Ultra Precision Reference LTZ1000

[Andreas]

Hello,

the cirquit is very close to the one on page 15 of this thread:
so I have already external compensation.
And a additional FET for the current power stage to
reduce the necessary amplification of the current loop.

With best regards

Andreas

[Andreas]

Hello,

SMD parts are mounted on soldering side.
(example with LTC2057 as OP).

And some hours later low profile parts on component side.
(example with LT1013A except for output buffer)

With best regards

Andreas

[Andreas]

Hello,

carefully soldering the precision resistors.
(cooling by a "helping hand" with aligator clips during soldering).

First PCB done for the critical resistors.

With best regards

Andreas

[Macbeth]

DB25? 

[TiN]

What will be going on with large empty area at side?

[Andreas]

Hello,

DB25?

I like D-SUB connectors: they are cheap, have golden contact surface, and neighboured contact pins have low thermal differences (=EMF) (also because of the metal shell around the contact area).
So I have the unbuffered LTZ-voltage and the NTC outputs and the power supplies on the DB25 connector.
Perhaps I will have later a 7 to 10 volt translation cirquit attatched to this connector.
see also similar cirquit diagram:
https://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/msg249123/#msg249123

What will be going on with large empty area at side?

The batteries (12 NIMH cells) see picture on page 17:
https://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/msg250759/#msg250759

The whole is put into a compact aluminium case.

The design is not really different from the previous design. (only a revision).
The main difference is the buffered output on the 4 mm banana jacks,
and the possibility to use different precision resistor types + OP-Amps.

with best regards

Andreas

[Alex Nikitin]

I am planning to build LTZ1000 reference on the AD5791 Evaluation board.  I'll be using LTZ1000 plain, non-"A" version and somewhat changed resistors values (87K in place of 70K and 100 Ohm in place of 120 Ohm), plus I'll lower the IC temperature by changing the divider values in the regulator. Any comments on changing these resistors values or experience with that particular board layout?

Cheers

Alex

[acbern]

Hello,
I like D-SUB connectors: they are cheap, have golden contact surface, and neighboured contact pins have low thermal differences (=EMF) (also because of the metal shell around the contact area).

D-Subs are perfectly fine, they are closed (no air flow) and have a surrounding metal case ensuring low thermal differences. Datron also used it on its rear panel inputs on the 1281 DMM.
As a general remark, the EMF issue often seems to be overestimated here with lots of approaches fighting it (frankly, sometimes reminds me a little on certain high end audio type theological  discussions). I have actually not seen any considerable differences once equilibrium has been reached after connecting metals, at least not on the level we are talking here (say 0.1ppm/1uV, things change when we talk nanovolt).

[plesa]

I am planning to build LTZ1000 reference on the AD5791 Evaluation board.  I'll be using LTZ1000 plain, non-"A" version and somewhat changed resistors values (87K in place of 70K and 100 Ohm in place of 120 Ohm), plus I'll lower the IC temperature by changing the divider values in the regulator. Any comments on changing these resistors values or experience with that particular board layout?

Cheers

Alex

I have build my 4x LTZ1000 with focus on minimal long term drift. Based on recommendation from Bob Dobkin R2/R3 is 100k, die temperature to 45°C (12k/1k). For zener current change from 5mA to 7,2 mA I have no opinion. Like Ken mentioned LTFU is using higher zener current (20mA) and according to Bob LTZ1000 max zener current can be increased to 20mA.
But who know the long term and stability effects? Layout comparison between LTZ1000 and LTZFU can give us some hint, maybe.

[Andreas]

I have build my 4x LTZ1000 with focus on minimal long term drift. Based on recommendation from Bob Dobkin R2/R3 is 100k, die temperature to 45°C (12k/1k). For zener current change from 5mA to 7,2 mA I have no opinion.

With 12K/1K + higher zener current the temperature controller will fall out of regulation at a lower temperature.
(this will limit environment temperature range further).

