Upgrading LM399AH ref opamp - Page 1
Forum section: electronics / general

author: jonpaul
author_rank: Super Contributor
timestamp: December 26, 2021, 02:42:02 pm
content: Bonjour a tous: Restarting an old topic I had posted back in the Spring, after acquiring Keysight 34465A new.TheLM399AH is on the PCB with an LT1001ACN8 opamp and the appropriate resistor divider for 10.000V out.Reads 10.00014 V on 15V power, after warmup of Keysight 34465A and REF 2 hrs.Leads are using gold plated bananas to the DVM.Am reworking adding 15V LOD preregulator, LM2940CT-15,Is there Any better pin compatible opamp in 8 pin DIP than the LT1001ACN8?Many thanksJon

author: Kleinstein
author_rank: Super Contributor
timestamp: December 26, 2021, 03:10:35 pm
content: There are few quite similar OP (OP177, OPA177, OPA277) that are also avialabel in DIP. There are slight differences, but not really much.The noise and likely also the drift from the LM399 is quite a bit higher than the OPs noise and drift. So getting better performance is not really about a better OP, it would be more about a 2nd ref in parallel of a better (maybe LM1399) reference to start with.

author: jonpaul
author_rank: Super Contributor
timestamp: December 26, 2021, 05:12:33 pm
content: Dear Klienstein! You are always the first to respond, and have the authoritative answers. many thanks !Since last shots, after 3 hours, V settled to 10.00005, LM399 and the Keysight 34465A are agreeing.....1/ ACK re opamps, will use the original LT1001 or the others you mention. You idea if instead of soldering the IC to the PCB , to use a machined pin gold plated IC socket?2/ Power source either PS 20V or 2 x 9V batteries, >> LDO regulator LM2940-15 >> 15 V>> LM399AHC. Comments?Best Regards and Happy Holidays!Jon

author: Kleinstein
author_rank: Super Contributor
timestamp: December 26, 2021, 07:43:15 pm
content: For the heater the LM399 needs quite some power, something like 200 mW and thus a bit over 10 mA at 18 V. So it would not really last very long on 9 V batteries. The LM399 is more something to be mains powered, at least most of the time. If both are used some kind of seamless cross over (e.g. diodes before the regulator) may be a good idea. So one could do a 2 hour warm up from mains power and only for the actual measurement switch over to battery. An LDO is OK and 15 V maybe about the maximum to get from 2 near empty 9 V alkaline batteries.A socket may add some extra thermal EMF, though usually not really much. Still the OPs listed are about equal performance and it is very hard to tell if the OP behaves well or not. So there would be very little need to change the OP. It is not that one could change the OPs and look for lowest noise or drift. They would measure essentially the same with essentially no way to tell from the measurements which one is better. Best chance may be to go lowerst supply current if battery operated.The more critical part than the OP are the resistors, if the 10 V output is used. One may consider an additional ouput of the 7 V (possibly with an extra buffer) as this is more stable than the 10 V and hardly effected by the resistors.

author: Andreas
author_rank: Super Contributor
timestamp: December 26, 2021, 08:06:18 pm
content: Quote from: jonpaul on December 26, 2021, 05:12:33 pmHello,for battery operation I would use a LDO which needs less quiescent current than the LM399 itself.My preferrred solution is the LT1763 configured to 14V for 12 AA NiMh cells.At 14V the cells are "empty" and should be switched off by a battery monitor.At re-charging with maximum 17.4V the maximum supply voltage of the LT1763 is not exceeded.Cirquit example see here:https://www.eevblog.com/forum/metrology/ad587lw-10v-precision-travel-standard/msg1449488/#msg1449488with best regardsAndreas

author: David Hess
author_rank: Super Contributor
timestamp: December 26, 2021, 10:40:54 pm
content: Quote from: Kleinstein on December 26, 2021, 03:10:35 pmThe LT1001 is an improved OP-07, and awfully good. Its DC characteristics can be further improved through offset nulling which will reduce offset *and* offset voltage drift.A good way to divide these operational amplifiers into groups of similar parts is by input noise and bandwidth because these depend on the input stage collector current. Input bias current is less useful because of input bias current cancellation. These and the OP-07 are all in the same class and I doubt replacing an LT1001 with any of the improved OP-07s will make a performance difference.There might be something to be gained with a different class of part, like the noisier but lower input bias current LT1012, or by buffering the output since precision operational amplifiers are sensitive to loading, but this would require detailed analysis of the circuit.QuoteI agree; the reference noise and drift are greater than that of an LT1001 class operational amplifier, so there is just not much performance improvement to be gained with a better operational amplifier even if it existed.

author: jonpaul
author_rank: Super Contributor
timestamp: December 27, 2021, 10:05:22 am
content: Rebonjour Klienstein, Andreas and David Hess:Wow, mille mercis for your fine advice and ideas.The portable ref of Andreas is a great design, a fine project, but my needs are much more modest:self contained, int/ext power, small box 4.5" x 2.75" x 1.75", 18V ext PSU or internal 2X 9V batteriesIt is used usually on external power, the batteries are intended ONLY for momentary use, away from a PSU.Has a switch for the power and Schottky diodes to select PSU or battery. After the power source switching, an LDO reg LM2940-15. Instead off the LT1001, OPA277, no socket needed.Great suggestion for a second output direct from the LM399!Last question: I have blown up similar by misconnecting power to the opamp output.Any suggestions for short ckt/power reverse protections without affecting the voltage of the REF out?Schottky catch diodes?Bon Fêtes d'hiver et nouvelle année!Jon

author: Kleinstein
author_rank: Super Contributor
timestamp: December 27, 2021, 10:34:32 am
content: For the output some series inductor als an LR parallel element is likely a good idea, to decouple the OP from capacitive load. Diodes against revere voltage are good too and should not effect the normal operation. The OP277 to drive the 10 V output should be OK with a short. An extra (e.g. NPN) driver would need a short circuit protection (e.g. resistor at the collector). Higher voltage up to the supply could be an issue. Here a diode to the supply and some clamp at the supply should help.

author: jonpaul
author_rank: Super Contributor
timestamp: December 28, 2021, 08:21:10 am
content: Hello again:Buying the Linear Tech LT100, For the LT1001 on ebay , sold by Chinese firms, any tips on how to tell genuine orignal LT from illegal Chinese copies?They all look the same?Many thanks again,Jon

author: magic
author_rank: Super Contributor
timestamp: December 28, 2021, 09:29:33 am
content: You will get recycled chips in unknown condition, probably not worth it for a $4 opamp still in production.Look for local sources, sometimes NOS ICs can be had for very good prices.

author: David Hess
author_rank: Super Contributor
timestamp: December 28, 2021, 09:44:40 pm
content: Quote from: jonpaul on December 28, 2021, 08:21:10 amI agree with magic; it is not worth the cost savings to buy a recycled and likely fake LT1001 given its relatively low price and easy availability.If you do have to test, then input offset voltage, input bias current, and input offset current are easy to check with a simple circuit. Then check the 0.1 to 10 Hz noise with the circuit recommended by Linear Technology.