EEVblog #730 - Thin Film Resistor Networks - Page 1
Forum section: electronics / general

author: EEVblog
author_rank: Unknown
timestamp: April 04, 2015, 05:51:40 am
content: How ceramic backed precision thin film resistor networks are constructed and laser trimmed.Examples come from the resistor networks in the 7.5 digit Keithley DMM7510 Multimeter, manufactured by Fluke.

author: Willy.nebula
author_rank: Newbie
timestamp: April 04, 2015, 06:55:23 am
content: Ok is it me or does that first resistor network in the video look like it has a face in the top right corner

author: VK3DRB
author_rank: Super Contributor
timestamp: April 04, 2015, 07:06:29 am
content: Excellent video Dave!You didn't waffle on - it was both informative AND interesting.Years those, the bonding technology between the pins and the ceramic circuit was not near as good as today. The old AWA Thorn 4KA colour TV chassis was plagued by these connections going open circuit intermittently, causing "random" intermittent CRT convergence problems. The English 4KA's were a dreadful design, generally ending up on the rubbish tip only a few years after they were bought. A quick and dirty attempt to fix the ceramic contacts was hitting the lead with a soldering iron for about 1 minute. Sometimes it worked, sometimes not.The algorithms for how to trim resistor networks quickly would be interesting in itself.

author: SNGLinks
author_rank: Regular Contributor
timestamp: April 04, 2015, 08:16:00 am
content: I wonder if they use glass so they can laser trim them after encapsulation?

author: nitro2k01
author_rank: Frequent Contributor
timestamp: April 04, 2015, 08:27:36 am
content: Just to nitpick a little. Around 3:15 you said that the portion of metal on the left adds resistance. It really adds conductivity/decreases resistance compared to if that part was isolated, though, since it adds however many micro or nanoohms in parallel. (Not trying to claim you don't know of course. Only a minor objection to the wording.)Otherwise a great video.

author: Dr. Frank
author_rank: Super Contributor
timestamp: April 04, 2015, 08:36:50 am
content: Dave, very interesting and informative video!Some remarks, though:FLUKE made these glass covered resistor networks for their high grade calibrators and references in first place.. You'll find these in the 732B, the 5720A and also in the 8508A 8 1/2 digit "Reference Multimeter".These networks give these instruments their superior stability.The reason, it's an open, glass covered design, is simple.. The LASER trimming is done after the sealing of the element, through the glass plate, and in some cases obviously in situ, in the finalized instrument at the final testing place.Imagine, you use that network for final trimming of the 732B output, i.e. absolute 10.00000V and 1.00000V.The High Voltage divider should have exactly 9.9M / 100k .. I assume that there was some protection circuitry inside the instrument, giving these few kOhm instead.The reason, Fluke manufactured this special network, has two reasons or advantages over usual design ocompany Caddock, where they bond 2 distinct resistors back-to-back.First, both resistors have identical T.C., as they have been sputtered from the same alloy target (NiCr or TaN), so the T.C. tracking is already near perfect.But they also overcome the power difference T.C. effect in a usual HV divider..The line width of each resistor seems to be the same everywhere.The length of the 9.9M resistor path should be exactly 99 times the 100k resistor path.So the power dissipation on each resistor element is equal.That means, that the 9.9 M and the 100k resistor will heat up to the exactly same temperature when 1000V were applied. Additionally, the high thermal conductive ceramic substrate equalizes any temperature gradients, but on a slower time constant.That means, that the 100:1 ratio will be constant under all circumstances, to < 1ppm..Also dynamically, if you want to digitize an alternating 1000V level on a short time scale.Most other 6 1/2 ... 8 1/2 DMM suffer from that power dissipation effect.They need a quadratic compensation, by calibrating at 1000V, 500V, and maybe 100V, to compensate later numerically for this change in ratio over power.The 3458A does not have this compensation, because maybe it is not feasible to calculate the quadratic compensation for 100kHz digitizing at these high voltages, but it therefore has a mediocre 12ppm additional error at 1000V.So, this Fluke Thin Film network really is absolutely perfect.Frank

author: EEVblog
author_rank: Unknown
timestamp: April 04, 2015, 08:38:41 am
content: Quote from: VK3DRB on April 04, 2015, 07:06:29 amYes, I'd love to see the manufacturing and cal facility and procedure for these.The laser machines that do this are not trivial either.

author: EEVblog
author_rank: Unknown
timestamp: April 04, 2015, 08:42:40 am
content: Quote from: nitro2k01 on April 04, 2015, 08:27:36 amI assume you means 13:15. Yes, my goof, I was trying to say adds resistance in parallel as you say. Not the best wording at all.

