Linear LM399 low-cost version on XDevs.com - Page 1
Forum section: electronics / projects

author: dgminala
author_rank: Contributor
timestamp: July 22, 2017, 12:46:10 am
content: I've started the Linear LM399 low-cost version 10/20 volt reference as presented onhttps://xdevs.com/article/kx-ref/. I was getting ready to start ordering parts for the project, and finding the specified resistors a bit difficult to locate, I started analyzing the circuit to see if I could find more easily sourced resistor values. Using the resistor values shown on the schematic, I can't make any sense out of the circuit.The values on the simplified schematic of the LM399 circuit on that page work out well, but the values on the final schematic seem to generate an output voltage of about 94 volts... not quite what it should be..Can someone work out that circuit and help me figure out what I'm doing wrong? I've ordered the PCB from OshPark, but don't want to order a lot of expensive resistors that won't make the circuit work.Thanks for any help!Dave M

author: MK
author_rank: Regular Contributor
timestamp: July 22, 2017, 07:12:17 am
content: Your right, it seems to have a ratio of about 22.4:1 for 20V out, and about 11:1 for 10 V out, you need to inspect the gerber to work out if it is the same as the complete schematic first off, then LTspice will help you work out some corrected values.You could also ask TiN as xdev.com is his site.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 22, 2017, 10:15:26 am
content: The circuit shows a lot of optional resistors. There are also lots of parallel / series connections that are nor really needed if you can order suitable resistor values.In the simplified circuit, there is no need for R1 to be really low TC and stability. The LM399 is not very sensitive to that resistor so something like normal 100 ppm/K resistor should normally be good enough.Similar in the "full" curcuit R23 and R3 used for the parallel connection are not all critical - these can be normal resistors.

author: dgminala
author_rank: Contributor
timestamp: July 22, 2017, 10:20:23 am
content: Thanks for the confirmation of my math. I posted the question on xdevs.com, waiting for a reply from him. After mulling over the issue, I tried switching places with the resistors around the op amp, but that still didn't make it work.I'll wait for a response from TiN and see what he says. In the meantime, I'll do a bit of math and see if I can come up with some values that will fit the PCB layout.Best I can determine from the gerbers, the PCB agrees with the schematic; it's just that the given resistor values don't work.Thanks,Dave M

author: Kleinstein
author_rank: Super Contributor
timestamp: July 22, 2017, 03:34:18 pm
content: With just one LM399 one does not need more than the simplified circuit. More resistors are just there to use values at hand and to allow for fine adjustment, e.g. to get exactly 10 V at the output. However even the large number of resistors in the full circuit does not look like they are made for fine adjustment. So there might be a rather limited value in using that board.With two references one would need another 2 resistors to do the averaging. Also grounding can get a little tricky - this does not look that good in full circuit.For the LM399 I am not sure the LTC2057 is a good choice. It adds some high frequency noise, EMI. Good conventional OPs (like OPA177) should have less drift than the LM399 and especially less than the typical resistors (unless you spend a fortune there) - so there is no real need for an AZ OP. The main critical parts in this circuit is the 7 to 10 V scaling and thus the two resistors that set the gain. So usually the point is getting two really good resistors (e.g. 1-3 ppm/K range, PWW or precision foil). Additional less critical resistors are for adjustment - here it will take individual values depending on the reference voltage, but these don't need to be that stable.

author: dgminala
author_rank: Contributor
timestamp: July 22, 2017, 04:56:55 pm
content: Yes, I know that the simplified circuit will work as an absolute minimum circuit. The big deal with this project is an attempt to eek out the very best performance from an LM399-based voltage reference. That's why the low tempco resistors. And I'm aware that the circuit will work with lower quality resistors. The LM399 reference circuit is a MUCH lower cost than the LTZ1000 circuit described on TiN's web site. And that's why I am interested in building it.Getting back to my original post,, I can't make any combination of the specified resistor values around the opamp give a 10V output. That's why I posted the question, just to make sure that my analysis was correct. And yes, I can work out resistor values to make it work; I'm just confused about TiN's resistor specifications.The LTC2057 was chosen to minimize input offset voltage drift over temperature. It's a good selection for a low drift amp. In fact, the only parameter in which the OPA177 beats the LTC2057 is high freq noise. That noise can be reduced by the inclusion of the integrating capacitor shown in the schematic. Still waiting TiN's response to my question about his recommended value for that capacitor (capacitor values are not specified on the schematic).I wonder if TiN had actually built the LM399 circuit, or if it's just a conceptual design. He went to the trouble of designing a PCB for the circuit, so he must have intended to actually build it for testing.Thanks for your response. Always good to get other viewpoints on a project.Dave M

author: Kleinstein
author_rank: Super Contributor
timestamp: July 22, 2017, 05:27:47 pm
content: The larger number of resistors does not really help. It does not get significantly better with 10 resistors with a 25 ppm/K stability. One could consider using 3 equal resistors as a starting point to give amplifier a gain of 1.5 as a starting point. The correct way is using 2 resistors with a really good TC matching (e.g. 1 ppm/K range) and long time stability. Other resistors (to set the current, for averaging and fine adjustment) are not critical and could be in the 50-100 ppm/K range- maybe slightly better for the fine adjustment.Stability of the 10 V would be mainly set by the 2 critical resistors, with smaller contributions from the reference and the OP.Filtering at the OP (like shown in the full circuit) would be mainly high frequencies and for added stability, so maybe something in the 10 nF. For real filtering of the reference the capacitor would be from the OPs non inverting input to ground and it only works in combination with the averaging resistor (likely slightly larger and even with just one reference). The capacitor can be rather large, depending on the requirements - so could be 1µF or slightly larger (likely polyester,like MKS series as other types get bulky and expensive with little advantages). Still this would only provide filtering above about 10-100 Hz.To make it a useful circuit it might help to make the output more tolerant to capacitive load.

author: cellularmitosis
author_rank: Unknown
timestamp: July 24, 2017, 05:08:17 am
content: Dave, do you need 10v output? The 7v output will be much more stable, especially with "normal" resistors.

author: cellularmitosis
author_rank: Unknown
timestamp: July 24, 2017, 05:15:06 am
content: See also this threadhttps://www.eevblog.com/forum/metrology/influence-of-resistors-in-lm399-reference-circuit/