Modified LM399 reference - Page 1
Forum section: electronics / general

author: Grandchuck
author_rank: Super Contributor
timestamp: May 02, 2019, 02:35:45 pm
content: An LM399 voltage reference from eBay has worked well (considering the cost) but was too temperature sensitive. After acquiring some Vishay metal foil resistors (eBay lots of 100) it was decided to modify the reference. All of the surface mount resistors (except R1 and R3) were removed. The op-amp feedback resistor was replaced with a 9.09 kohm and the divider resistor with two 9.09 kohm resistors in series. Picture 1 shows the arrangement. Also, the divider was trimmed with a 1 kohm variable resistor in series to ground. This allowed the output to be adjusted to 10 volts. The performance was promising, but the adjustment was touchy with occasional jumps in voltage.Picture 2 shows the second modification. Three Vishay 1 kohm resistors were added as shown in the schematic. Now, the output trimming is smoother and the jumps are eliminated. A one hour data set has been included. Another modification could be to replace the trimmer with a wirewound type from Bourns.

author: iMo
author_rank: Super Contributor
timestamp: May 02, 2019, 03:28:42 pm
content: Does your kit allow to feed the 399 from the opamp's output?I also use an NPN transistor (with 2resisotrs) at the opapmp's output (in order to deload the opamp).

author: mvanocht
author_rank: Contributor
timestamp: May 02, 2019, 03:35:19 pm
content: What meter are you using?

author: Grandchuck
author_rank: Super Contributor
timestamp: May 02, 2019, 05:38:47 pm
content: There is not much room for a transistor. I wanted to keep most of the mods confined to the area under the shield lid (not shown in the photos).The meter is a Keysight 34465A

author: 1audio
author_rank: Frequent Contributor
timestamp: May 06, 2019, 05:00:01 am
content: I have the same module and noticed the same issues, plus the numbers on the lid did not seem to correspond to the actual voltages or even close. but it does function pretty well. I am modifing my to get approx 12V out for an old instrument (JRL Volt-a-vider) that need a reference. The original was lifted and something much poorer was grafted in.In my application I discovered I can use a depletion mode Mosfet (DN2535) as a supply buffer with its gate tied to the reference voltage out. The 3V is enough for the internal circuitry to work and eliminate the need for a preregulator for the supply. I still need to work out the correct resistor values and trim value.I have not seen Vishay foil resistors in small round cases. They used to be available in round plastic cases about 1/4" diameter. Where did you find those in your pictures? Are there more?

author: Grandchuck
author_rank: Super Contributor
timestamp: May 06, 2019, 01:43:17 pm
content: You can find the data sheets for the Vishay resistors online. Search for RN55D and RNC55H.Good luck with your project. You should be able to come up with a suitable 12V reference.If you want some resistors to play with, PM me.

author: 1audio
author_rank: Frequent Contributor
timestamp: May 06, 2019, 03:34:56 pm
content: I found those resistors- the RN55D is a metal film with 100ppm tempco. They are good parts but could limit the ultimate performance of the LM399. They are not the metal foil parts-http://www.vishaypg.com/foil-resistors/list/product-63066/with as little as 2 ppm tempco. Vishay bought almost all passive part vendors including a number of competitors and lead to much confusion on which parts are the foil parts. Once i determine the correct values I may go to Texas Componentshttp://www.texascomponents.com/store/TXCCproducts.aspto get them.

author: Grandchuck
author_rank: Super Contributor
timestamp: May 06, 2019, 04:41:08 pm
content: The RNC55H resistors (9.09k) are 50 ppm. It is working well enough, considering. The 1k trimmer is the weak link at this point. My lab has AC which helps a lot.

author: iMo
author_rank: Super Contributor
timestamp: May 06, 2019, 06:04:18 pm
content: Absolute tempco is not critical as it is the matching (ratio) tempco of the divider.Of course, the smaller tempco of the individual resistors the better chance to get good matching tempco.

author: 1audio
author_rank: Frequent Contributor
timestamp: May 09, 2019, 04:47:23 am
content: Here is my tweaked LM399 board. I have a good size collection of Vishay S102K and a lot of the dual 10K Vishay resistors just visible under the S102K. I managed to get my target voltage with this and the trim resistors (around 6 megOhms). The voltage seems really stable. In 2 days I could not see a PPM change in the output on my Fluke 8506A. The board layout is not ideal and could be significantly better but its not an impediment to getting close to PPM stability and accuracy if you are patient enough. I'll share more of the host instrument on the TEA thread.