Author Topic: LM399AH at 5v?  (Read 3113 times)

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Offline alex-shTopic starter

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LM399AH at 5v?
« on: April 24, 2019, 06:25:20 pm »
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?

Thanks
Alex
 

Online Kleinstein

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Re: LM399AH at 5v?
« Reply #1 on: April 24, 2019, 06:38:36 pm »
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.
 
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Offline iMo

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Re: LM399AH at 5v?
« Reply #2 on: April 24, 2019, 07:23:02 pm »
The best way is to mutliply by 3/4 by an LT1043. And use 5.2V instead 5V as your reference..
 

Online Kleinstein

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Re: LM399AH at 5v?
« Reply #3 on: April 24, 2019, 07:34:59 pm »
Depending on the application 2/3 or 7 V could also be a good option.
 

Offline iMo

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Re: LM399AH at 5v?
« Reply #4 on: April 24, 2019, 09:06:51 pm »
LM399 * 3/4 = 5.2V
 
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Offline alex-shTopic starter

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Re: LM399AH at 5v?
« Reply #5 on: April 25, 2019, 08:35:36 am »
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.

Indeed, 0.71 is a very peculiar factor.
 

Online Andreas

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Re: LM399AH at 5v?
« Reply #6 on: April 25, 2019, 08:45:50 am »
LM399 * 3/4 = 5.2V

Hello,

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/#msg304819

did you ever test the cirquit in real life?
Up to now I have only simulations in LTSPICE.

with best regards

Andreas
 

Offline ycui7

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Re: LM399AH at 5v?
« Reply #7 on: April 25, 2019, 09:17:12 am »
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/#msg2319243

The tempco is roughly 1ppm/K. Long term stability not tested.

Offline alex-shTopic starter

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Re: LM399AH at 5v?
« Reply #8 on: April 25, 2019, 09:17:48 am »
I suppose two resistors 200k and 499k can do a voltage divider with necessary ratio  ~ 5V
 

Offline iMo

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Re: LM399AH at 5v?
« Reply #9 on: April 25, 2019, 09:21:56 am »
LM399 * 3/4 = 5.2V
did you ever test the cirquit in real life?
Up to now I have only simulations in LTSPICE.
Nope, 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..
« Last Edit: April 25, 2019, 09:39:47 am by imo »
 

Offline alex-shTopic starter

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Re: LM399AH at 5v?
« Reply #10 on: April 25, 2019, 09:29:12 am »
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/#msg2319243

The tempco is roughly 1ppm/K. Long term stability not tested.

1ppm 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?

Thanks
Alex

EDIT: 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?

« Last Edit: April 25, 2019, 09:45:31 am by alex-sh »
 

Offline iMo

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Re: LM399AH at 5v?
« Reply #11 on: April 25, 2019, 09:46:38 am »
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?
 

Offline alex-shTopic starter

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Re: LM399AH at 5v?
« Reply #12 on: April 25, 2019, 10:11:40 am »
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?

The 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
 

Offline iMo

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Re: LM399AH at 5v?
« Reply #13 on: April 25, 2019, 10:16:53 am »
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..
« Last Edit: April 25, 2019, 10:24:01 am by imo »
 

Online Kleinstein

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Re: LM399AH at 5v?
« Reply #14 on: April 25, 2019, 04:01:44 pm »
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.
 

Online Andreas

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Re: LM399AH at 5v?
« Reply #15 on: April 25, 2019, 09:00:12 pm »

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..

Hello,

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 regards

Andreas
 

Offline iMo

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Re: LM399AH at 5v?
« Reply #16 on: April 25, 2019, 09:20:02 pm »
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.
« Last Edit: April 25, 2019, 09:24:18 pm by imo »
 

Online Andreas

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Re: LM399AH at 5v?
« Reply #17 on: April 26, 2019, 04:24:31 am »
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 cirquit
works 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 regards

Andreas
 

Offline iMo

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Re: LM399AH at 5v?
« Reply #18 on: April 26, 2019, 05:53:14 am »
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 capacitor

deltaV = 3.5e-12/1e-6 = 3.5uV

With above schematics we have several switches involved.

« Last Edit: April 26, 2019, 09:25:15 am by imo »
 
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Offline alex-shTopic starter

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Re: LM399AH at 5v?
« Reply #19 on: April 28, 2019, 11:16:46 am »
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/#msg2319243

The tempco is roughly 1ppm/K. Long term stability not tested.

Interesting.
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?
 

Offline ycui7

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Re: LM399AH at 5v?
« Reply #20 on: May 03, 2019, 04:25:22 am »
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/#msg2319243

The tempco is roughly 1ppm/K. Long term stability not tested.

Interesting.
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?

There 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.
 
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