Test Equipment Recommendations for Measuring USB Cable Resistance

[reborn1212]

I am looking for tools with somewhat decent accuracy (about 2% in the few dozens to over a hundred milliohms range). After some studying, I listed some choices (in conjunction with USB breakout boards);

1) Micro-Ohmmeter
2) LCR Meter
3) Power Supply and Electronic Load

I am a hobbyist and have about a sub-2000 USD budget for this. I am aware of DIY methods to achieve what I want, but I prefer tested products with known specifications across all measured ranges.
Also, I am looking to expand my toolset with good value for money items. Recommendations of tools with make / model are highly appreciated.

On a side note: Using USB breakout boards inevitably adds significant error when measuring. Any advice on how to mitigate this?

[srb1954]

I am looking for tools with somewhat decent accuracy (about 2% in the few dozens to over a hundred milliohms range). After some studying, I listed some choices (in conjunction with USB breakout boards);

1) Micro-Ohmmeter
2) LCR Meter
3) Power Supply and Electronic Load

I am a hobbyist and have about a sub-2000 USD budget for this. I am aware of DIY methods to achieve what I want, but I prefer tested products with known specifications across all measured ranges.
Also, I am looking to expand my toolset with good value for money items. Recommendations of tools with make / model are highly appreciated.

On a side note: Using USB breakout boards inevitably adds significant error when measuring. Any advice on how to mitigate this?

A good quality bench multimeter with 4-wire resistance measurement capability would also be suitable and you could easily obtain reading resolutions to 1milli-ohm or lower. The ever popular HP 34401A would be a good choice and is fairly readily available on the second-hand market for reasonable prices.

The secret of getting reliable, accurate measurements of low resistances is to use Kelvin probes, which eliminate the errors due to the resistance of your test leads and variations in the connection resistance of your test clips. A good set of Kelvin probes won't be cheap and a name brand might cost almost as much as second-hand multimeter. However, you can buy the special Kelvin clips reasonably cheaply and make up your own probes.

If your are testing cables already terminated with connectors you will still have to use a USB socket to connect to but a break-out board shouldn't really be necessary as suitable Kelvin probes should be able to make direct connection to a THT USB connector.

I suggest you do some research on 4-wire resistance measurement techniques. The Keithley handbook "Low Level Measurements" is a good starting point for information on this and many other tricky measurement problems.

[Martin72]

What micro/milli-ohm-meters concerns, I´m happy with my used bought hioki 3450.
At work I´ve bought a calibrated one from GwInstek with deliver up to 1A into the testobject.


https://www.reichelt.de/de/en/gw-instek-gom-804-dc-milliohm-meter-gom-804-p176003.html?r=1

[Tjuurko]

Using USB breakout boards inevitably adds significant error when measuring. Any advice on how to mitigate this?

Make a homemade ultra-short cable with USB-connectors and use it as a "zero".

[reborn1212]

The secret of getting reliable, accurate measurements of low resistances is to use Kelvin probes, which eliminate the errors due to the resistance of your test leads and variations in the connection resistance of your test clips.

I know about 4-wire measurements with Kelvin probes and the reasoning behind it, just am unsure which benchtop DMMs with this capability and the accuracy I require are within my budget. I'll look up the HP 34401A.

If your are testing cables already terminated with connectors you will still have to use a USB socket to connect to but a break-out board shouldn't really be necessary as suitable Kelvin probes should be able to make direct connection to a THT USB connector.

I found some THT USB-A connectors, but are THT USB-C connectors commonplace as well? I have several A-C and C-C cables I want to test. Also, do Kelvin probes come in sizes or are one-size-fits-all?

[reborn1212]

What micro/milli-ohm-meters concerns, I´m happy with my used bought hioki 3450.
At work I´ve bought a calibrated one from GwInstek with deliver up to 1A into the testobject.

https://www.reichelt.de/de/en/gw-instek-gom-804-dc-milliohm-meter-gom-804-p176003.html?r=1

I think you mean the Hioki 3540. The GW Instek one in the link seems to suit my applications; will consider.
Side question; will a micro-ohm meter's performance outstrip a LCR meter / benchtop DMM by a lot in my current budget range?

