Author Topic: LM399 based 10 V reference  (Read 316657 times)

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Online imo

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Re: LM399 based 10 V reference
« Reply #1025 on: August 28, 2019, 08:35:38 pm »
I've been bending the leads a little bit (using 2.54mm pitch socket). Hopefully that stress does not contribute to the popcorn too..
 

Offline Andreas

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Re: LM399 based 10 V reference
« Reply #1026 on: August 29, 2019, 08:53:30 am »
Hello,

I think that is no issue as long as you do not damage the glass seals.
I have here one LM399 which is affected by humidity.
(Seasonal changes of the output voltage).
So I think the hermetical seal is broken.

with best regards

Andreas
 
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Online imo

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Re: LM399 based 10 V reference
« Reply #1027 on: August 29, 2019, 11:29:43 am »
Interesting.. Pretty hot the 399 package and the humidity has such an effect..  :phew:
 

Offline FriedLogic

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Re: LM399 based 10 V reference
« Reply #1028 on: September 03, 2019, 12:11:07 am »
I've been bending the leads a little bit (using 2.54mm pitch socket). Hopefully that stress does not contribute to the popcorn too..

I got some 2mm pitch sockets (2x2 Preci-Dip 833 Series) for an LM399 ageing box since the spacing is closer to correct and the insertion force looked reasonable. Although unlikely to be a problem, I thought that the less mechanical stress there is the better.
It's not built yet so I can't comment on how well they work (has anyone else tried them for this type of application?)

Even in the few LM399's that I have running I've seen occasional popcorn noise appear or disappear over months, so I'd expect that ageing has some effect, but it could be good or bad.
« Last Edit: September 03, 2019, 12:12:43 am by FriedLogic »
 

Online imo

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Re: LM399 based 10 V reference
« Reply #1029 on: September 03, 2019, 09:45:46 am »
I was using the 2.54mm rounded precision ones and my observation had been the touching the 399 while in the socket changed the output voltage by dozens of uVs.
 

Offline Robert763

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Re: LM399 based 10 V reference
« Reply #1030 on: September 05, 2019, 09:09:29 am »
There is a proper socket available that the 399 will fit (0.1" 4 pin square. It's a TE connectivity 8060-1G4
Mouser part No. 571-8060-1G4  Farnell order code 1608447
https://www.te.com/usa-en/product-1-1437504-7.html
Drawing here:
https://www.te.com/commerce/DocumentDelivery/DDEController?Action=showdoc&DocId=Customer+Drawing%7F1437504-3%7F99%7Fpdf%7FEnglish%7FENG_CD_1437504-3_99.pdf%7F1-1437504-7

Robert.

 
 

Offline free_electron

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Re: LM399 based 10 V reference
« Reply #1031 on: September 06, 2019, 04:52:01 am »
if anyone needs real lm399's ( not from china ) . i just found about 20 or so in my garage. brand new , NOs , nat semi (old logo )
10$ a piece.
Professional Electron Wrangler.
Any comments, or points of view expressed, are my own and not endorsed , induced or compensated by my employer(s).
 

Online imo

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Re: LM399 based 10 V reference
« Reply #1032 on: September 28, 2019, 11:03:38 am »
What would be the "optimal" value for the RS1..RS4 and CLP1 in case I want to average 4x LM399s?

I plan to use an OPA277 (bip) or OPA2210 (low noise bip), or OPA187 (AZ) there (all are with drift <=0.1uV/C).

PS:

OPA2210  0.4pA/sqrtHz      2.5nV/sqrtHz
OPA277    0.2pA/sqrtHz    12nV/sqrtHz   (fyi - OP07  0.35pA/sqrtHz   11nV/sqrtHz)
OPA187    0.16pA/sqrtHz   20nV/sqrtHz

PPS:

added some calcs, it seems 4x RSx=47k + OP create around 1uVpp (CLP1 not connected).

OPA2120 below should be OPA2210.
« Last Edit: September 28, 2019, 04:20:54 pm by imo »
 

Offline Kleinstein

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Re: LM399 based 10 V reference
« Reply #1033 on: September 28, 2019, 03:00:25 pm »
The LM399 has relatively high noise. So it is relatively easy to get get resistors and an OP with lower noise. If filtering is used, it is more the filter function that sets the suitable resistance. To high resistance, bias current drift, leakage and resistor noise set an upper limit. I would consider the 47 K about the upper useful limit, maybe better only 10 K to be conservative with high bias OP.
Without a filter one can go lower - like 1 K if one wants, as there is still not much voltage across the resistors. In most cases I would add at least some filtering, as it's low effort in this position.

