51:54 am
content: 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.

author: s.mosfet
author_rank: Contributor
timestamp: December 13, 2013, 10:43:04 am
content: 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.

author: tszaboo
author_rank: Super Contributor
timestamp: December 13, 2013, 12:34:24 pm
content: Quote from: s.mosfet on December 13, 2013, 09:41:08 amI 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.

author: robrenz
author_rank: Super Contributor
timestamp: December 13, 2013, 12:49:17 pm
content: 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!

author: Jon Chandler
author_rank: Frequent Contributor
timestamp: December 13, 2013, 01:14:49 pm
content: Geez oh grief you guys are being unhelpful.In a BNC cable, the center conductor is connected to the pin and the shield is connected to the shell. Connecting two BNC jacks together with a standard BNC cable connects center pin to center pin and shell to shell.A properly designed instrument with banana jacks has them spaced at 0.75" centers. The adapter shown above from Pomona can be used to connect a BNC cable to a pair of banana jacks. The shield of the jack goes to banana plug nearest the ridge, the center conductor to the other banana plug.Many instruments for sensitive measurements feature 3 binding posts: signal high, signal low and a separate guard jack which is generally connected to signal ground. If you were using a shielded, twisted pair cable, the shield of the cable would be connected to the guard terminal. The guard terminal is often fitted with a movable link so that it can be tied to the signal low binding post. If the Keithly has such an arrangement, you could link the guard terminal to the signal low terminal for low level measures if noise is a problem.There are exceptions to the rules. Grounding and shielding is a huge topic on its own. With multi-channel systems, say 12 channels of amplifiers feeding a 12 channel tape recorder, it may be desirable to connect the shield of only a single channel between the amplifiers and tape recorder to prevent ground loops. The shields of the other channels are connected at one end only (probably the signal source end). When this is done, it's usually with a custom-built wiring harness.

author: robrenz
author_rank: Super Contributor
timestamp: December 13, 2013, 01:46:14 pm
content: Quote from: Jon Chandler on December 13, 2013, 01:14:49 pmHow?I already covered your first 2 paragraphs in my posts and your second 2 paragraphs he is not interested in as statedQuote from: s.mosfet on December 13, 2013, 10:43:04 am

author: s.mosfet
author_rank: Contributor
timestamp: December 13, 2013, 04:28:15 pm
content: OK, so I will use a simple cable with a banana jack.Should it be a 50 ohm, right?

author: KJDS
author_rank: Super Contributor
timestamp: December 13, 2013, 05:07:52 pm
content: Quote from: s.mosfet on December 13, 2013, 04:28:15 pmThe impedance is irrelevant. Please have another read of the Keithley document mentioned earlier and hopefully you'll start to understand what is and isn't important.