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I'm a student doing research in a physics lab and we often have the need for a certain type of BNC cable splitter. This splitter has a BNC jack which accepts a single coaxial cable as input. It has two jacks for output. It routes the center pin of the input to the center pin of one of the outputs and it routes the shield of the input to the center pin of the other output.

To give an example, one thing we use this type of splitter for is to amplify a voltage (center pin) with respect to ground (outer pin) using a purely differential amplifier which takes two BNCs as input.

My questions are: 1. Is there a name for this type of splitter? 2. Is there anywhere that sells nice, high quality splitters of this type? We can homebuild them but it would be better if we can find a place to buy them from.

I've tried searching for this, but I haven't turned anything up. I appreciate any and all answers!

Edit: I should note that another thing we use this type of splitter for is connecting a two point resistance measurement from a multimeter to our breakout box. The multimeter has banana plugs and we want to connect the positive wire and the negative wire to different jacks on the breakout box (corresponding to different contacts on our sample inside a dilution refrigerator). It seems to me that a splitter like I described is necessary for this.

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  • \$\begingroup\$ how (if at all) are the shields of the outputs connected? \$\endgroup\$ – Yuriy Nov 13 '13 at 3:17
  • \$\begingroup\$ For our purposes they would be left floating. \$\endgroup\$ – William Scales Nov 13 '13 at 3:25
  • \$\begingroup\$ Unsure of what you are actually measuring, I do want to note that you're messing up impedances and shield/reference. This doesn't sound like a reliable method to measure signals. \$\endgroup\$ – jippie Nov 13 '13 at 7:02
  • \$\begingroup\$ I'm not sure about impedances (what do you mean that we're messing them up?) but in our setup all of the grounds are connected and so we actually want to avoid ground loops. It seems like the ground gets messed up, but I believe this actually avoids a ground loop. \$\endgroup\$ – William Scales Nov 13 '13 at 8:18
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I don't think anyone really makes a connector like that. It really defeats the purpose of a differential amplification. I think a better solution would be to run the BNC cable straight into the + input of the amplifier and then terminate the - input or short it to GND. Or flip flop that if you want the output inverted. This will do exactly the same thing and you should get better noise performance as you will be bringing in less of the noise that will be coming back up the shield.

Edit: As for connecting to a multimeter, there are BNC to banana adapers available that will split out the center contact and shield on separate banana plugs. A couple of these in combination with a couple of banana cables would allow connections between the multimeter and any combination of signal and shield connections. However, this likely will not be very effective for performing sensitive measurements.

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  • \$\begingroup\$ Thanks for the answer. I have a question about your suggestion for the differential amplifier. Our voltage preamplifiers are LM1167 in-amp ICs. In an effort to avoid ground loops (which cause us big problems), we use insulating BNC jacks for the inputs and leave the ground tabs on the jacks disconnected. What would you reccomend for this case if we want to amplify a signal with respect to ground? \$\endgroup\$ – William Scales Nov 13 '13 at 3:21
  • \$\begingroup\$ Yeah, I suppose ground loops could be a pretty big problem with a complex setup. I have seen connector shields connected to circuit grounds with parallel RC circuits. That sort of a configuration would limit the amount of current that can flow if there is a ground potential difference. What is at the other end of the BNC cable in your setup? You may get better performance if you can run a pair of BNC cables and take a differential measurement of the center conductors. \$\endgroup\$ – alex.forencich Nov 13 '13 at 3:38
  • \$\begingroup\$ The other end of the BNC is plugged in to a breakout box which is connected to the sample inside a dilution refrigerator. Most of the time we do connect a pair of BNCs like you say. \$\endgroup\$ – William Scales Nov 13 '13 at 8:07
  • \$\begingroup\$ Then what is the purpose of only connecting one BNC cable if there are provisions for two? Do some experiments require single-ended measurements? \$\endgroup\$ – alex.forencich Nov 14 '13 at 6:47
  • \$\begingroup\$ We always plan to connect two BNCs, but sometimes contacts on the sample break after we've cooled it down and we still want to measure something, for instance. Also, see my edit to the question. \$\endgroup\$ – William Scales Nov 14 '13 at 7:39
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I haven't seen that particular style of adapter but if you don't have any luck tracking one down a solution I use personally is a pair of BNC receptacle to banana plug adapters along with a BNC to banana socket adapter. They are fairly widely available in a variety of styles such as the following:

BNC banana adapters

Those particular parts are a Pomona 4435175 and Pomona 1894 - I've given links to them on Element 14 but similar adapters are available from a wide range of manufacturers and suppliers. Just try a search on "BNC banana" to find them, I just did a search on Digikey and it looks like they have a good range as well.

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We have a similar requirement it to take the output of an isolated ground system and plug it into a differential amplifier (so that the ground of the amplifier doesn't create ground loops with the isolated ground system). For now we have just been making our own with a box with two grounded BNCs (which go directly to the inputs of the differential amplifier) and an isolated ground BNC. Inside the box is simple -- two wires. It works, but it would be nicer to just have something that looks like a Y splitter.

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  • \$\begingroup\$ Even since it's first revision, this has contained an answer. \$\endgroup\$ – Chris Stratton Mar 6 '14 at 0:27
  • \$\begingroup\$ Sorry I never got around to accepting this answer. I moved on from the lab in question after December '13. But this is pretty much what we were doing at the time. \$\endgroup\$ – William Scales Apr 22 at 23:09

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