Author Topic: Ultra Precision Reference LTZ1000  (Read 986004 times)

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Offline Kleinstein

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Re: Ultra Precision Reference LTZ1000
« Reply #1375 on: March 27, 2016, 09:09:05 am »
Not having a capacitor at the base of the sensing transistor should not be such a big problem with noise, at the low impedance there you can't effectively filter the LF noise from the zener reference anyway. Here the capacitor at the 7 V level might be more effective than at the sensing transistor. Still a capacitor there might be a good idea for EMI reasons.

The 330 K resistor in feedback makes this a proportional type controller - no more integral part left. The gain is rather high so it may not be a problem after all.  Still it is well possible that the quality if temperature control is simply limited on how well the heater and sensor are placed inside the LTZ1000. You can't do better than that.

Still I don't think this modified version of temperature control is an improvement, more like a poor example. For example short time fluctuations on the temperature of the driving transistor and the diode would require the controller to react - so this may be a reason why a real integral part is not helping with that type of output stage. Also the 2.2 K resistor from the divider to the OP will slightly shift the temperature setpoint and in this sense makes the whole circuit sensitive to the temperature of the diode and transistor. 

If I were to improve on the regulator part, I would try to compensate the square law of the heater, not to reduce regulator gain when the power goes down - this might give improvements at low heater power and should allow for twice the gain in the normal case, as there is no need for extra stability reserve at high power. With improved performance at low power one might consider a second level of regulation if really needed. With the often rather high temperature inside instruments this might be well something like a temperature controlled fan and suitable shielding against direct air flow of the sensitive parts.

 

Offline Andreas

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Re: Ultra Precision Reference LTZ1000
« Reply #1376 on: March 27, 2016, 12:47:28 pm »
Not having a capacitor at the base of the sensing transistor should not be such a big problem with noise, at the low impedance there you can't effectively filter the LF noise from the zener reference anyway.

Hello,

my measurements tell something other for the LF noise.
I get a reduction from 6mVpp to 0.6mVpp with the capacitor.
And also the sporadic dips of -40mVp are gone on the heater voltage.

with best regards

Andreas



 

Offline Andreas

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Re: Ultra Precision Reference LTZ1000
« Reply #1377 on: March 27, 2016, 01:18:18 pm »
Hello,

summary of my measurements with shortened legs on LTZ#4:

power supply current
edit: increased from 22.6mA  to 24.0 mA (+6%) for LT1013A
increased from 22.6mA  to 25.2 mA (+12%) when changeing from LT1013A to LTC2057

PSRR 14-18.5V battery voltage
no significant change: from 0.076 ppm/V to 0.084ppm/V

Tilting LTZ#4
no significant change: from 0.25-0.31ppm to 0.26-0.29 ppm

Noise LTZ#4 unbuffered output:
from 1.19 uVpp to 1.05 uVpp in average (15 measurements over 100 secs)

T.C. of reference (without R9).
decreased from -5.1 ppm over 22 deg C  (-0.23 ppm/K) to -2.1 ppm (-0.095 ppm/K)

so all in all:

higher power consumption, slightly lower noise (could be within statistics) and large reduced amount of T.C.
so I will try wether R9 = 402K will give near zero T.C.

attached the T.C. before shortening the legs and T.C. after shortening the legs.

Also attached the FFT of a noise measurement.
nothing unexpected here.
Above 10Hz the edge of the filter amplifier is seen. And of course some 50Hz mains hum.

with best regards

Andreas

« Last Edit: March 28, 2016, 05:26:16 am by Andreas »
 

Offline Kleinstein

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Re: Ultra Precision Reference LTZ1000
« Reply #1378 on: March 27, 2016, 04:46:48 pm »
Is there an explanation for the change in voltage due to the shorter leads  (130 µV noted down below) ? 

I am surprised to see so much change, as normally the slight higher thermal current should not change much. There is thermal EMF at the leads, but this should compensate to a very good approximation. The design must be terribly off if differences as large as 3 K exist.