author: EEVblog
author_rank: Unknown
timestamp: April 04, 2015, 08:48:00 am
content: Quote from: Dr. Frank on April 04, 2015, 08:36:50 amI had suspected that but didn't think it would be the best approach, perhaps due to heat buildup or other issues with the sealed glass. It didn't occur to me it could have been trimmed after installing in the product.QuoteYep, likely.QuoteI thought about going into that, but the video was already much longer than what I had planned.QuoteGreat added detail, thanks.

author: G7PSK
author_rank: Super Contributor
timestamp: April 04, 2015, 08:49:16 am
content: My guess is that the areas where you can see what looks like burning is metal vapour on the glass if the trimming is done after the glass is sealed onto the resistor network so the lines that look clear and sharp are cut before hand or nichrome vapour is transparent.

author: EEVblog
author_rank: Unknown
timestamp: April 04, 2015, 08:50:07 am
content: Quote from: Dr. Frank on April 04, 2015, 08:36:50 amDo you know if it's the same part reused, or custom values/config for the DMM7510?

author: EEVblog
author_rank: Unknown
timestamp: April 04, 2015, 08:56:00 am
content: Quote from: G7PSK on April 04, 2015, 08:49:16 amYep, could well be the case.

author: HighVoltage
author_rank: Super Contributor
timestamp: April 04, 2015, 09:03:04 am
content: Very nice video and really good comments hereThanks!

author: Dr. Frank
author_rank: Super Contributor
timestamp: April 04, 2015, 09:27:47 am
content: Quote from: EEVblog on April 04, 2015, 08:50:07 amI assume, they used a similar 100:1 divider in the 5720A..Actually, it's a 70k / 7M there, for a 100 fold amplification of 11V reference to 1100V output.I always wondered, how they could use a single component for that crucial circuit element in the 5720A.. And how Fluke could make an Artefact High Voltage Autocalibration to that degree of uncertainty,where HP in the 3458A failed at the exactly same feature.. Now I know!See also the Fluke 5720A manual / schematics and the Theory of Operation...For the other network.. There are several pictures of the similar components from the 732A, 8508A, but I doubt that they reused these in the Keithley instrument.. As there the networks were used for the LTFLU or the LTZ1000 references..Maybe you can determine, for which mode Keithley uses this network, as current shunts, or for the resistance mode.Frank

author: German_EE
author_rank: Super Contributor
timestamp: April 04, 2015, 09:36:15 am
content: Thank you for an interesting video. I'm tempted to try this for myself using the laser cutter at my local Maker Space but I don't think it will cut nichrome or copper.

author: fcb
author_rank: Super Contributor
timestamp: April 04, 2015, 10:40:35 am
content: Great video. Where's the teardown/review of the 7.5 digit mm?Gunhaver posted a comment on the YT comments "Danaher no longer owns Tektronix! The sale may not be completely final yet, but they sold off the Tektronix brand and part of Fluke Networks along with a bunch of other assets."Any news? Couldn't find anything recently on Google.

author: EEVblog
author_rank: Unknown
timestamp: April 04, 2015, 11:07:13 am
content: Quote from: fcb on April 04, 2015, 10:40:35 amNot finished yet. Already got almost an hours worth of raw footage, it's getting out of hand.QuoteAll I could find was that they sold off Tektronix networks in Oct:http://www.oregonlive.com/silicon-forest/index.ssf/2014/10/danaher_sells_communications_b.html

author: smoo
author_rank: Newbie
timestamp: April 04, 2015, 12:52:13 pm
content: Quote from: EEVblog on April 04, 2015, 08:38:41 amLaser trim machines are not that complicated. First you have the laser itself, which does require a big old power supply and water cooling, also some circuitry to pulse the laser. Then the laser pulse has to be directed where you want it, either using prisms mounted on a table that can be moved in the x, y and z axis's, or mirrors mounted on galvos.For the final in circuit trimming it may even be done manually with an operator moving the laser beam one pulse/step at a time while watching the readout on whatever instrument in use.

author: dentaku
author_rank: Frequent Contributor
timestamp: April 04, 2015, 12:53:36 pm
content: Thumbs up for an interesting video and knowledgeable forum comments.

author: Prime73
author_rank: Regular Contributor
timestamp: April 04, 2015, 01:05:02 pm
content: thanks for an interesting video Dave.On a side note a quick question - I've noticed in your videos you use different DMMs all the time. What's your reasoning when you choose a DMM to measure something? For instance in this particular video you've used Brymen BM257. Why this particular one and not say yours Fluke 85V ?

