Issue with my ltz1000 circuit

[julian1]

Out of curiosity, why did you opt for the non-A version?  Did I read that right?

A couple of *ideas* (not fully justified or thought out) I had about the non A version were,

- that non-insulated means that it can cool faster, and I felt that was likely to be more balanced/symmetrical to the heating side of regulation which is reasonably powerful. So hopefully the pid controller would have an easier job and spend equal time on both sides as it converges on the equilibrium regulation point.

- Faster cooling means it's faster to do the tempco trimming search in the version of the circuit that is not temperature regulated. That's because after applying a heating pulse, there's less time to wait before it cools and is ready for the next pulse.

- I associated the A versions with higher temperatures - at least it was my observation that many using the A version were running them hotter (including hp3458). From an engineering point of view - if there is sufficient headroom over ambient conditions to guarantee stability then that should be sufficient. Also the better long-term drift characteristics of the ltz1000 at lower temperatures.

[Andreas]

At the moment, I suspect the RF issue really needs to be dealt with. 

Hello,

now when I have seen the other thread with the scope ...
Yes you have a massive 100 MHz RF problem.
The RF gets rectified on PN-junctions and generates a "offset-voltage".
Would be interesting if the source is radiated or conducted via the mains line.


I fear a capacitor alone will not help much.
At 100 MHz a 2.2nF to 10nF might be better than a 10nF to 100nF due to self resonance of the capacitors.

But there are too many potential problems on the PCB:
Long legs of LTZ1000 which can generate a loop antenna (similar to the scope).
(so the filter cap may be too far away to filter out the RF).
The "pot" has also a large capacitance to the environment (your finger).
I´d suggest additional using of a metal housing and at least filter the power supply lines with feed through like capacitors to the housing.

The standard cirquit of LTZ is likely to oscillate when you try to filter the zener voltage too ...

with best regards

Andreas

[julian1]

At the moment, I suspect the RF issue really needs to be dealt with. 

Hello,

now when I have seen the other thread with the scope ...
Yes you have a massive 100 MHz RF problem.
The RF gets rectified on PN-junctions and generates a "offset-voltage".
Would be interesting if the source is radiated or conducted via the mains line.


I fear a capacitor alone will not help much.
At 100 MHz a 2.2nF to 10nF might be better than a 10nF to 100nF due to self resonance of the capacitors.

But there are too many potential problems on the PCB:
Long legs of LTZ1000 which can generate a loop antenna (similar to the scope).
(so the filter cap may be too far away to filter out the RF).
The "pot" has also a large capacitance to the environment (your finger).
I´d suggest additional using of a metal housing and at least filter the power supply lines with feed through like capacitors to the housing.

The standard cirquit of LTZ is likely to oscillate when you try to filter the zener voltage too ...

with best regards

Andreas

Good, thanks. I suspect there may be some rectification on the output of the op-amp by the 1n4148 as well. Out of *all the things* I have experimented with - removing it has been the only thing that had a definitely noticeable affect.

I don't know much about RF. I'll try to buy a fully enclosed aluminum project box, and dunk it inside that. It should be an interesting test anyway.

[Kleinstein]

A closed metal can definitely a good idea. Besides of keeping RF out, it also helps to keep temperature gradients constant - so you have only one temperature to care about and not additional external temperature gradients. A case also helps keep the whole circuit at a slightly elevated temperature and thus lower and slightly more stable humidity level.

With extra capacitors one has to be careful not to make the circuit unstable and this way potentially damage the reference (e.g. gets to hot).

Even the non A version is rather fast in temperature response. Usually the same temperature regulation circuit is used - though the thermal response is not the same. Even without a very exact adaption to the thermal regulation is already very good. Still this is one of the very few points with a little room for optimization (e.g. to work better at low heater power). The main disadvantage of the A version is the slightly higher minimum temperature (and thus slightly more aging). Theoretical there could be a slight effect of the case at a different temperature as the chip itself, as there could be a minute IR radiation effect, but this should be a really small effect unless the temperature is really high.

Using the LT1013 as an OP would makes things much easier as one would not need a negative supply. There is no good reason not to use it: it is way good enough and readily available.

[Andreas]

I'll try to buy a fully enclosed aluminum project box, and dunk it inside that.

