Torn between "sexy" old school metal can packages and SMD for old AN cct - Page 1
Forum section: electronics / projects

author: AlfBaz
author_rank: Super Contributor
timestamp: September 06, 2017, 03:45:36 am
content: So I'm putting together a board from an old Article Note (AN-260)Purely for curiosities sake, see if it gets anywhere near 20bit.Whilst looking for parts (never realized how hard precision resistors are to buy) I come across the availability of a lot of the parts specified still available in the old metal can packagesShould I stop being a nostalgic dick and just do SMD all the way or is there something to be said for sporting the old look?More interestingly it would be good to do both to see if there are any major performance differences between the old and new packages

author: AlfBaz
author_rank: Super Contributor
timestamp: September 06, 2017, 12:25:35 pm
content: Thanks for that Wilfred, aside from the Hewlett Packard high speed opto which seems to have mutated into the HCPL-7720/7721, the LM121 was a tad difficult to find info on as it was also available as the LM321 variant, which unfortunately had it's number stolen and used for a standard op-amp.Everything else seems to be readily available in metal can packages. Some parts are only available in the aforementioned packages and are quite expensive compared to the more modern counterparts.Since Mouser seems to be the only one with the precision resistors in stock (weeks and weeks lead time on RS, E14 and digikey) I decided to order the parts all from them. So far I'm upto $180 in components!! (admittedly bought enough to have some spares as seeing the word "obsolete" next to a part makes you want to buy more) and tha'ts not including the PCB or the LM399

author: ejeffrey
author_rank: Super Contributor
timestamp: September 06, 2017, 06:08:14 pm
content: When you are talking about precision DC, surface mount components are very likely to be worse than the same component in metal can packages. They are exposed to high heat during reflow and are more susceptible to mechanical stress transmitted from the PCB. Whether it makes a difference depends on the part in question and the application.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 06, 2017, 07:31:19 pm
content: Metal case parts are usually higher stability (better DC performance), but not all parts are really that critical. One advantage of that circuit is that the amplifiers don't need to be really DC stable. So I Don't think one would need the old style metal cases.That circuit uses really old parts. I doubt one would need the old LM121 / LM108 combination. A good modern OP like OP27 / OPA177 should do that better or as good. One does not even DC precision from this OP. What is the mystery LM121 ?The circuit is strange in using an extra fast opto-coupler for detecting the end of the slope, but a normal, rather slow one to start the slope.If you don't need an exactly -10 V full scale to get a simple result one should get away without scaling the reference to -10 V. It should work as well (or even better) with a -7 V reference, even for signals larger than 7 V. So one could start without the expensive resistors for scaling.Likely protection from stray 50/60 Hz is more important than the choice of OPs. This also applies to the input signal. So the ADC might not be that practical, as it offers essentially no 50/60 Hz suppression and a high sensitivity to noise. I would expect filtering of the LM199 /LM399 reference would be a really good idea - much better than scaling it to 10 V.

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: September 06, 2017, 10:53:24 pm
content: Quote from: Kleinstein on September 06, 2017, 07:31:19 pmEarly zero-drift amplifier. And AFAIK the first available as milspec. See also NS AN-79.1 on stock

author: AlfBaz
author_rank: Super Contributor
timestamp: September 06, 2017, 11:09:30 pm
content: Quote from: Kleinstein on September 06, 2017, 07:31:19 pmHi Kleinstein, attached is the data sheet for the LM121/321 "Preamplifier". It has very low input offset current (0.5nA max) and input bias current (15nA max) for an "Rset" of 70k.I read somewhere, can not find where at the moment, that the very low input current of the LM121 and the low output impedance and relatively large output current of the output would cause temperature gradients across a single IC and this is why typical applications for the LM121/321 show the use of a secondary output amp.As to why dc precision is needed here, I can only speculate at the moment. The input and of course the output, are constantly switching from full scale to zero and then to the external signal to be measured. From the attached ltspice cct you can see the output of this combo (I just used an lt1007 in the sim) changes from near zero to 2mA. This change in current may be enough to cause thermal gradients which incite unpredictable driftsQuoteI was also perplexed by this at first. The 4n28 opto's start the processes and reset the integrator, the ultra-fast opto output is to measure the time between falling and rising edges of the comparator. I did some rough calculations and the orders of magnitude difference in propagation delays between input and output and very slow rise/fall times of outputs would effect timing measurements drastically if trying to achieve 1ppm measurementsQuoteThank you for this, I will definitely look into it.One of the main reasons for taking on such an obscure "precision" analog project is simply to deal with things you often read about in other's ventures. It's one thing to read about such things, it's entirely something else to attempt them yourself

author: floobydust
author_rank: Super Contributor
timestamp: September 06, 2017, 11:15:50 pm
content: The old cans are better in a few ways.Plastic packaging for semiconductors allows flux/moisture ingress.SMT packaging transfers more mechanical PCB stress to the part, compared to leads on through-hole parts. Bending TH leads is a one-time stress event, but PCB expansion/contraction due to temperature is repetitive.I'll never forget SOIC-8 op-amp's offset moving around as I gently flexed a PCB.LT came up with a special hermetic SMT package for theirLT6655 voltage referenceto solve these two issues.I'd go old school if you want low PPM stability.