author: EEVblog
author_rank: Unknown
timestamp: April 04, 2015, 01:10:41 pm
content: Quote from: Prime73 on April 04, 2015, 01:05:02 pmWhichever one is closest to hand that has the probes I need usually, unless I have a specific need for a certain meter. Meters get thrown around the lab all the time, probes go missing and magically reappear etc. Even more so now with Dave2 in the lab a lot.

author: Tek_TDS220
author_rank: Regular Contributor
timestamp: April 04, 2015, 01:38:27 pm
content: He has a name: "Dave2" !!!!

author: FrankBuss
author_rank: Unknown
timestamp: April 04, 2015, 02:33:45 pm
content: Interesting video. First I didn't understand why the resistance is ohms/square, but I think I found the solution: Let's assume one 1mm^2 square resistor, which has a resistance of n ohm. Now if you have a 2mm^2 square, you could divide it into two 1x2mm^2 parallel resistors. Obviously each one has a resistance of 2*n ohm, and in parallel it is again n ohm. That's neat and not obvious at first glance.But why do they need to calibrate the resistors at all? As a software guy, I would just use some low temperature drift resistors, measure them and calculate the rest in software.

author: TiN
author_rank: Super Contributor
timestamp: April 04, 2015, 04:36:10 pm
content: Hm, LTFLU voltage reference in DM7510? That's surprisingExpected to see LTZ there.Thanks for video, we desperately want moooore!

author: zapta
author_rank: Super Contributor
timestamp: April 04, 2015, 05:18:30 pm
content: Quote from: FrankBuss on April 04, 2015, 02:33:45 pmThis was the most fascinating part of this episode. I presume it's measured from an entire edge of the square to the entire opposite edge.You can also mention in your proof that the voltages between the two parallel resistors are identical at each point and thus there is no current between them. Otherwise, it would affect the analysis.Also, I presume that the resistance/square depends also on the thickness. Is the resistance per cube (face to opposite face) also fix? If so it may be an even more canonical characteristic.

author: FrankBuss
author_rank: Unknown
timestamp: April 04, 2015, 06:11:34 pm
content: Quote from: zapta on April 04, 2015, 05:18:30 pmYou can use the same idea for cubes: 2mm^3 can be divided in four 1x2m^3 resistors, so if a 1mm^3 resistor has n ohm, then a 2mm^3 resistor of the same material has n/2 ohm (and a 3mm^3 cube has n/3 ohm).

author: MisterDiodes
author_rank: Frequent Contributor
timestamp: April 04, 2015, 06:33:04 pm
content: Thanks for the Video, Dave!Actually, I need to jump in here: The example you show here is a somewhat "low-end" example of recent designs by Fluke / Danaher. This technique of laser trimming through glass is not new (been around since at least late 60's / early 70's) and one of the main problems with what you see here is the fly-ash contamination that results from the laser trim process. That ash deposited on the inside glass is not completely inert, and eventually finds its way back onto the network circuit (light, vibration, static fields etc all promote migration). Let alone the mechanical stress on the substrate when you burn into it with the laser. All of this can show up years down the road as a bit of instability in the instrument if its not calibrated often. This is no longer considered "best practices" in precision measurement circles. A stable circuit is a clean circuit!Very stable resistor networks are done on a substrate using transparent sapphire, not the white (probably) alumina as shown here. Both materials are in the same "ceramic" family, but sapphire can offer a superior end product with better thermal properties. (Its very expensive to work with though and a pain to cut it apart). If you want the resistor network to remain stable, you -don't- touch it with laser trim, no matter what the substrate. The resistor assembly would be cleaned of -all- contaminates, welded into a shielded light-tight / humidity-proof and vibration proof hermetic metal package, and mounted onto a PC board with something besides that wobbly, cheap SIP technique. That's how we do it for military & aerospace applications anyway.Watch out for thermals around the pins of that SIP package the in the example shown here. This might be only for high voltage input, but if you were trying to attenuate lower level voltages with any sort of precision, the resistor network would not be mounted on the PC board like that. In general, those pins all need to be at the same temperature for low level signals. It could be they weren't after that, since that ~9M ohm resistor is going to make all sorts of thermal noise on its own.As another poster pointed out, if you have to trim a resistor network to this level, you're not doing the software right somewhere else, maybe. For the last decade or so we have cheap 32 and 64 bit MCU's to handle calibration duties much easier than this, and it results in a much more stable, reliable product. You want a STABLE resistor divider; trying to hit some cardinal divide ratio isusuallynot necessary at all if you calibrate in software. Just build the resistor divider very close to what you need, then take care of as much as possible for trimouts during software calibration. We haven't needed to use a precision-trimmed resistor divider in years...kind of obsolete like CD's and cassette tapes. And we build precision test / measurement devices all the time.Of course if low noise is what you need, then good precision Wire Wound resistors, coupled with a thermocouple for temperature compensation at the controller end, can easily outperform printed or diffused resistors used on front-end attenuators for freqs < 250kHz or so.Thin / thick film network circuits were used extensively by Beckman Instruments, General Instruments, Bourns and others even back to the mid 50's that I know of. The substrates were on glass, sapphire, alumina and any other hard, temperature stable material. In pre-software days the resistors would be trimmed with abrasive techniques (sand blast, bead blast), water jets, diamond saws, microwave / maser, ultrasonic probe, radiation sources, etc. Generally these were for aerospace & military equipment, and in use for many years before Fluke started selling calibration equipment.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 04, 2015, 06:33:13 pm
content: Resistance per cube is called bulk resistivity (e.g., copper is around 17 nOhm m).It's not "per cube" because it *does* vary with a dimensional unit.Interesting fact about quantities: when you see a bulk parameter that's got a single order unit (m or m^-1), it's usually a cubic dimension, not simply a linear dimension. That is, you factor the length of the region in the same proportion as the given unit, and its cross-sectional area inverse to that. So, bulk resistivity has meters on top, so multiply by length (= resistance is proportional to length) and divide by area to get ohms. Conversely, conductivity is S/m, so divide by length and multiply by area.Tim