A casted case or a tin plated steel case is the way to go.
but at 100 MHz also the 1 oz copper foil from a PCB will be ok if carefully soldered at the edges.

Examples:
http://www.amplifier.cd/Technische_Berichte/Spannungsreferenzen/Spannungsreferenz_verbaut.html
http://www.amplifier.cd/Technische_Berichte/Netzteil/Versorgung_Spannungsreferenz.html

with best regards

Andreas

[VK5RC]

+1 re EMI,  I got around 100uV changes with a smpsu from a battery charger or led light driver,  probably conducted via ground. 
There is a standard Ham trick of tracking EMI of putting the DUT on full battery power,  switching everything mains powered off (some even switch off the whole house) ,  then switch it back on one circuit at a time!

[julian1]

Placing the pcb in a heavy die-cast aluminium box, with a gnd run to the case helps enormously. There is little detectable change above background noise doing the hand test. The general noise also seems lower.


 

[Andreas]

Hello,

fine,

with a 10 nF capacitor at the heater setpoint you should now get below 1uV change instead of 20-30uV.

with best regards

Andreas

[nns]

I had extremely similar issues with an LTZ design. Large changes in output voltage when I was in proximity to the board. I also have local RF noise in the 100MHz range in parts of the office, so I double shielded it to no avail, leaving me to believe that it is magnetic instead of electric noise pickup. No mumetal box big enough to try, so I spent a significant amount of time minimizing loop area in the main zener path. It seems this path (particularly the grounding location of the 120 ohm) can have large effects when there is local RF noise.

A simple method that might help is a small 100p cap directly on the output voltage (pin 3 to pin 7). This may help mute the RF pickup. You could also do some hacking to the board (what I had to do on my early prototype) to adjust the grounding location of the 120 and pin 7 locations.

After optimizing layout I was able to obtain steady output to within about 1-2uV independent on board/body position and no other consideration to shielding.

As an FYI, I've also including an "Andreas" capacitor and did find that helped a little, although not nearly as much as my other comments.

[MisterDiodes]

..Minimizing loop area on critical circuits is always a good idea! Keep the connections short and optimized.

That brings to mind something else:  Be careful adding capacitors to LTZ's and other delicate analog gadgets, especially ceramic smt's - they are very microphonic and can cause surprising problems - and turn your LTZ into a seismic detector.  We have seen those pickup local mechanical cooling fan motor vibrations.

Or like the day we had a delicate circuit that was making noise only at around between 1pm to 3pm on weekdays:  Turns out it was the train locomotive a few blocks away making a local pickups or dropoff at a rail siding... It was slowiy shaking the building enough that we didn't feel it, but an LTZ's ceramic bypass caps did.

Just keep an eye out for those types of issues.

[julian1]

Minimizing loop area on critical circuits is always a good idea! Keep the connections short and optimized.

I basically did the opposite - and tried to match the schematic as closely as possible to make it easy to debug if needed. Lots of room for improvement!

[MisterDiodes]

OH yes - the more you play with LTZ-type references, the more you realize you can make all sort of stuff unintentionally: Radio receivers, thermometers, seismic and vibration detectors, board stress detectors, barometers, board cleanliness measurement units, resistor drift test equipment, etc. 

That's why I was asking about the choice of Non-A LTZ - remember those especially can be micro-barometers and board stress detectors - those internal wire bonds are more  sensitive to the outside world mechanical stresses than the 'A version.  Also you will need to adjust that extra TC resistor for your thermal flow situation - and if you're not using the datasheet resistor values you'll need to also adjust the TC resistor for best operation at -your- ambient temperature, enclosure and environment.

In the long haul over years and decades, we've never seen one LTZ type - be it A or Non A - outperform the other in terms of drift or noise.  Either one can work well and we've plenty of examples where they just run very well inside datasheet specs.  The non-A does require a bit more fiddling but can be good for very controlled environments, and the 'A is easier to get running on a production basis - and a lot more forgiving in real world apps.  That's why a lot of manufacturers stick with the 'A.

I'm looking at a hard-working 3458a that has an 'A in it, the usual 15k over 1k heater ratio and LT1013 in plastic DIP - and it doesn't move a PPM or so annually, and has worked that way for years.  The datasheet circuit values is a very good starting place for learning how these work. 

And a PPM drift is almost impossible to -accurately- measure anyway in any real-world situation, if you add up all measurement uncertainty.