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Cheap SMD LCR Fixture, the Good, Bad and Ugly
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ExaLab:
If your main goal is to measure low impedance components, the "open" calibration assumes a marginal role and in some cases can be avoided.
Anyway, it is always possible to use a high value resistor to perform the open calibration.
As example, if you intend to characterize resistors with a value lower than 10 ohms and you carry out the open cal using a 10 Mohm resistor, the induced error on the measurement should be less than 1 ppm...
mawyatt:
That's a good idea just using a much larger Z device that "bridges" the gap.

Curiosity got to me and I got a couple 100 ohm 1206 resistors and placed them in the "teeth" of some Kelvin Clips, this separates the Force and Sense functions by 100 ohms. Did a quick Open Cal on our TH2830 (can't use the IM3536 at this movement, fan makes too much noise and bothers family!!), then normal Short Cal and measured 0.1 and 1 ohm Dale 1% resistors (large aluminum cased type), and a Caddock 10K 0.01%. Then repeated this using a normal Open Cal.

This wasn't anything other than to "see" how the Open Cal responded to the Force and Sense being separated by 100 ohms, and suspected little variation, which is what we got!!

So now we got the "bug" to roll a version with separate Force and Sense lines to the DUT and shunted by a couple resistors. Since our PCBs are in the production "Que" at the fab over the weekend, we might sneak a quickly prepared version of the LCR Fixture PCB with separate Force and Sense DUT PADs. Maybe a version without the on PCB BNCs, where these would be in a box rather than on the PCB (thinking here is better mechanical stability when pushing the Lever Arm). Might even split the DUT PADs uneven to give emphasis to the Force side which handles the current.

Anyway, thanks for the dialog :-+

Best,
ExaLab:

--- Quote from: mawyatt on September 03, 2023, 12:47:06 am ---This wasn't anything other than to "see" how the Open Cal responded to the Force and Sense being separated by 100 ohms, and suspected little variation, which is what we got!!

--- End quote ---

I have no idea what the input impedance of the sense line might be. If this were ideally infinite, the error that emerges by carrying out the open cal using two 100 ohm bridge resistors would essentially be linked to the capacitance of the measurement cable and should decrease reducing the test frequency.

If the discrepancy you observe is also present in the DC measurements, this means that the instrument's equivalent input resistive component has its weight!
mawyatt:

--- Quote from: ExaLab on September 03, 2023, 11:13:08 am ---
--- Quote from: mawyatt on September 03, 2023, 12:47:06 am ---This wasn't anything other than to "see" how the Open Cal responded to the Force and Sense being separated by 100 ohms, and suspected little variation, which is what we got!!

--- End quote ---

I have no idea what the input impedance of the sense line might be. If this were ideally infinite, the error that emerges by carrying out the open cal using two 100 ohm bridge resistors would essentially be linked to the capacitance of the measurement cable and should decrease reducing the test frequency.

If the discrepancy you observe is also present in the DC measurements, this means that the instrument's equivalent input resistive component has its weight!

--- End quote ---

Exactly, and we got what was expected. Little variation in DC and a small variation at 10 and 100KHz due to capacitive (Kelvin cable) effects. Since the intent of these custom LCR fixture(s) (working on PCB layout of the split Force/Sense DUT Pad version now) is for low impedance devices and the fixture doesn't have the long cable and capacitance of the Kelvin Leads we expect the overall results to be better than our cheap LCR SMD fixture for low impedance SMD devices.

If this behaves as expected, then "tempted" to cannibalize one of the cheap LCR SMD fixtures to get those nice male BNCs with the lever arms (can't find these anywhere) and maybe use the fixture base with the plunger mechanisms removed as the base for the low impedance SMD fixture, or just mount the fixture PCB on top of a metal case with female BNCs and use BNC male to male adapters to interface with the LCR Meter female BNCs.

Anyway, we'll see how this progresses as time permits, as we have a "paying" project with PCBs in fab now which must take precedence.

Best,
mawyatt:
Update.

We got a change to try the split contact area concept for the Lever Arm type SMD fixture. Still needs some work, as this was just a kludge setup using a split BNC cables for the Force and Sense leads to the LCR meter. Later when time permits we may roll a direct BNC to LCR meter fixture with the split contact area.

Here's some results with low valued 2512 resistors using a zero ohm 2512 for short cal. on the TH2830, and for comparison using a SMD fixture on the IM3536. Number is () is repeated measurement by removing DUT and later reinserting such.

Type                                    TH2830                                       IM3536

2512 1% 10mohm                9.949mohm (9.951)                     -9mohm (6.68) unstable readings
              40mohm                39.840mohm (39.848)                 21.38mohm (31.54) somewhat erratic
              50mohm               49.993mohm (49.994)                  42.09mohm (45.39)
              100mohm              100.317mohm (100.332)             81.52mohm (93.57)   

Getting good repeatable results is much easier, altho still a task, with the Lever Arm Fixture for these low valued resistors.

Here's what things look like and we'll be working with this later when time permits, and likely rolling a direct BNC connecting PCB.

Anyway, just an update, hope some folks find this interesting as we've found getting good repeatable low Z measurements with SMD is not trivial.

Edit: These are DCR measurements. Also no disrespect for the standard SMD fixture, these aren't intended for low Z measurements. If one studies the design the Kelvin type connections are terminated on a long thin brass bolt and screws into the bottom of the brass plated plungers, so everything after the thin brass bolt head is not part of the Kelvin measurement. This includes the plungers, and the plunger to DUT contact surfaces, which can only be compensated for by the Short Cal. Whereas the Lever Arm structure with the split contact area, the DUT contact "ends" make the Kelvin termination, and thus should improve results for low Z components, which apparently works is some cases as we've demonstrated.

BTW, if some folks want to participate we might roll the next PCB with gold contacts, which should improve contact area performance.

Best,

                     
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