With best regards

Andreas

[TiN]

Also higher current does help for lower noise but for long-term stability it's opposite (similar to aging, faster happening with high current and/or temperature). So you may want actually lower current and lower temperature, to get better long-term perf. Also if hysteresis not a problem for you, it worth to think about keeping references OFF until you need them.

[Alex Nikitin]

I have build my 4x LTZ1000 with focus on minimal long term drift. Based on recommendation from Bob Dobkin R2/R3 is 100k, die temperature to 45°C (12k/1k). For zener current change from 5mA to 7,2 mA I have no opinion.

With 12K/1K + higher zener current the temperature controller will fall out of regulation at a lower temperature.
(this will limit environment temperature range further).

With best regards

Andreas

Hi Andreas, reducing the zener current sensing resistor from 120 to 100 Ohm would increase the current from 5 to 6mA. It is just I have already some 100 ohm Bulk Foil resistors, that is all. Also I use "non-A" version so the additional 7.5mW dissipation should not be a problem. If required I'll just increase the temperature a bit. I may use a metal can LM35 to accurately measure the case temperature of the LTZ1000.

Cheers

Alex

[Kleinstein]

There is no real need to measure the case temperature of the LTZ1000. The important part is the inside temperature. To see if the thermostat is still in the active region, just look at the heater output - if this goes to zero, or very close regulation is lost. At the very low power end the regulation may not work so good and could need some help to compensate the square law for the power from a resistor.

If you want to measure temperature, the more imortant one in likely that of the resistors.

The change from 70 K to 87 K will also give a slightly lower temperature (e.g. 5 K). Otherwise this change should not be that important. Changing the resistor ratio (e.g. 12K/1 K ) to set the temperature will also change the TC a little bit - the transistors TC of U_BE is smaller (less negative) at a higher voltage used to set a lower temperature. So the overall TC will be slightly more positive. Depending on the unit this might be positive or not.

[TiN]

There is no real need to measure the case temperature of the LTZ1000

+1, it's will not even indicate correct relative temperature. I tested before with 50C and 75C setpoints, difference on case was only ~11C (readout by Fluke Ti32 on black matte tape on top of LTZ can).

[Alex Nikitin]

There is no real need to measure the case temperature of the LTZ1000

+1, it's will not even indicate correct relative temperature. I tested before with 50C and 75C setpoints, difference on case was only ~11C (readout by Fluke Ti32 on black matte tape on top of LTZ can).

Was it on LTZ1000 or LTZ1000A?

Cheers

Alex

[TiN]

ACH.

[Alex Nikitin]

ACH.

OK, thanks. That is hardly surprising as the thermal resistance on the "A" version is five times more and you have to run it hotter to get the temperature to stabilize. I am not sure that an internal heater is such a great idea. The only real advantages compared to an external "oven" approach are a quick start-up time and cost?

Cheers

Alex

[Andreas]

I am not sure that an internal heater is such a great idea.

Hello,

I am very shure that the LTZ _lives_ from the internal heater.
The zener cirquit has a raw TC of 50 ppm/K.
So the oven needs a stability of 0.001 K for the 0.05 ppm/K specced for the LTZ.
With a external oven you would need a much more stable zener with a "zero TC".

With best regards

Andreas

[TiN]

Internal heater allows way smaller power and better accuracy, as you need to keep only die temperature constant. OCXO is a good example of external heater design, at best, yet still bulky and power hungry beasts.

[Alex Nikitin]

I am not sure that an internal heater is such a great idea.

Hello,

I am very shure that the LTZ _lives_ from the internal heater.
The zener cirquit has a raw TC of 50 ppm/K.
So the oven needs a stability of 0.001 K for the 0.05 ppm/K specced for the LTZ.
With a external oven you would need a much more stable zener with a "zero TC".

With best regards

Andreas

Hmm, the Fluke 732B uses that approach, with a low tempco reference and an external oven, isn't it? I have achieved <1ppm/K tempco on my JVR with a metal can FET and wire-wound Ultrohm resistors and plan to use an external oven approach so I could test the long-term stability (so far it did drift less than 2ppm over 700 hours powered up and measured at the same temperature). I plan to run it against the LTZ1000 as my Fluke 731 has a noticeable (~1.5ppm/K) temperature drift (thought the long-term stability seems to be excellent).