author: Dave
author_rank: Super Contributor
timestamp: April 04, 2015, 09:38:55 pm
content: Quote from: T3sl4co1l on April 04, 2015, 06:33:13 pmIt was actually resistance per square.It's quite simple, really.R = rho * L / AThe thickness of the copper layer that gets deposited on the ceramic substrate is fairly accurately controlled, so you can divide resistivity by that thickness and you get what is commonly called "sheet resistance".So:R = Rsh * L / WIf you look at a square of copper, L / W = 1 / 1, so the resistance of the opposite sides of that square are exactly Rsh.But here's the trick:opposite sides. You can't simply add squares like Dave added them in the video. The resistance of a 90° corner square would be 0.56*Rsh, for example. The resistance of a square, where the current enters and leaves on the same side of the square (the top square in Dave's example), would be something different entirely.

author: EEVblog
author_rank: Unknown
timestamp: April 04, 2015, 10:37:07 pm
content: Quote from: TiN on April 04, 2015, 04:36:10 pmI thought it was indentical to the LTZ? Just labeled for Fluke, and perhaps binned better?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 04, 2015, 10:37:19 pm
content: Quote from: Dave on April 04, 2015, 09:38:55 pmYeah, when you have one dimension of the cross sectional area predefined, it becomes per square (dimensionless, so depends only on the geometric ratios, not the scale). For example, copper at 17 nOhm m / 35um as in PC board = 0.48 mohm/sq.Tim

author: EEVblog
author_rank: Unknown
timestamp: April 04, 2015, 10:42:01 pm
content: Quote from: DiligentMinds.com on April 04, 2015, 05:59:53 pmThat was my guess, yes, just like in the new Keysight.

author: jippie
author_rank: Unknown
timestamp: April 05, 2015, 08:48:58 am
content: Looking at the image of those networks, I see loads of parasitic capacitors and inductors. The AC response would be dramatically complex. Could you elaborate a bit on that Dave? I can imagine an RMS to DC converter is placed before the resistor network, but then that converter would need similar accuracy (or at least stability so an error can effectively be calibrated out) as the resistor network.

author: zofz
author_rank: Regular Contributor
timestamp: April 05, 2015, 03:34:44 pm
content: There is a current density and potential distribution for one of the resistors in the video.I did only a single layer resistance - constant value.

author: DanielS
author_rank: Frequent Contributor
timestamp: April 05, 2015, 04:24:50 pm
content: Quote from: jippie on April 05, 2015, 08:48:58 amThe DMM7510's AC performance specification only goes up to 300kHz, at which its accuracy is only within 4% reading + 0.5% range.I doubt the 9.9M resistor has more than 1nF and 1nH worth of overall capacitance and inductance. That would affect accuracy on a 300kHz input signal by less than 0.001%. Even if my guestimate was off by two orders of magnitude, it would still be well within tolerances.

