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Measuring current (via sense resistor) with a coaxial cable assembly is unecessary?

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grizedale

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Hello,

Please could you tell me if the following current sense resistor voltage measurement technique, which is the standard procedure at a HUGE international telecoms company, is correct?

It concerns the measurment of current (~16A) consumed by the FPGA at the output of a 500KHz, 0.9V buck converter.

The current is measured by using a current sense resistor of 2 milliohms in series with the buck inductor…….(we cannot place the current sense resistor anywhere else because the PCB trace is too wide to cut away…..so the sense resistor “sees” the buck inductor ripple current)

Anyway, the basic method used is to get a 1 metre coaxial cable assembly and connect it to the current sense resistor, and then put this into a Agilent 34401A Bench multimeter….via a BNC to 4mm-plug adapter.

Agilent 34401A meter:
**broken link removed**




The exact method used was:

Get 1 metre length of 50 Ohm coaxial cable.
Solder a 47R axial resistor to the innner conductor at one end of the cable.
Then solder the free end of this 47R resistor to one end of the current sense resistor.
Then "peel away" some of the outer conductor of the cable, and solder that to the other end of the current sense resistor.
Then put a BNC connector on the other end of the cable.
Then put this into a BNC to 4mm plug adapter.
(-An RCR filter is then added, and this is because page 218 of the 34401A meter datasheet states that it can only measure frequencies up to 300KHz.)
-then finally connect this to the input of a Agilent 34401A bench multimeter.




Anyway, the input impedance of the 34401A multimeter is a lot more than 50 Ohms, (page 219 of manual says its 1Megohm) so why do we bother putting the 47R resistor at the end of the coaxial cable?…it surely won’t provide a matching function?




Also, sometimes it's fiendishly difficult to solder the "peeled away" outer connector of the cable to the current sense resistor, so a short, 1 inch piece of twisted pair wires is then used at the end of this coaxial cable assembly to make it easier to solder-connect it to the current sense resistor.....is this acceptable procedure?,
or would that 1 inch piece of twisted pair wires infect the reading with too much noise?

Also, since we are using a heavy RC filter to filter away the ripple, is it really worth us using a coaxial cable assembly?, after all, all the high frequencies are getting filtered out of the measurement, so why don’t we just twist some wires together and run them from the sense resistor to the 34401A inputs?
 

Sounds like a mixture of wideband current sense setup (e.g. to be used to display waveforms on an oscilloscope) and a simple DC measurement.

You can try yourself by simply probing the resistor voltage with regular multimeter probe cables, optionally twisting them. I would expect identical results in all cases.

Personally I don't feel motivated to fight other people's unreasonable process specifications as long as they do no harm or cost my time or money.
 
OK thanks, but using the coax does take time and money, because its a pain to get the tool that you use to connect the BNC connector to the coax cable, and also soldering the other end of the coax to the 47R resistor and to the PCB is a real fiddle.

also, coax cable in small lengths is hard to buy and has to be bought in big reels which is very expensive.
 

Apart from necessity for the present problem, all required tools are available from a qualified electronics DIY shop or mail order supply, I think. You even get all coaxial cable types in single meter quantities from some professional catalog distributors, at least in my country.
 
Ok Thanks, but you do agree that this is "unecessary process specification" and that there is simply no need for using coxial cable, and a simple pair of twisted wires is all that's needed?

After all, the sense voltage is getting heavily filtered, so coaxial cable, which is designed to pass microwave frequencies, is completely unecessary as you seem to say?

Bringing , for the first time, common mode noise into the discussion now......
Common mode noise tends to afflict measurements generally, but coaxial cable certainly doesnt get rid of common mode noise, -just look at any scope probe measurement when there's an smps around, its festooned with common mode noise.
 
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Are you only trying to measure the average current, or the ripple as well? For average current a LPF and a precision voltmeter should be all that's necessary. Coax usually becomes necessary when you want to see ripple.
 

i am only wanting to measure the average current......and do you agree that twisted pair wires are all thats needed?..ie not coax?
 

Twisted pair probably isn't even necessary. Any kind of test lead or wire should do for DC signals, so long as it's filtered properly.
 

so coaxial cable, which is designed to pass microwave frequencies, is completely unecessary as you seem to say?
Yes, as I said in my first post. Apparently the probe has been originally designed for wide band measurements.
 

so this huge(!!!) telecoms company, one which you will have heard of, are doing things wrong?
 

I wouldn't say it's wrong per say. Just unnecessary. I use spliced coax often for testing circuits. But when I do that I always terminate the other end with 50ohms, otherwise the measurement will be screwed up at high frequencies.

But for DC high impedance measurements, the cabling shouldn't matter at all (unless you're going for extreme precision where thermocouple effects become an issue... but that's probably not the case for you).
 

thanks mtwieg,

but thermocouple's tend to use those green and white twiste pair wires, and not coaxial cable?

I presume you mean that you add the matching 50 Ohms in situations where you are not connecting to a scope which already has the 50 Ohm impedance in its input BNC port?
 

but thermocouple's tend to use those green and white twiste pair wires, and not coaxial cable?
When I say thermocouple effects, I'm talking about parasitic thermocouples created from solder joints, alligator clips, etc. When temperature differentials exist they can create DC voltage offsets in the range of a few nV to a few uV.

I presume you mean that you add the matching 50 Ohms in situations where you are not connecting to a scope which already has the 50 Ohm impedance in its input BNC port?
Right, when using coax for high frequency measurement you need to have the destination terminated (though not necessarily the source).
 

Right, when using coax for high frequency measurement you need to have the destination terminated (though not necessarily the source).
As discussed in a related thread, source side series termination without load termination (high impedance oscilloscope input) can make sense though. It's pretty standard for a number of problems in instrumentation. You are probably using it without thinking when connecting a 50 ohm pulse generator output to an 1 MOhm oscilloscope input through coax cable. Please consider how the waveform would look like with zero source impedance.

https://www.edaboard.com/threads/254629/
 
Yes, terminating the source only can work as well, though for me it's usually more convenient to terminate at the load (adding a BNC T and a terminator, vs having to solder a 50 ohm resistor on the board). I usually AC couple at the source with a blocking capacitor for AC measurements, so this works fine. But I suppose if you want to also see the DC component of the signal, then source termination makes more sense.
 
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