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Sensitive LCR Meter Measurements
Martin72:
--- Quote ---3 ohms is considerable, try measuring between the static plunger and the BNC shell for each. The lever arm doesn't really matter as it's not conducting nor shielding any signal. The BNCs provide the connection back to the LCR Meter are important wrt any shield currents getting back to the instrument.
--- End quote ---
I had measured everything, that was just the most “drastic” difference, which I then photographed.
I am no stranger to good shielding in principle.
mawyatt:
Along the lines of sensitive measurements with typical Kelvin Clips utilized often with LCR Meters and DMMs, here's some information that users might find interesting wrt to very low Z measurements.
A note clipped from a Hioki data sheet illustrates the importance of "Proper" Zero/Short Meter/Accessory Compensation. We've added some notes of our own to help illustrate the effects of reversing the Kelvin Clips and why the proper orientation for a quality Zero compensation is important for quality low Z measurements.
With a proper Clip orientation with the Kelvin tips Hcur and Lcur directly on top each other as shown, the LCR Meter Forcing Current flows thru from the High Force side (Hcur) directly to the Low side (Lcur) and little current flows towards the Sense terminals (Hpot & Lpot) because of the high impedance these terminals present. In our notes because of this Ix ~0 since Id and Ic ~0. Under these conditions then Lpot and Hpot sense the same voltage Vx which is the mid terminal connection point between Hcur and Lcur, and the meter computes the effective Z as ~0 since Z = Vdut/Idut and Vdut is VHpot-VLpot or 0 since they each read as Vx as shown in the schematic.
When the Kelvin tips are reversed with Hcur now directly connected on top to Lpot, then Hpot, and Lcur, the full DUT Force current flows thru all 4 Kelvin Tip ends and introduces a voltage drop between Hpot and Lpot which creates a DUT effective impedance as shown as Rx in our notes.
Of course things get a little more involved and as frequency increases things can get way more involved, but this serves to illustrate the concept.
To show these effects we've employed our Tonghui TH2830 LCR Meter with TH26011CS Kelvin Clips. These images show the Kelvin Clip connections on a Guard Plate, proper and reversed orientation and the subsequence meter readings.
First a Zero/Short Compensation was preformed with the Proper Clip orientation, then the Clips disconnected and reconnected to show the Zero Reading, then Clips were reversed (L Kelvin Clip flipped over) and measurements repeated. 16 averages were used at results shown at 100Hz, repeated measurements with connect and disconnect showed reversed connection variation from 2 to ~4.5 milli-ohms, whereas with proper orientation these variations were under 20 micro- ohms (Note the Th2830 resolution is 10 micro-ohms).
Would be interesting to see what others find with their LCR Meters and Kelvin Clips.
Anyway this is a subtle point wrt to Kelvin Clip usage for low Z measurements, and hope some find this useful.
Best,
mawyatt:
We've added results for the TH2830 Rdc Measurements and the Keithley DMM6500. The TH2830 larger Rdc 46 micro-ohms with Proper Orientation is due to the Thermal EMF in the Kelvin Clips and Instrument, this effect is negated with AC measurements.
Note negative readings with reversed Kelvin Clip Zero measurement, reason is left as reader exercise ;)
Best,
EC8010:
I had to stop and think a bit about those Kelvin clip strictures.
Seeing Martin72's internal photographs, I had to open up my SMD fixture. It's identical to his expensive fixture (although the screen is steel) but has Hameg HZ188 written on the outside because it came with a Hameg 8118. I seem to remember seeing a Digimess LCR tester from before 2007 that had a similar jig.
mawyatt:
Thanks to Martin for showing the ST version :-+
Think the ST and many others from Tier 1 sources (R&S) are directly rebrands from OEM Tonghui, the lower Tier sources are various clones of the Tonghui with varying degrees of quality and various rebrands.
We have a couple clone fixtures, one is very well made, the other not so!! At lease on the inferior SMD fixture, the plungers seem the same and good quality, can't say so for the case tho :P
We decided to "repurpose" the inferior SMD fixture into a self contained SMD fixture which will allow external DC Bias applied to the DUT. This works really well and is compact and more convenient for SMD devices than using the DC Bias Adapter we developed for LCR Fixtures, which works well with other LCR Meter fixtures such as, Leaded, Kelvin Clip & Tweezers.
Also check out the Split-Kelvin SMD Fixture here:
https://www.eevblog.com/forum/projects/different-type-lcr-smd-fixture/
So we've pretty much got all the bases covered for SMD devices ;)
BTW if you use Kelvin Clips please note posts 71 and 72, the Zero Ohm Compensation does make a difference with Kelvin Clip Tip Orientation as shown :-+
Best,
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