author: SeanB
author_rank: Super Contributor
timestamp: April 05, 2015, 04:48:27 pm
content: Quote from: DanielS on April 05, 2015, 04:24:50 pmThe layout does it's best to make power dissipation per unit area as close to equal as possible, and the long and convoluted tracks are going to have low self inductance, and reasonably low capacitance between tracks with large voltage potential, and as they all are in series with each other it will be quite non inductive and reasonably low capacitance. Designed to be as easy to compensate as possible with only a single low value high voltage capacitor or to have a easy to compensate frequency response.

author: DanielS
author_rank: Frequent Contributor
timestamp: April 06, 2015, 02:14:14 am
content: Quote from: SeanB on April 05, 2015, 04:48:27 pmI doubt there is any frequency compensation to be done here: if each loop is 20pH and there are 200 of them, that's 4nH and at 300kHz, that's 7.5mOhm vs 10Meg or 1ppb of error. That's five orders of magnitude beyond the meter's 900ppm accuracy on its best AC ranges, never mind the 45000ppm accuracy spec at 300kHz. The unnecessary AC compensation components would likely have a much worse detrimental effect on DCV accuracy and drift specs than any AC accuracy gain it could provide.I doubt they would risk losing 1ppm on DC accuracy to improve AC accuracy by an effectively non-detectable amount.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 06, 2015, 02:42:55 am
content: The complex LC response of such a network will be much worse than you'd imagine, and far more involved than a simple lumped capacitance and inductance figure.Hairpin loops do have low inductance, but it's not zero. I'd have to guess more like 100nH. But this will be utterly negligible, because with R so large, the L/R time constant is minuscule (e.g., 0.1u / 10M = 10 fs!). Likewise, stray capacitance will be more like 10pF, but it's distributed over the whole thing, so it's more like a chain of RC filters, half to ground and half bridging across various points in the path itself. It will have extreme "hook" -- there's a Tektronix appnote on this subject, as it applies to oscilloscope attenuators and probes.They may well simply add a dominant pole-zero (i.e., swamping the stray capacitance with external capacitors), giving a significantly reduced input impedance at AC, in exchange for an at least reasonably predictable frequency response.Tektronix' preference for this sort of thing is a chain of small resistors with capacity hats, keeping everything as small and simple as possible, while still maintaining sufficient voltage standoff and power rating to handle the high voltage. The, P6015 probe is it? has a dielectric fluid (freon??) backfill, to maintain the voltage and power rating of its components while keeping the frequency response right.Tim

author: DanielS
author_rank: Frequent Contributor
timestamp: April 06, 2015, 06:00:29 am
content: Quote from: T3sl4co1l on April 06, 2015, 02:42:55 amIn any case, my point to the original post I responded to was: the DMM7510 has nowhere near the AC precision and bandwidth necessary for the resistor network's parasitics to have any meaningful effect on AC measurement accuracy even if my guesstimates (or yours) were a few orders of magnitude off.While I have no doubt there are some very fancy resistor networks and construction techniques in high-frequency, high-voltage probes and oscilloscope front-ends, the DMM7510 is a multimeter with only 4.5% AC accuracy at 300kHz. Its AC accuracy is already becoming miserable decades before the network's parasitics are likely to become remotely significant.

author: jwm_
author_rank: Frequent Contributor
timestamp: April 07, 2015, 04:58:53 am
content: Quote from: zofz on April 05, 2015, 03:34:44 pmWhat did you use to simulate that? looks pretty neat.

author: Macbeth
author_rank: Super Contributor
timestamp: April 07, 2015, 09:35:54 am
content: I found this using google search for Resist2d (loads of useless results), then display by image (spotted it right away):http://www.zofzpcb.com/Matrix2DR.html

author: Marco
author_rank: Super Contributor
timestamp: April 08, 2015, 01:13:39 am
content: So how much does this kind of network gain the instrument? That resistor network likely costs more than all the components in the 2002 put together.

author: TiN
author_rank: Super Contributor
timestamp: April 08, 2015, 08:15:56 am
content: Not really, 2002 have hermetic foil (which are the most expensive) and ceramic networks as well.Main cost is in aging and training for these custom parts to ensure their stability.

author: Marco
author_rank: Super Contributor
timestamp: April 08, 2015, 08:37:53 am
content: Quote from: TiN on April 08, 2015, 08:15:56 am3 individual ones for the resistance and current measurement ... but they are not bespoke and soldering stuff into metal cans is only artificially expensive. I seriously doubt they get anywhere near the cost of this monster.