Ultra Precision Reference LTZ1000

[Andreas]

Hello,
with a dual supplied OP-Amp or a OP-amp which can go below 0.5V at the output there is always risk to forward bias the substrate diodes in the LTZ1000. I would not want to remove this diode.

with best regards

Andreas

[julian1]

Hello,
with a dual supplied OP-Amp or a OP-amp which can go below 0.5V at the output there is always risk to forward bias the substrate diodes in the LTZ1000. I would not want to remove this diode.

with best regards

Andreas

Thanks, I think I agree.

[RoadRunner]

Hello to all,

I am looking to make a reference board with LTZ1000. I wonder where do people find these impossible to find resistors?
If any one already have these resistors for sale, I would love to buy them.

Is there a group buy planned? Are there other people looking for them? May be we can plan a group buy.


Regards

[Ole]

Currently cou could get the LTZ1000CH#PBF from mouser.com, but not the 1000A
If we could organize a group buy for the Resistors and/or for the 1000A (once it is availible again from analog.com or other vendors),
I would very much be interested to participate.

Edit: The resistors are mostly from http://www.vishaypg.com/foil-resistors/hermetically-sealed/ for the hermetically sealed ones.
And yes the ordering from Vishay is not the simplest. I haven´t placed a order with them yet.

Edit2: The datasheet specifies a 13k/1k divider, does anyone know the resulting temperature from the upper resistor across multiple values?

Ole

[maat]

In Germany the Vishay distributor is BADER GmbH & Co. KG (https://www.bader.net/). Send them an email, be prepared for hefty prices and long wait times (14 Weeks or so).

[Kleinstein]

At least for the first test one does not need the absolute highest stability resistors.  E.g. Mouser has 1 K and 13 K resistors in the S102 version (plastic case metal foil).

Some of the divider ratios can also be obtained with resistor networks: e.g. 1:12.1 from a dual 10K/1K. This is at the lower temperature end, but may still be OK for the non A version. A smaller resistor for R2 can shift the temperature up a little if needed.
The 8 or more equal resistor networks can also do some rations like 1:12 and 1:15 and 1:14 (AFAIR) and maybe a fre more.

[martinr33]

I have had good results from these folks:

http://webdirect.texascomponents.com/SearchResults.asp?Cat=1842

You have to call / email to get quotes, and they do have a small premium for single parts. They are a Vishay licensee. They also used to have VHP devices, but they do not show them on their web page which means they could have a supply problem. Also, if you call them, they may have parts not on the web site. Generally, I have had them ship in a couple of weeks.

Also:
https://www.ebay.com/itm/121777259014?hash=item1c5a7d8206:g:x8wAAOSwVL1WD3Em
for example.

[Andreas]

Hello,

according to the datasheet:

"With the values given in the applications, temperature is normally 60°C."
From there you can extrapolate the resistors with the typical -2mV/deg C of the base emitter diode.

with best regards

Andreas

[branadic]

The 8 or more equal resistor networks can also do some rations like 1:12 and 1:15 and 1:14 (AFAIR) and maybe a fre more.

And even more ratios: https://xdevs.com/guide/rnet_ratio/ just pick the one you need.

-branadic-

[Ole]

"With the values given in the applications, temperature is normally 60°C."
From there you can extrapolate the resistors with the typical -2mV/deg C of the base emitter diode

I would presume that the average voltage across R4 and R5 would be 7,7V which would give UR5 with 13k on R4 a value of 550mV approximately.
Since I don´t yet know the normal UBE of Q2 nor the normal temperature, I am gonna presume a temperature of 20°C and 0.65V.
My target oven temperature is between 40°C and 50°C which would mean that the UBE of Q2 would be at 0.6V so a 1k/12k Divider should do the job.
Although I might want to add a spot for an SMD resistor besides each part of the divider os that I can adjust the temperature by addding higher resistance values in paralell.

Ole

[julian1]

Are there any reports of improvement in low frequency noise (current leakage, da, acoustic pickup) with film versus c0g/x7r  - for the 22nF or 100nF compensation cap (if used)?

[Andreas]

I would presume that the average voltage across R4 and R5 would be 7,7V

No the output voltage is 7.2 V per data sheet.

by the way I published all operating points of some of my references here:
https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/msg874560/#msg874560

the 0.53x V give around 52 deg C heater temperature which is enough for about 40 deg C environment temperature.
Note that the "environment temperature" of the reference internal of a housing is higher than room temperature.

with best regards

Andreas

[Kleinstein]

Are there any reports of improvement in low frequency noise (current leakage, da, acoustic pickup) with film versus c0g/x7r  - for the 22nF or 100nF compensation cap (if used)?

The usual choice is using film caps - this was at least before higher capacitance C0G were available. The reference is static and dielectric absorbtion would have essentially no effect.  By now the price for C0G caps even with 100 nF is at an acceptable level and there is no real need to take the risk with X7R or similar with possible acoustic pickup and leakage.   I don't think X7R was ever a good choice.