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: September 06, 2017, 11:20:21 pm
content: Its the abovementioned AN-79 you're looking for!

author: AlfBaz
author_rank: Super Contributor
timestamp: September 06, 2017, 11:40:57 pm
content: Here's the ltspice circuit.If you're using XVII copy lib files to documents\LTspiceXVII\lib\suband asy files to documents\LTspiceXVII\lib\sym

author: AlfBaz
author_rank: Super Contributor
timestamp: September 06, 2017, 11:50:27 pm
content: Quote from: floobydust on September 06, 2017, 11:15:50 pmThanks for that. Interesting how slots not only provide thermal isolation but prevent stress as well. The data sheet also refers toAN-82which talks about this and other error inducing issues as well

author: AlfBaz
author_rank: Super Contributor
timestamp: September 06, 2017, 11:59:23 pm
content: Quote from: Neomys Sapiens on September 06, 2017, 11:20:21 pmAhh,found itthank you

author: X
author_rank: Regular Contributor
timestamp: September 07, 2017, 12:43:07 am
content: I would go old-school where possible. Through-hole components in general can be more physically stable than their SMT counterparts, and metal cans provide not only more RF protection, but also a thermally conductive surface which reduces temperature gradients across the chip (keeps the all portions of the chip at approximately the same temperature). That, and better heat dissipation.For example LM329 (TO-92) has a lower operating temperature range (0-70deg C) than LM129 (metal can) at -55 to 125deg C.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 08, 2017, 03:08:24 pm
content: The old LM321 is an interesting circuit, but I don't think it would perform much better than an LT1007 OP. If one is really concerned about the heating effect one could also use a 2 OPs (or OP and transistor) combination here. However it is only some 2 mA maximum and thus not that much power in the OP. With something like 20-50 ms for a single conversion, the heating effect is not only DC, but could be just with a time constant comparable to a conversion. The same current is also flowing through the A1 amplifier - so the compound amplifier for A4 is only a partial solution.Switching between zero, the signal and reference may do more conversion of the signal and zero than for the reference. It somewhat depends on the noise levels and stability of the ramp, if this helps. Due to noise it might need more than one conversion to get ppm level resolution. Effective bandwidth of the comparator can depend on parasitic capacitance at A2, especially in parallel to the 499 K resistor. For experiments one might consider some extra capacitance here to slightly lower the noise (trough less BW), at the cost of slightly lower accuracy at (low) positive voltages.A possible source of nonlinearity not considered is the capacitance of the FET used for the reset. The FETs capacitance is in parallel to the critical capacitor. It also helps to have a very stable -15 V supply, as the FET might couple some supply noise to the ramp.There seems to be an interaction of the reset signal to the output opto-coupler (though missing a connecting dot in the diagram). With this connection the single fast OK should be ok, as the start if from switching the reset signal and not from the comparator.I don't think there is need for higher grade metal case components - except for the LM399 of cause. I would take some care with the 100 nF capacitor at A1 - definitely not an SMD version for this one, maybe higher capacitance. In stead of the hard to get metal case version for A1 and A2 I would prefer a lower noise OP, even if SMD, but just the DIP version of the 356 should be OK.