The choice of OP also depends on the load to the OPs output - usually no need for something really fancy and the OP07 may be good enough.
 

Online imo

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Re: LM399 based 10 V reference
« Reply #1034 on: September 28, 2019, 03:18:15 pm »
When using RSx=100k and CLP1=1uF the BW(-3dB) is 1.6Hz. That lowers the resistor's noise, and let the OPamp noise BW stays the same (??) (say <10Hz due to DMM's sampling period). Sure it is at the edge.

Re noise the OP-07 is similar to the OPA277, the only issue I see with the OP-07 is up to 10x higher TC drift.

PS: fixed RS_CLP1 BW.
« Last Edit: September 28, 2019, 04:22:27 pm by imo »
 

Online antintedo

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Re: LM399 based 10 V reference
« Reply #1035 on: September 28, 2019, 04:03:49 pm »
When using RSx=100k and CLP1=1uF the BW(-3dB) is 1.6Hz.
1-pole filter is far from a brick wall filter, so the effective noise bandwidth is 1.57 * 1.6Hz. Not very important here but good to remember. Also watch for 1/f current and voltage noise of the opamps - parameters are specified at 1kHz but you are most concerned about 0-10Hz region. AD app note MT-048 has some relevant information. To be able to ignore resistor 1/f noise make sure to use good quality metal film or wirewound resistors, crappy thick film ones can be problematic.
 
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Offline Kleinstein

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Re: LM399 based 10 V reference
« Reply #1036 on: September 28, 2019, 04:15:25 pm »
The averaging resistors see rather low voltage. Thus resistor excess noise is not really relevant for these resistors. This is different for the resistor(s) in the 7 to 10 V stage, that see some 7 V.

The 1/f noise can in deed be relevant. Especially the current noise of the BJT based OPs tends to be higher at low frequency.
 

Online imo

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Re: LM399 based 10 V reference
« Reply #1037 on: September 28, 2019, 04:25:19 pm »
Therefore the OPA2210 - from DS
Quote
The OPA2210 precision operational amplifier is built on TI's precision superbeta complementary bipolar semiconductor process, which offers ultra-low flicker noise.. SiGe bipolar process..

The Q is how is the current noise at 0.1-10Hz.
A: aprox 22pA p-p (3.3pA rms).
« Last Edit: September 28, 2019, 06:02:09 pm by imo »
 

Online antintedo

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Re: LM399 based 10 V reference
« Reply #1038 on: September 28, 2019, 04:58:18 pm »
Current noise is specified in the datasheet: page 7, figure 4. With 1/f corner frequency and white noise density it is possible to approximately calculate the noise. The app note I mentioned does exactly that.
« Last Edit: September 28, 2019, 05:20:35 pm by antintedo »
 

Online imo

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Re: LM399 based 10 V reference
« Reply #1039 on: September 29, 2019, 10:03:11 am »
FYI - the noise analysis of an OP07 and an OPA2210 in a traditional Vref circuit.
The noise of the LM399 not included here.

PS: interestingly, the built-in noise model of the OP07 is pretty optimistic, at least when comparing the OP07 and OPA2210 datasheets :)

PS: added a few popular opamps
« Last Edit: September 29, 2019, 11:01:36 am by imo »
 
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Online imo

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Re: LM399 based 10 V reference
« Reply #1040 on: September 29, 2019, 01:03:52 pm »
A summary of the above noise simulations..

PS: added Vn_out with RS=10k

PPS:
BTW - the chopping frequencies:
                  Datasheet           Simulation peaking at
LTC1050     2.5kHz                2.5kHz
LTC1052     330Hz                 140Hz (model without external 100n caps)
LTC1250     5kHz                   none
LTC2057     30kHz                 30kHz
ADA4522    4.8MHz/800kHz   none (sim up to 10MHz)
« Last Edit: September 29, 2019, 04:20:13 pm by imo »
 
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Online imo

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Re: LM399 based 10 V reference
« Reply #1041 on: November 02, 2019, 12:28:49 pm »
FYI - Interesting reading for all of us who are using 10T RES TRIMMERS in their Vrefs ::)

 


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