It might be interesting to compare to the change when changing the thermal shield - this can also have an influence on the heater power.
 

Offline Andreas

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Re: Ultra Precision Reference LTZ1000
« Reply #1379 on: March 27, 2016, 06:52:34 pm »
Is there an explanation for the change in voltage due to the shorter leads  (130 µV noted down below) ? 

The design must be terribly off if differences as large as 3 K exist.

130uV are around 18 ppm so less than 0.4K for the zener  (50-55 ppm/K without thermal regulation).
So how do you calculate 3K difference?

With best regards

Andreas


 

Offline Kleinstein

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Re: Ultra Precision Reference LTZ1000
« Reply #1380 on: March 27, 2016, 08:11:17 pm »
The 3 K change would be from the about 40 µV/K thermal EMF. So this estimate only shows that this is not the source.

If you calculate back to the zener temperature, this would be 0.4 K - but this would be a huge change against a temperature regulator.  As the feedback loop itself has rather high gain and ideally (e.g perfect position of sensor/heater) should get the temperature stable to mK or better. So I really doubt the hole chip changes by 0.4 K - this would suggest a surprisingly poor thermal design. The more likely part would be having a change in the difference for the zener and compensating transistor. So more like a change in temperature difference by 0.06 K. Still this does not sound like a good thermal design for the non A LTZ1000.  In this case a different thermal isolation cap could also have quite some effect.

With so much sensitivity to the thermal design / heat flow one might really consider using a second layer of thermal stabilization.
 

Offline Galaxyrise

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Re: Ultra Precision Reference LTZ1000
« Reply #1381 on: March 27, 2016, 08:16:40 pm »
The 3 K change would be from the about 40 µV/K thermal EMF. So this estimate only shows that this is not the source.

If you calculate back to the zener temperature, this would be 0.4 K - but this would be a huge change against a temperature regulator.  As the feedback loop itself has rather high gain and ideally (e.g perfect position of sensor/heater) should get the temperature stable to mK or better. So I really doubt the hole chip changes by 0.4 K - this would suggest a surprisingly poor thermal design. The more likely part would be having a change in the difference for the zener and compensating transistor. So more like a change in temperature difference by 0.06 K. Still this does not sound like a good thermal design for the non A LTZ1000.  In this case a different thermal isolation cap could also have quite some effect.

With so much sensitivity to the thermal design / heat flow one might really consider using a second layer of thermal stabilization.

Or perhaps that the old (long-leg) design was a poor choice for the non-A LTZ1000.  The datasheet does explicitly call for short legs, after all.
« Last Edit: March 27, 2016, 09:12:46 pm by Galaxyrise »
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Offline Kleinstein

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Re: Ultra Precision Reference LTZ1000
« Reply #1382 on: March 27, 2016, 08:40:00 pm »
I see that the data for the short legs look better (lower TC) - but this still means the point with a higher heater power and thus more internal temperature gradients. 

This makes me think, if if might be even better not to use the internal heater, but only the internal sensor and an external heater instead. As the heat flow would be only due to the rather constant internal loss, there should be much better temperature stabilization. I know an external heater would need more power and is slower, but who cares in a 24/7 line powered application.
 

Offline Andreas

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Re: Ultra Precision Reference LTZ1000
« Reply #1383 on: March 27, 2016, 08:51:16 pm »
Or perhaps that the old (long-leg) design was a poor choice for the non-A LTZ1000.  The datasheet does explicitly call for short legs, after all.

Hello,

where have you found this?
Up to now I have only found that "thermal layout"  is essential to get full performance.

And in my opinion you get lesser thermal EMFs (more equal temperatures on solder joints) when leaving the leads long.