author: AlfBaz
author_rank: Super Contributor
timestamp: September 10, 2017, 11:07:43 pm
content: Hi Keinstein, thank you kindly for looking at the circuit in detail.My apologies for the late reply, long days at work during the weekend saw me too tired to give your post the attention it deservedQuote from: Kleinstein on September 08, 2017, 03:08:24 pmIf, once the circuit is built, I get anywhere near the stated accuracy I may start looking into current flows as source of heating. My main thought here is the A4 combination of the LM121 and LM108 may well have been a "standard circuit trick" for precision circuits back in the day and used simply because you could. The handful of milliwatts, low duty cycle and relatively large masses of the components may not have enough of an effect to warrant concern.Having said that I'll design the board layout to allow for the "current buffering" in A4 using the techniques you've outline above. Once the board is built I can make measurements with and without the buffering to see what, if any, the differences areQuoteDuly noted and easily implemented. I'll check these methods and others once the board is built. The simulation shows some large current transients during integrator reset. Different spacings between resets and signal source switching may be interestingQuoteGiven the requirements for integrating capacitor selection (see below) the fet is a concern.This may be a good opportunity to "mess around" with the integrating capacitor in the simulation to see how these parasitics effect linearity. The problem with that though, is I am unsure how to meaningfully measure this linearity. Is it simply a comparison of the ramp voltage with a straight line and the amount of deviation from that straight line?Thanks also for the tip regarding the supplyQuoteI had forgotten about that connection since drawing the initial schematic. I took it on face value that the missing junction dot on their schematic meant no connection. On reflection why else would they draw those resistors at that location rather than just hanging off the supply rail. I'll look into that interaction furtherQuoteAccording to the text dielectric absorption of the 100nF integrating cap should be very low as evidenced by their selection of a teflon type capacitor. An initial search seems to indicate polypropylene caps are the closest and most readily obtainable low dielectric absorption capacitors compared to ptfe especially for such "large" capacitance. AVX seem to make an extremely low absorption type with their MLO range but they are designed for rf and as such are only available in the picofarad range.As for the original intention of this post, which was aesthetics, I now see that accessibility to components for test and measurement is probably far more important. Can packages are by far the easiest to lift pins on so I'll get those where possible, dips next and avoid smd's. If the circuit proves useful and/or practical then perhaps an smd version may be worthwhile

author: edavid
author_rank: Super Contributor
timestamp: September 10, 2017, 11:31:37 pm
content: In case it's not obvious, this is a classic Jim Williams app note from his National Semi days.

author: AlfBaz
author_rank: Super Contributor
timestamp: September 11, 2017, 01:07:03 am
content: Thanks for that, I always try looking for the authors name on app notes, especially old ones but could see anything on this one.Although I posted a link to the pdf, I found it in my "Linear Applications Handbook". It's seen better days but is a gold mine of great analog application notes

author: cat87
author_rank: Regular Contributor
timestamp: November 04, 2017, 12:13:00 pm
content: Any update on the current state of affairs with this project ?I also want to have a play with this circuit so I ordered some parts like the L399 and some precision resistors.For the cap, I found I have some old caps 100nF, 1% in my parts bin, that appear to be some sort of metalized foil (sawed one in half out of curiosity). I plan to build something with which to measure the dielectric leakage, to see if they're alright for the job.Also, for the JFET that shorts the cap, I found some Reed relays in an old russian bit of kit (pic attached). I'd like to measure the leakage through that one too, maybe that'd be more appropriate than the original JFET sollution.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 04, 2017, 01:45:07 pm
content: A relay can not substitute the FET used to discharge the capacitor. The timing on the FET is used to start the measurement.The 10 nF integrating capacitor is really critical to get good linearity. So not every foil cap would give good results. It should be very low dielectric absorption, ideally an air or vacuum dielectric, teflon and polypropylene are acceptable too.To check for dielectric leakage a better method would be likely to use a high resolution dual slope converter like the ICL7135. The error when measuring the reference is supposed to be mainly due to the capacitor.

author: cat87
author_rank: Regular Contributor
timestamp: November 06, 2017, 07:54:16 am
content: A fair point regarding the timing. But I thought I might get away with the fact that delays with the relay switching would be constant, so in the end, after a conversion cycle, I can just take into account the timing offset (same value offset for every conversion) and still get accurate results..Though, thinking about it some more, I might be barking up the wrong tree. Timing offset might be just too large.Thanks for jogging my neurons Kleinstein

author: AlfBaz
author_rank: Super Contributor
timestamp: November 06, 2017, 03:01:21 pm
content: Quote from: cat87 on November 04, 2017, 12:13:00 pmI've put most of my projects on hold due to my wife's recent ill health and what little time and enthusiasm I have has had to go into an unrelated "proof of concept" project that might generate some form of profitQuote from: Kleinstein on November 04, 2017, 01:45:07 pm+1 My initial research into this was mainly about dielectric absorption of the integrating cap. Decent info on teflon caps where hard to find amongst the noise generated by audiofiles. They seem to gravitate toward components with the slightest air of exoticism.The lowest DA caps I found were from AVX using their propriety "Multilayer Organic" (MLO) technology. Unfortunately they on go up to a handful of pF. Teflon or PTFE caps (at least easily sourced ones) also seem to be limited to tens of picofaradsPoly-Propylene seem to be the next best choice in dielectric material with respect to availability, appropriate capacitance and low DA