Using USB breakout boards inevitably adds significant error when measuring. Any advice on how to mitigate this?

Make a homemade ultra-short cable with USB-connectors and use it as a "zero".

That can work, but the above suggestion of using THT connectors that have pins I can access with Kelvin probes seem to be a more elegant solution.

[srb1954]

I know about 4-wire measurements with Kelvin probes and the reasoning behind it, just am unsure which benchtop DMMs with this capability and the accuracy I require are within my budget. I'll look up the HP 34401A.

Practically all bench multimeters of 6 1/2 digits or better provide 4-wire resistance measurement capability. Many 5 1/2 digit bench meters also provide for 4W measurements but this feature becomes quite rare on meters of 4 1/2 digit and below or for hand-held meters.

[srb1954]

What micro/milli-ohm-meters concerns, I´m happy with my used bought hioki 3450.
At work I´ve bought a calibrated one from GwInstek with deliver up to 1A into the testobject.

https://www.reichelt.de/de/en/gw-instek-gom-804-dc-milliohm-meter-gom-804-p176003.html?r=1

I think you mean the Hioki 3540. The GW Instek one in the link seems to suit my applications; will consider.
Side question; will a micro-ohm meter's performance outstrip a LCR meter / benchtop DMM by a lot in my current budget range?

A dedicated micro-ohmmeter often offers additional flexibility in selecting the test voltages and test currents to be applied to the circuit under test. This can be very useful when verifying the contact resistance of connectors, especially at low voltages and low currents where surface contamination of the connector pins can have a major adverse impact on the connector resistance.

A multimeter usually doesn't offer any ability to select test conditions for resistance measurements i.e. for any resistance range the test voltage and test current are fixed in the design of the meter.

[tautech]

I know about 4-wire measurements with Kelvin probes and the reasoning behind it, just am unsure which benchtop DMMs with this capability and the accuracy I require are within my budget. I'll look up the HP 34401A.

Practically all bench multimeters of 6 1/2 digits or better provide 4-wire resistance measurement capability. Many 5 1/2 digit bench meters also provide for 4W measurements but this feature becomes quite rare on meters of 4 1/2 digit and below or for hand-held meters.

Yet the $ 399 4.5 digit SDM3045X has 4W just like its more expensive 5.5 and 6.5 digit siblings.

[Henrik_V]

While getting a 4W (milli) Ohmmeter is easy , you want to measure the cable , not the connector
While applying a 4W measurement on a USB C connector with it's build in Kelvin contacts , connecting a USB A/B (mini/micro/...) pin with two contacts will need some DIY connector mechanics. Not impossible, but assuming some 100 cycles not an easy part.


Another testing method: Apply more current and apply a thermometer (FLIR) 

[reborn1212]

While getting a 4W (milli) Ohmmeter is easy , you want to measure the cable , not the connector
While applying a 4W measurement on a USB C connector with it's build in Kelvin contacts , connecting a USB A/B (mini/micro/...) pin with two contacts will need some DIY connector mechanics. Not impossible, but assuming some 100 cycles not an easy part.


Another testing method: Apply more current and apply a thermometer (FLIR) 

There are USB C connectors with built-in Kelvin contacts??

[alm]

I don't have a datasheet handy, but I'd think that a general purpose bench meter with its low test current won't be very stable or accurate in the dozens of milliohms range. With the low voltage across the DUT noise and thermal EMFs might become an issue. Dedicated milliohm meters, or solutions with a current source and sensitive voltmeter (SMU, separate current source), can mitigate this with a higher test current and by either switching polarity of the current source or pulsing it on and off.

On a side note: Using USB breakout boards inevitably adds significant error when measuring. Any advice on how to mitigate this?

