Electronics > Metrology

PCR versus TCR

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--- Quote from: Zeranin on April 21, 2016, 12:50:04 pm ---The Kelvin sensing appears to be incredibly primitive. It looks as if the sense connection is made to the high-current conductor, just beyond where the wires enter the package. The sense connections should be at the resistive foil, not several inches back along the tinned-copper high-current connecting wire. What am I missing?

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One current conductor is on top side of all the chips, covering with thick tin. partially can be seen.
The other current conductor is similar, go thru all the chips at bottom side, torn from the chips.
Kevin sensing are two thin wires(one can be seen) soldered to the middle of the two current conductors.

Alex Nikitin:

--- Quote from: Zeranin on April 21, 2016, 01:01:19 pm ---
--- Quote from: Alex Nikitin on April 21, 2016, 11:10:44 am ---I still think that your amplifier noise can be improved and the shunt voltage (and dissipation) can be lowered, reducing all power-related effects. The self-noise of the shunt is very low compared to 40 Ohm or so of the amplifier equivalent noise. Thermoelectric effects may be easier to deal with.

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We appear to be at cross purposes. The Johnson self noise of the 0R1 shunt itself is so low as to be neglected. If we lower the shunt resistance, then the shunt noise is of course even less, but that's irrelevant. It doesn't matter how you cut it though, if you lower the shunt resistance, then you get less signal, and this degrades SNR at the shunt preamp output. Reducing the shunt to 0R05 (which I have) wouldn't degrade the noise performance dramatically, and may provide a better all round balance.

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What I've meant that your amplifier appears relatively noisy (~80nV RMS noise in 10kHz BW, or 0.8nV/rtHz, equivalent of ~40 Ohm resistor noise at 300K) . If you use multiple amplifiers in parallel to achieve a lower noise, this figure looks excessive.



The Kelvin sensing looks a little primitive, but it is not that bad as one might think at first. The copper / solder part is mainly responsible for spreading the current over the parallel connected resistor elements. As the length of the copper path is the same for all elements the current is evenly divided in the resistors, even if the copper part changes resistance. Then there is not much tin /copper in the path sensed by the voltage sense connectors. I agree it could have been done better, but as long as the parts stays stable this should not be such a bad thing. The copper/tin part might add a little to the TC, but not much. 

The main part I would be afraid of would be aging in the tin part, as this is a relatively low meting point alloy with normally a fine structure, it can change even at room temperature or slightly above. As it's potted there should be no tin whiskers growing.

As for the amplifier a value of 0.8 nV / Sqrt(Hz) is not that bad, but a parallel connection of 4 amps could give you about halve the noise and thus allow for the reduced shunt size. It's a trade of in spending money for the shunt or the amplifiers.


--- Quote from: Zeranin on April 21, 2016, 03:13:43 am ---If we agree that the zeranin is effectively unconstrained in the Z-direction, and I insist this is the case, then at first glance the explanation for the 5ppm drift is easy. The zeranin is mechanically constrained in the  X &Y directions, in the plane of the material, because it is mechanically bonded to an aluminium substrate that is much thicker than it is. Easy peasy. Zeranin expansion coefficient is +18ppm/K. The zeranin heats by 0.25K with respect to the substrate, and as a result, expands by 4.5ppm (18x0.25) in the Z direction, but is prevented from expanding in X and Y, and this will have the result of decreasing the resistance by 4.5ppm, just as observed.

Unfortunately there is a serious problem with this explanation. What’s good for the goose is also good for the gander. We would expect the same behaviour when measuring the R-T curve, but not so in practice. When the zeranin temperature is changed in unison with the heatsinking aluminium plate, as per the R-T curve, there is essentially no change in resistance, yet there will indisputably still be the same expansion of the zeranin in the Z-direction, with any increase in temperature. The zeranin will still be constrained in the X & Y directions, though there will now be a small change in length in X and Y due to the different COEs of zeranin and aluminium. However, this won’t have any effect on the zeranin resistance, because an equal change in X and Y (a length and crosss section term) cancel, the increase in length cancelling the increase in width as far as resistance is concerned. Drats! You can try until the cows come home, but this explanation just won’t work. Sure, you can explain as above why the zeranin changes resistance when it self-heats above it’s substrate, but then you won’t be able to explain why the same thing doesn’t happen when both are heated in unison.
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Zeranin- Thanks for bringing your problem to the forum and for your detailed descriptions and responses.  Everyone loves a mystery!

I would like to question your assumption that the Zeranin is constrained in the X and Y axis.  As-built, the Aluminum X & Y are locked to the Zeranin X & Y through the heat-bonding, electrically insulating film.  But that film has mechanical compliance and its own TCE (which is safe to ignore).  The mechanical compliance can be thought of as an array of stiff springs connecting each Al X-Y coordinate to the same Zeranin X-Y coordinate at room temperature (as-built).  Now when heated uniformly in an oven (no current), the Al and Zeranin want to grow to two different sizes because of the differing TCEs but they can't because they are constrained by all these stiff springs.  So, does the Al get compressed to match the Zeranin hot dimensions or does the Zeranin get stretched to match the Al hot dimensions?  My guess is that the Al, even though it's softer, is thicker and ultimately has the higher stiffness.  So the Zeranin gets stretched when the assembly is heated.  But the springs of the insulating film also get stretched such that the hot X-Y coordinates no longer match; the Zeranin is stretched to slightly smaller X-Y dimensions than the Al.  In this regard, the rubbery thermal interface may have and advantage because it has more "give" (weaker springs).

Also recognize that in the dimension of the zig-zags of the Zeranin, there are gaps which will take up a lot of the differential TCE in that dimension.  However, these put additional strain on the corners where the zig bends to become a zag.  But I agree, that within each 25mm zig and zag the Zeranin is still getting stretched in both dimensions.

Now apply current.  The Zeranin gets 0.25K warmer and would have slightly larger dimensions, if it weren't constrained, by an amount I'll call dx and dy.  In this case, the Al X-Y coordinates stay the same (assumed constant temperature) but what will the Zeranin's X&Y's become?  I assume the spring constant of the insulating film remains constant and stretches the Zeranin by the same amount.  It follows then that the differential heated dimensional difference of dx and dy shows up as Zeranin's X&Y coordinates growing by dx and dy (in reality, slightly less than dx & dy due to slightly less spring force).

I believe there is also a slight temperature difference between the cooled side/face of the Zeranin and the top clamp side of the Zeranin.  This means the clamped face area expands slightly more than the cooled face causing the Zeranin to want to curl or cup (convex on the warm side, concave on the cool side).  Given the thinness and good thermal conductivity of the Zeranin, this is probably a second-order effect at best.  But it also results in different stress/strain and dimensional changes that may explain the powered vs un-powered difference in R-T.


--- Quote from: zlymex on April 21, 2016, 01:35:07 pm ---One current conductor is on top side of all the chips, covering with thick tin. partially can be seen.
The other current conductor is similar, go thru all the chips at bottom side, torn from the chips.
Kevin sensing are two thin wires(one can be seen) soldered to the middle of the two current conductors.

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I feel much happier now we can clearly see that the sensing has been done properly. I could not believe that it was done as badly as first appeared. Thanks so much for providing these pictures.


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