Cheers

Alex

[Kleinstein]

An external heater usually requires more power and one might have difficulties to bring down temperature differences as well. The slower system also makes low drift analog control more difficult. On the up side the slower system smooths out fast variation - however the Zener referene has quite some 1/f noise anyway, so not a problem here. Also other parts like resistors could be kept at temperature to.

A large heat source in an instrument makes life hard for the rest of the circuit.

Even with an internal heater, there is still the option to add a second layer of temperature control, e.g. for the whole circuit. Though you would usually still keep the internal heat for the internal temperature. But there usually is no option for an internal heater if not allready there.

[alanambrose]

Hi,

As a belt-and-braces kind of guy, I wonder whether all 3 together might work well.

I'm also wondering whether there might be a much more sophisticated approach to maintaining a constant thermal environment. I don't have all the pieces figured out in my head yet, but I'm thinking the level of sophistication of a good filter design (but in the thermal domain) where the 'input signal' is the lab temperature environment and the objective is v low pass filter giving near 'thermal DC'. As EE-types, we have a tendency to bodge a bit of arbitrary plastic / foam / copper foil around a voltage ref and call it a day. But I'm imagining a modelled thermal environment with 3D printed / CNCed / lasercut materials chosen with the right characteristics to maintain optimum minimal temperature variation with fairly low power. A basic start would be say picking a temperature for the outside of the ref (maybe halfway from the die temp to max ambient) and designing the right thermal environment with some sort of iso-thermal case surrounding the key electronics - and then that surrounded by the right insulation to get the appropriate thermal heat flow out together with a means of targeting the temperature of the iso-thermal case. Am I making any sense?

Alan

[Andreas]

Hello,

next step:

All the high profile through hole parts.

1st PCB with LT1013A in CERDIP hermetically case.
2nd PCB populated with 2 choppers instead.
So only the LTZ1000A is missing.

Mmh, shall I populate it immediately?
Perhaps its better to check the new design with a "crash test dummy".
A 6V2 zener and a small signal transistor would give a
appropriate load to test at least the current regulator.

With best regards

Andreas

[splin]

The power required to maintain the temperature equilibrium, i.e. just to compensate the leaks, is : P=S.Lambda.Delta_T, which is roughly (0.05*0.05)*6*0.04*(50-20) = 0.018W. It's not that bad

18mW isn't bad if you have 1m thick insulation but if it's only 1cm you're looking at 1.8W. Though you probably don't need 50mm height - 10mm should be enough. And improving the insulation using, for example 20mm thick aerogel (.014W/m/K), would reduce power requirements somewhat - 200mW may be achievable.

[Edit] corrected aerogel thermal conductivity from .14 to .014W/m/K

[Kleinstein]

A good thermal isolation is a two sided thing: The circuit inside will produce some heat, and this heat will give a minimum temperature rise above the enviroment. So the better the insulation, the higher will the minimum temperature be. The same happens to the LTZ1000: the A Version has higher internal thermal resistance and thus a high minimal set temperatur to work.

There is a minimal difference from enviroment to internal temperature with the LTZ1000 - so the outer box temperature temperature should be choosen just low enough for the inner heater to work. You don't want much extra power from the internal heater to heat up your outer oven. Also one should keep out all heat sources that don't have to be on the inside (e.g. the driving transistor for the heater, possible current buffering transitor for the output). Only than one can add isolation to the point to keep the power requirement low. Normally you have to consider something like 4 times the power of the internal circuit for the heater, so you have some room to go up and down in power.

One difficult with the outer temperature regulation might be that the time constant is rather long so the regulator needs to include long time constants ( > minutes) - this migh be difficult analog and require a digital (e.g. µC) control. Also to keep the innertemperature as low as possible one might consider a controlled fan if the ambinet temperature is rather high. It might be an interesting option but I think this is something for a different thread.