I think the best you could do is design your own breakout board with Kelvin connections right from the connector pins. This doesn't eliminate the resistance of the connector but does eliminate the resistance of the breakout board and connections.

You don't need 4W probes. Any way that separates current carrying conductors from voltage sensing conductors will work equally well.

[Someone]

I don't have a datasheet handy, but I'd think that a general purpose bench meter with its low test current won't be very stable or accurate in the dozens of milliohms range.

Just read the data sheets and pick one that does....
34461, 100Ohm range, 1mA, +/- 4mOhm error
DMM6500, 1Ohm range, 10mA, +/- 0.2mOhm error

Its getting to the corners of "normal" but still pretty comfortable.

[alm]

34461, 100Ohm range, 1mA, +/- 4mOhm error
DMM6500, 1Ohm range, 10mA, +/- 0.2mOhm error

So for 10 mOhm, the uncertainty would be +/- 40% for the 34461A. And the DMM6500 2%. So the 34461A is clearly not sufficient for dozens of mOhms with 2% accuracy, and the Keithley barely. The Keithley has offset compensation and a higher test current, so could probably barely do this measurement. But a dedicated milliOhm meter will likely be better.

I have a Ballantine 3205B (up to 1A test current) and Keithley 580 (up to 100mA test current, thermal offset calculation). The Keithley would measure 10 mOhm with 0.24% uncertainty. I'm not sure about what is currently on the market. I think Keithley wants to sell you a current source + nanovolt meter.

[Someone]

So for 10 mOhm, the uncertainty would be +/- 40% for the 34461A. And the DMM6500 2%. So the 34461A is clearly not sufficient for dozens of mOhms with 2% accuracy, and the Keithley barely. The Keithley has offset compensation and a higher test current, so could probably barely do this measurement.

The OP asked for 2% in "few dozens" milliohms, and there is at least one bench meter that gets there. That's not barely, that's guaranteed within worst case.

All that was needed was a quick perusal of data sheets. So this:

I don't have a datasheet handy, but I'd think that a general purpose bench meter with its low test current won't be very stable or accurate in the dozens of milliohms range.

Pure guessing, and shown to be wrong (also with an example of one bench meter that is poor in the same range despite having 4 wire mode). All it takes is picking a suitable bench meter. The bigger challenge is probing/fixturing to isolate the actual characteristic wanting to be measured.

[switchabl]

I think the test current is really the elephant in the room here. There has been some discussion about contact resistance issues at low currents but the resistance will change at high currents as well. The cable will heat up and copper has a temperature coefficient of about 0.4%/K. Well, there goes your error budget.

So what does that mean? First and foremost that you need to make a decision about your desired test conditions. This will also inform your choice of test setup and equipment. The USB-IF specifies the allowed voltage drop in the cable assembly at maximum rated VBUS current. This makes a lot of sense because that is usually the scenario you are worried about in actual use. Depending on the cable that could be as low as 500mA (USB 2.0) and as high as 5A (100W USB-PD).

A normal multimeter will not do 500mA (let alone 5A). You can definitely get SMUs that do 5A (and probably some specialized milliohm meters as well) but maybe not within your budget. A more budget-friendly solution could be a power supply with accurate current readback (e.g. used Agilent 66332A is readily available and does ± 0.5mA ± 0.3%) along with a multimeter for measuring the voltage drop. Really any proper bench multimeter would do, even a cheap old 34401A (DMM6500 is nice if you want a more modern one).

Of course, you are not doing actual compliance testing, so you are free to measure at low current if that is what you care about. In that case a DMM6500 in 4-wire mode might be all you need.

BTW the USB specification also includes "representative receptables" in the measurement along with the cable assembly. In that case, the fixture could simply be a USB connector on a PCB along with 4x banana jacks connected to the power/GND pins in a Kelvin configuration. You might want to compare different connector models to figure out exactly what is a "representative receptable" (or buy a USB-IF approved test fixture). You also need to test the repeatability of the connection. Your 2% error budget may turn out to be too optimistic.