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Sensitive LCR Meter Measurements

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mawyatt:
Following up on the issue with the Kelvin Type clips we decided to do a quick experiment to see how well the quality Tonghui TH26009B Shielded Tweezers would compare to a direct connected LCR Meter SMD fixture shown here:

For some reason link doesn't work, so search for:

"cheap-smd-lcr-fixture-the-good-bad-and-ugly!"

Honestly, we don't have a quality SMD fixture (know we should have, but don't), however we do have a pair of cheap clone types that have been modified (see above) that produce highly repeatable results. Like < 0.01pF with small capacitance SMD chips, so no fear in utilizing these SMD fixtures as a reference to compare the TH26009B Tweezers with.

First off we setup the SMD fixture and did the usual Short and Open Cal with the IM3536. For the Open we pried the plunger Opening ~ same as 0603 device, to Cal out the parasitic capacitances at that spacing (important if one is doing small capacitance measurements like 1pF!!). Precision C0G type 0603 values of 1, 10, 100 and 1000pF were selected for comparisons.

After the SMD fixture measurements we setup the TH Tweezers on top of a conductive plate (PC Bite) connected to the IM3536 Guard Terminal. Operating with the IM3536 Guard Enabled has shown better repeatability with all cable type clips with high Z measurements, so we use this when necessary. We also spaced the Tweezers ~ same as 0603 spacing for Open Cal for same reasons.

Here's the IM3536 results at 1MHz:

Capacitance             SMD Fixture                   TH26009B
1pF                         1.0004pF                       0.9937pF
10pF                       9.9692pF                       9.9303pF
100pF                     99.9551pF                     99.7126pF
1nF                         992.236pF                     992.838pF

The results produced were stable and little variations.

This shows that quality Tweezer Kelvin Types like TH26009B which employs shielded conductors right to the tweezer tips can produce good results with small DUT capacitances with carefully attention to setup and measurement details.

However, because the Kelvin connection is not made at the very tip end, one would expect results with very small impedances to suffer. Here even the SMD fixtures mentioned above aren't going to be optimum as the Kelvin connection on these types is at the plunger body and not the plunger tip. This was one reason we developed the experimental fixture type shown here and it's various refinements:

https://www.eevblog.com/forum/projects/different-type-lcr-smd-fixture/msg5061028/#msg5061028

Anyway, seems the Quality Tweezers like the TH26009B which carry the signal shields to the tips are capable of quality, repeatable measurements with small capacitance DUTs (high Z), even at higher frequencies.

Best,


mawyatt:
This effort created another where we wanted to see how well things worked with low Z SMDs, so selected some precision low ohms 2512 SMDs.

After a few futile efforts we gave up on the TH26009B Tweezers, too difficult to create a good repeatable Zero Ohms Cal, and the readings were unstable jumping around too much for our liking. We needed calibrated hands ???

Maybe if we had more patience, but were somewhat in a hurry so proceeded to the SMD Fixture mentioned above in "Good, Bad and Ugly" which for some reason link didn't work, so you'll need to do a search for the thread. This fixture directly mounts to the IM3536 and should produce better results, which it did but far from what we would have liked!!

Some difficultly with good Zero Ohms Cal, remember we're talking about sub-milliohms now!! Readings took awhile to stabilize also, so room for improvement!!

Then we engaged the Special SMD Fixture approach indicated here:

https://www.eevblog.com/forum/projects/different-type-lcr-smd-fixture/msg5061028/#msg5061028

This technique works with the Split Kelvin Connections Force-Sense right up to the DUT SMD and allowing the DUT SMD end caps to make the Force-Sense Kelvin connection. Getting a good Zero Ohm Cal is still tricky, but nowhere as difficult as with the Standard SMD Fixture, or the Tweezers, and getting good stable readings is a breeze comparatively :-+

We first did the Special SMD Fixture, then the Standard SMD Fixture. Afterwards we decided to reinstall the Special Fixture, do a quick Zero Cal, and rerun the test DUT Resistors, note the repeatability in the Special SMD Fixture results.

Resistor Value
in Milliohms       Special SMD Fixture  (Repeat see text above)                      Standard SMD Fixture     

1                       0.80                           (0.80)      {0.82}     <0.81>                       ~1.23
5                       4.84                           (4.82)      {4.82}     <4.86>                     ~5.45
10                     9.90                           (9.87)      {9.86}      <9.92>                     ~11.98
15                     14.70                         (14.71)     {14.72}    <14.73>                     ~13.29
20                     19.83                         (19.84)     {19.85}    <19.85>                    ~17.84
40                     39.87                         (39.88)     {39.88}    <39.88>                      44.87
50                     49.87                         (49.91)     {49.91}    <49.93>                       48.29
100                   100.21                       (100.23)    {100.21}  <100.27>                       99.28 
   
"~" indicates some variability in readings

Anyway, the Special SMD fixture seems to be a good candidate fixture for low Z SMD LCR Meter measurements and curious to see if the Guard Plate under the SMD measurement provides additional benefit (see link above).

Edit: 4~5 hours later redid the measurements shown as {#} for additional results.

Edit: From <#> post 24 below.

Best                       

exmadscientist:
This seems like a good thread for dumping one of my old observations:

When using an Agilent E4980A LCR meter or its predecessor the 4284A, with leads longer than I'd like but difficult to make better, doing the open and short corrections twice (so open-short-open-short or vice versa) gave significantly better results, especially in terms of repeatability. I can only guess that it's easier to dial in a small correction than a large one, and the first round correction makes the second round smaller?

Either way -- I've done that ever since when using an LCR meter and trying to get any level of precision.

mawyatt:
Good point, will give that a try next time and see if it improves the Calibrations. Not sure how these Calibration Measurements are utilized internally within the LCR Meter, but doing it twice certainly can't hurt!!

Best,

mawyatt:
Just turned power on to the IM3536, no Calibration (using previous Calibration from yesterday), and started measuring the pervious 2512 Precision Resistors.

This is with no warm up, no Calibration, and just quickly making measurements.

Note results added in <#> in above post number 21.

Here's what we use for Zero Ohm Calibrations, short lengths of #12 Cu wire with ends filed smooth for Standard SMD Fixture. For Special SMD fixture is a short flat rectangular conductor slightly bent and ends rounded, also shown is an "L" #12 Cu wire we use for Kelvin Clips.

2512 test resistors are 1,5,10,15,20,40,50 & 100 milli-ohms.

Best,

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