LM299 cost - Page 1
Forum section: electronics / general

author: IconicPCB
author_rank: Super Contributor
timestamp: May 04, 2017, 11:30:25 am
content: Whats a decent price on NOS lm299?

author: necessaryevil
author_rank: Regular Contributor
timestamp: May 05, 2017, 11:31:25 am
content: Brainfart: compare it to a LM399 (about about 10 USD ex vat.) and decide if the price is fair to you. Let's say twice the price of an LM399 would be fair, ten times not.Beware of fakes.You might also consider using an LM399 instead, but wait for comments from the experts.

author: chris_11
author_rank: Contributor
timestamp: May 05, 2017, 01:19:26 pm
content: If not fake that is most likely an old National Semiconductor/TI part. They speced it with 20ppm long term stability. LTC spec on their LM399 8ppm. Go figure.I would buy a new one LM399 LTC from an authorised distributor (digikey etc.).

author: Edwin G. Pettis
author_rank: Frequent Contributor
timestamp: May 05, 2017, 03:22:53 pm
content: The main differences between 199, 299 and 399 versions was the range of operating temperature. The 199 was spec'd for military -55 - 125°C, the 299 for industrial and the 399 was commercial. There were some some variances in the specifications between them as well. Since the 199/299 were obsoleted a few years ago, the 399 was basically spec'd to cover the commercial temperature range of 0-70°C. As far as I know, LT was the last one to make a LM199 version. The LM399AH is available from LT.

author: IconicPCB
author_rank: Super Contributor
timestamp: May 05, 2017, 09:58:45 pm
content: Good point Chris,I was aware of NS prefix sequence ( 1xx, 2xx, 3xx depending on the temperature range) and my next port of call is to compare the the data sheets line by line.I have no reason to doubt the authenticity of thelm299 offering ( a local electronics supplier ) selling the LM299 in low twenties ( Australian dollars).

author: IconicPCB
author_rank: Super Contributor
timestamp: May 07, 2017, 12:27:08 am
content: A superficial compariosn of spec sheets for NS ( TI ) lm299 and LT lm399 devices suggests LT write a better spec sheet.I guess an LT lm399 will be a "better" bang for bucks choice.And now for additional bang for bucks discussion:Given that there is talk of improving reference performance by runing a number of them in parallel....How many lm399 in parallel does it take to meet an ltz1000 expectation?

author: Edwin G. Pettis
author_rank: Frequent Contributor
timestamp: May 07, 2017, 03:35:26 am
content: The LTZ1000/A has about 1.2uV of noise on average, by the time you add up all the extra components and the number of LM399s in parallel to get close to that noise level, you might as well just use an LTZ, there is really no advantage to paralleling a bunch of LM399As to try and equal an LTZ.

author: Dr. Frank
author_rank: Super Contributor
timestamp: May 07, 2017, 06:30:14 am
content: Quote from: IconicPCB on May 07, 2017, 12:27:08 amYou should first define your design goals, in terms of a) noise and b) stability.The LTZ1000 circuit is about 5 times less noisy, and about 20 times more stable (1 year, 45°C oven) as the LM399.I doubt, that you can really average so many LM399 to equal the noise, in theory about 25 EA.You might select the most stable LM399 out of a batch of about 25 for best stability, to approach the stability of the LTZ.That's also very difficult, as you need to monitor the whole batch over a long period of time (1/4 -1/2 year), find the average drift rate, and then select the one, which is exactly in the middle.The LTZ circuit is stable to about 1ppm/yr. (typically) from scratch.Frank

author: IconicPCB
author_rank: Super Contributor
timestamp: May 07, 2017, 11:22:48 am
content: Thanks Edwin and Frank.The issue of stability is understandable.However since noise is random fluctuations in voltage level the only way to minimise noise would be to hopefully introduce identical noise signal with say at most 180+/- 10 degrees phase difference and hope to achieve noise cancellation.If memory serves noise is typically not presented as voltage or current but rather as noise power, how does summation of similar noise power voltages eliminate noise?

author: Andreas
author_rank: Super Contributor
timestamp: May 07, 2017, 01:27:25 pm
content: Quote from: Dr. Frank on May 07, 2017, 06:30:14 amHello,from my experience the long term stability of LM399 is also in the ballpark of 2 ppm/year after 5-9 kHrs run in time.So not a factor 20 against LTZ1000.In the picture: ADC13 (around -1ppm/year) measuring different devices.So "real ageing" has to be corrected by -1 ppm/year.LM399 CH6 + CH7 are from "day zero" on.Other sources "well aged".LTZ#1 had a short on day ~860 (-1.9 ppm jump + increased ageing of green curve).You never should operate a LTZ1000 without buffering. The LM399 is insensitive to shorts against ground.LM399#2 had been accidently operated several days upside down "pins up" on day ~920.This created a increased ageing. (red curve).So keep the LM399 always in upright position.All in all the short term noise of a LM399 is much larger.But long term stability is usually (except of some "stinkers") not much more than a LTZ1000.Quote from: IconicPCB on May 07, 2017, 11:22:48 amThe only way is averaging several sources.Doubling the number of sources reduces (in theory) the noise by -3dB.Practically the noise of a LMx99 has a large stray (from device to device) compared to a LTZ1000.So I would recommend to select your LMx99 by noise.There are several measurement amplifiers to do this discussed here in the forum.With best regardsAndreas

author: necessaryevil
author_rank: Regular Contributor
timestamp: May 07, 2017, 02:11:09 pm
content: Do you have experience building high performance circuitry like that? It's not unlikely that the performance of your project is going to be limited by the circuitry and wiring, rather than the LTZ1000.What is the intended application?

author: IconicPCB
author_rank: Super Contributor
timestamp: May 07, 2017, 11:49:57 pm
content: I don't have any hands on.I doubt there are many instances in industry where one could glean such experience.I don't have a project per se.It is all of academic value presently and perhaps some ground work for future reference ( no pun intended).Looking at the requirement for multiple voltage references in order to conduct statistical analysis of quality of the instrumentation, it would seem that in terms of stability LM399 might be a challenger to the LTZ especially if as was suggested above multiple lm399 cold be "averaged" to achieve comparable noise performance.If the noise performance of the LTZ is say 14dB better than that of an LM399 then it stands to reason to suggest that 16 off lm399 would provide a matching performance.Costs....Digikey offerLTZ1000ACH AUD 81.82LTZ1000CH AUD 64.30LM399H 1 off AUD 13.72 => 16 pieces AUD 219.52LM399H 25 off AUD 9.18 = > 25 pieces AUD 229.61It is reasonable to suggest that if one were to "play" with a few LTZ1000CH references then the cost of 25 LM399 based references would be comparable if not lower.From the point of view of of quality of the resulting references ( a group of three may be four LTZs versus 25 LM399s, please comment on which of the above options would produce a " better " outcome.

author: TiN
author_rank: Super Contributor
timestamp: May 08, 2017, 10:28:51 am
content: You forgetting precision stable resistor cost in LTZ circuit, which can be double or triple of what LTZ chip itself costs.Even just two LTZ-based references will be better than 25 x 399's. Only benefit of 25 references is that you have larger sample size or you can send one ref to many different people for calibration and eventually get them back for comparisons.Also unlike 399 you can play with different oven temperatures, zener current and bias current levels with LTZ circuit, so it's a better learning tool instead of all-integrated 399.

author: necessaryevil
author_rank: Regular Contributor
timestamp: May 08, 2017, 11:24:44 am
content: It might also be interesting to use LT1021 or MAX6126 and build a mini-oven yourself.