[Ole]

So I´ve worked on a version of the LTZ1000(A) Reference Voltage Circuit that gives +10V and -10V as output.
Most of the circuit originates from the datasheet, I just added the ouput stage.
Edit: The added file is for the Falstad Simulator, I hope that is works

[Kleinstein]

So I´ve worked on a version of the LTZ1000(A) Reference Voltage Circuit that gives +10V and -10V as output.
Most of the circuit originates from the datasheet, I just added the ouput stage.
Edit: The added file is for the Falstad Simulator, I hope that is works

That circuit looks horrible:
The compensation for the temperature regulation is wrong in the main reference part.
The added part to generate the +-10 V is made would have poor performance. There is a good intention to get some kelvin sensing, but the difference ampfier would just add way too much drift, making things only worse.

The more practical way is to avoid the extra stage and use some compensation of the ground current to get a good apprixmation to kelvin sensing. Depending on the rest of the circuit / supply, one could also just have a seprate ground for the supply and output side, meeting at a star ground point near the LTZ1000 ground.

[Andreas]

Hello,

the scaling resistors are rather high-ohmic.
Depending on OP-Amp you may get high current noise.

No further comments as the schematic is near un-readable.

with best regards

Andreas

[Ole]

I´ve got a question about R2 and R3:
Can the values of these resistors theoretically be reduced to approx. 50kOhms?
Also, I´ve heard somewhere that R2 and R3 can theoretically be 5ppm/°C as well.
Is that an option or rather not?

[Kleinstein]

A different value for R2 and R3 is definitely an option. The data-sheet already has circuits with 30 K and 50 K as examples. There is also no real need to have the same value and type for R2 and R3 - the main reason is a simpler BOM. More current at the transistors gives slightly less temperature effect from the transistors (which is not good), but with only 50 K the differente is minute ( some 1% for the temperature sens and some 3 ppm/K for the unheated TC). On the upside the noise is marginally lower. A somewhat higher values (e.g. 100 K) would work as well, but chances are that there is not much difference. With a much different value (e.g. 300 K or 20 K ) one may have to adjust some of the capacitors too.

5 ppm/C, 10 ppm/C and maybe even 25 ppm/C would still be acceptable for the resistors. The relevant question is more the long term stability and a low TC is used as a overall parameter to judge resistor quality, as specs for the long term drift are rare and vague. The drift is attenuated by a factor of some 100 to 500. So 10 ppm/C for the resistor would lead to some 0.02 to 0.1 ppm/C for the voltage and thus likely only a small contribution. If the same resistor series is available in 2, 5 and 10 ppm/K grades it may not make a big difference to use the best selection there.

[Ole]

The reason why I would differ from the ususal 70k value is because there are 50k SMD resistors with a near zero tempco,
and those are in stock. Theoretically one could place two resistors in series but that could introduce thermal EMF.

[Andreas]

I´ve got a question about R2 and R3:
Can the values of these resistors theoretically be reduced to approx. 50kOhms?
Also, I´ve heard somewhere that R2 and R3 can theoretically be 5ppm/°C as well.
Is that an option or rather not?

Hello,

50k is possible.

Keep in mind that this will increase the sensitivity of setpoint temperature resistor ratio changes (1K/12K5) by about 10%.
So about 1.1 ppm / 100 ppm instead of 1 ppm/100 ppm ratio change.
Measured values should be somewhere at the beginning of this thread when I made measurements on my LTZ1000A #1 and #2.

The 70K resistor changes have low influence on overall T.C. so put your money mainly on the setpoint temperature resistor ratio.

with best regards

Andreas

edit: see here:

https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/msg436981/#msg436981

[cellularmitosis]

If we look at just the refamp portion of the 7V LTZ circuit, we have this:



This got me thinking, could we just add one additional resistor to produce 10V?  Has anyone tried this?

[dietert1]

The proposed circuit will work, yet it will be useless.
Ube of the transistor adds a -2 mV/K temperature coefficient. This is to compensate the TC of the zener, so it vanishes (mostly). Now you multiply that by 600R/120R = 5 and there will be a TC of -10 mV/K in 10 V = -1000 ppm/K.
Now you can argue the LTZ1000 has a near perfect oven, but still this won't work well. Unless you  find a way to compensate the TC once more, maybe using the temperature sensor inside the LTZ1000.

Regards, Dieter

[KT88]

No. Vbe is changing slightly with temperature which results in a change of Iz. There is one resistance value where the temperature drift will be compensated. You find this application example in the DS for an unheated LTZ1000. If the resistance is increased the over all drift worsons…
Also with the heated device there will be slight changes in the zener current which will be exaggerated by the additional resistance.

[dietert1]

No, the resistor value for fine tuning TC is around 10 or at most 20 Ohms.
The intention of the proposed circuit was to raise the reference output to 10 V.

Regards, Dieter

[KT88]

Increasing the resistor further than 20ohms would overcompensate the drift. You would need about 600 Ohms at 5mA Iz to get to 10V. That would mean you are overcompensating by 30 - 60x…