LM399AH at 5v? - Page 1
Forum section: electronics / general

author: alex-sh
author_rank: Regular Contributor
timestamp: April 24, 2019, 06:25:20 pm
content: Hello,I have several LM399 running at 10V and 7V. What's the best way to make it run at 5V please?10V with precision INA105 1/2 gain?ThanksAlex

author: Kleinstein
author_rank: Super Contributor
timestamp: April 24, 2019, 06:38:36 pm
content: The LM399 give out some 7 V. So it would need some precision divider with a factor of about 0.71. There is no need to go up to 10 V first. However this is not a very common divider ratio. There may be approximate values from some resistor arrays.

author: iMo
author_rank: Super Contributor
timestamp: April 24, 2019, 07:23:02 pm
content: The best way is to mutliply by 3/4 by an LT1043. And use 5.2V instead 5V as your reference..

author: Kleinstein
author_rank: Super Contributor
timestamp: April 24, 2019, 07:34:59 pm
content: Depending on the application 2/3 or 7 V could also be a good option.

author: iMo
author_rank: Super Contributor
timestamp: April 24, 2019, 09:06:51 pm
content: LM399 * 3/4 = 5.2V

author: alex-sh
author_rank: Regular Contributor
timestamp: April 25, 2019, 08:35:36 am
content: Quote from: Kleinstein on April 24, 2019, 06:38:36 pmIndeed, 0.71 is a very peculiar factor.

author: Andreas
author_rank: Super Contributor
timestamp: April 25, 2019, 08:45:50 am
content: Quote from: imo on April 24, 2019, 09:06:51 pmHello,there is a "bug" in the component values against my original cirquit which leads to overshoot and unnecessary long startup time:I have one 1.5uF capacitor to achieve better behaviour see:https://www.eevblog.com/forum/projects/building-a-7-decade-voltage-calibrator/msg304819/#msg304819did you ever test the cirquit in real life?Up to now I have only simulations in LTSPICE.with best regardsAndreas

author: ycui7
author_rank: Contributor
timestamp: April 25, 2019, 09:17:12 am
content: Attached picture shows my way of getting nominal 5V.I mentioned it in a separate post previously.https://www.eevblog.com/forum/metrology/lm399-based-2-5v-reference/msg2319243/#msg2319243The tempco is roughly 1ppm/K. Long term stability not tested.

author: alex-sh
author_rank: Regular Contributor
timestamp: April 25, 2019, 09:17:48 am
content: I suppose two resistors 200k and 499k can do a voltage divider with necessary ratio ~ 5V

author: iMo
author_rank: Super Contributor
timestamp: April 25, 2019, 09:21:56 am
content: Quote from: Andreas on April 25, 2019, 08:45:50 amNope, sim only.Btw, in the simulation there are ripples of about 40uVpp (it is to expect), thus additional filtering would be nice to have. With 1uF the ripple is 50uVpp.PS: the "Branadic's" ripple is lower, around 17uVpp..

author: alex-sh
author_rank: Regular Contributor
timestamp: April 25, 2019, 09:29:12 am
content: Quote from: ycui7 on April 25, 2019, 09:17:12 am1ppm tempco is a very good result in my view. I would really like to see the long-term stability.You are using two dual opams - LT1112. Why not using quad opamp LT1114 and terminating the forth one?ThanksAlexEDIT: You have a 50k pot. Finding a pot with a good temco is hard and they are much more expensive and only give 15ppm at best. What's R3 value please?

author: iMo
author_rank: Super Contributor
timestamp: April 25, 2019, 09:46:38 am
content: The tempco itself of the 50k 10turn pot is not important imho, the matching tempco between the 2 portions of the pot does..What could be a matching tempco of such A and B parts of the pot?

author: alex-sh
author_rank: Regular Contributor
timestamp: April 25, 2019, 10:11:40 am
content: Quote from: imo on April 25, 2019, 09:46:38 amThe datasheet does not say. They just say about tempco for the pot.This is prudent to assume that it would be not less than pot's tempco.http://www.vishaypg.com/docs/63056/12801285.pdf

author: iMo
author_rank: Super Contributor
timestamp: April 25, 2019, 10:16:53 am
content: It would be great if experts here would be kind enough and measure the matching tempco of, for example, a Vishay 100ppm 50k 10t pot trimmer in say 25k/25k and 5k/45k wiper positions, for example..

author: Kleinstein
author_rank: Super Contributor
timestamp: April 25, 2019, 04:01:44 pm
content: The pot's setting only has a reduced effect due to the resistors (10 K and 1 M). So the pots TK would also have a reduced effect, like some 1% of the pot's TK. So a 100 ppm/K (divider ratio) pot could contribute some 1ppm/K to the output. The ratio TK is often not that bad with pots.

author: Andreas
author_rank: Super Contributor
timestamp: April 25, 2019, 09:00:12 pm
content: Quote from: imo on April 25, 2019, 09:21:56 amHello,I think the higher ripple has something to do with your (unbuffered) LM399 model.I would try a 1uF capacitor in parallel to the LM399.with best regardsAndreas

author: iMo
author_rank: Super Contributor
timestamp: April 25, 2019, 09:20:02 pm
content: It comes from switching inside the LTC1043, see below. The red trace is the signal at the LTC1043 clock capacitor C1 (Cosc pin).An ideal 6.9V source used instead the LM399.

author: Andreas
author_rank: Super Contributor
timestamp: April 26, 2019, 04:24:31 am
content: Hello,now the large negative spikes from previous simulation have gone.-> we probably need some buffer or capacitor for a real world cirquit.The rest looks like charge injection.I always have the suspect that the charge injection cancellation cirquitworks in the "wrong" direction for the 2:1 divider cirquit from the data sheet.At least I get different gains when exchanging input and output ports for the 2:1 divider.(measured something like -9 ppm with datasheet pinning against -2ppm with reversed pinning)So it would be interesting what happens if we exchange S1 against S2 and S3 against S4 pin.By the way: if you use 10 nF for both OSC-Pins you get about the double switching frequency (1 kHz)against the typical LTC1043 400-500 Hz frequency. That is why I use 20 nF in my simulation.with best regardsAndreas

author: iMo
author_rank: Super Contributor
timestamp: April 26, 2019, 05:53:14 am
content: Looks symmetrical in sim. Mirrored Sx pins. 22nF clock cap.PS: added for reference divide by 2 (3.45V out). Ripple is 6.67uVpp.PPS: the charge injection for LTC1043 at 15Vcc and Vin=7V is 3.5pCb (DS).With a single switch involved it should create a voltage pulse on an 1uF capacitordeltaV = 3.5e-12/1e-6 = 3.5uVWith above schematics we have several switches involved.

author: alex-sh
author_rank: Regular Contributor
timestamp: April 28, 2019, 11:16:46 am
content: Quote from: ycui7 on April 25, 2019, 09:17:12 amInteresting.You have managed to get 2.5V reference from LM399 - I saw it in another post.Would you please post the full schematic - LM399 (~7V) - 5V - 2.5V?

author: ycui7
author_rank: Contributor
timestamp: May 03, 2019, 04:25:22 am
content: Quote from: alex-sh on April 28, 2019, 11:16:46 amThere isn't much secret. You can get 5V from the schematic I posted previously, and use another LT5400 to divide 5V to 2.5V and buffer it with a precision opamp such as OPA2277, AD8676, etc. For best stability, remove the trimming pot.