With best regards

Andreas
 

Offline Andreas

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Re: Ultra Precision Reference LTZ1000
« Reply #1384 on: March 27, 2016, 08:55:55 pm »
I see that the data for the short legs look better (lower TC)

This is only true for my sample LTZ#4.
On LTZ#5  I have "zero T.C." with long legs.
So I guess I would get a +0.15 ppm/K T.C. if I shorten the legs on LTZ#5.

with best regards

Andreas
 

Offline Galaxyrise

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Re: Ultra Precision Reference LTZ1000
« Reply #1385 on: March 27, 2016, 09:25:06 pm »
Or perhaps that the old (long-leg) design was a poor choice for the non-A LTZ1000.  The datasheet does explicitly call for short legs, after all.
where have you found this?

Whoops, I was looking at the wrong datasheet!  I didn't have enough of the filename showing, sorry.  :palm:
« Last Edit: March 27, 2016, 09:30:07 pm by Galaxyrise »
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Offline Andreas

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Re: Ultra Precision Reference LTZ1000
« Reply #1386 on: March 28, 2016, 05:24:08 am »
Mhm,

would be anyway interesting which device calls for short legs.

By the way: I did some error in comparison of the power consumption.
The 25.2 mA are for LTC2057 instead of LT1013A.
After changeing back to LT1013A I have only 24.0 mA.
So only a increasement of 6%.

with best regards

Andreas
 

Offline branadic

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Re: Ultra Precision Reference LTZ1000
« Reply #1387 on: March 28, 2016, 09:18:51 am »
Quote
would be anyway interesting which device calls for short legs.

Well, LM199 calls for short legs, refering to AN-161 and similar thermal mass at each pin:

"...Thermocouple effects can also use errors. The kovar leads from the LM199 package from a thermocouple
with copper printed circuit board traces. Since the package of the 199 is heated, there is a heat flow along
the leads of the LM199 package. If the leads terminate into unequal sizes of copper on the p.c. board
greater heat will be absorbed by the larger copper trace and a temperature difference will develop. A
temperature difference of 1°C between the two leads of the reference will generate about 30 ?A.
Therefore, the copper traces to the zener should be equal in size. This will generally keep the errors due
to thermocouple effects under about 15 ?V.
The LM199 should be mounted flush on the p.c. board with a minimum of space between the thermal
shield and the boards. This minimizes air flow across the kovar leads on the board surface, which also
can cause thermocouple voltages. Air currents across the leads usually appear as ultra-low frequency
noise of about 10 ?V to 20 ?V amplitude..."
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Offline Kleinstein

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Re: Ultra Precision Reference LTZ1000
« Reply #1388 on: March 28, 2016, 10:17:13 am »
There are several differences between long and short leads:

As advantage of short leads and thus not so good thermal insulation there is less temperature rise due to the internal power - so the set temperature might be chosen slightly lower (e.g. 2-5 K). Also thermal regulation might be slightly faster and thus could allow a faster regulation loop - but that is likely not an issue anyway. At least usual the thermal regulation is not build for ultimate speed.

Thermal fluctuations under the chip can also be avoided with a spacer or padding - so this is a rather weak argument for short leads.

With short leads one has a higher heat-flow through the leads and thus more sensitivity to unbalanced thermal layout. Also more power is needed to stabilize the temperature so chip internal temperature differences are also expected to be larger. This last point might be rather critical - at least for me it's the best explanation for the change in voltage just from changing the thermal setup. Often extra insulation is used to keep the thermal resistance high - so I see no good reason not to do that with leads too.

It might be interesting to check how much influence the thermal insulation has - it is well possible to have the TC separated into an effect of heat flow through the leads and heat flow through the cap part. I would also expect the TC is more an effect of the heat flows and less of the direct TC, as at least the sensor part of the chip should have a very stable temperature. With an effect due to heat flow it somewhat makes sense to have the 400 K compensation resistor from the heater voltage, though the heater power and thus the heater voltage squared should be more suitable.
 

Offline TiN

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Re: Ultra Precision Reference LTZ1000
« Reply #1389 on: March 28, 2016, 10:53:20 am »
I'm under impression that long leads cause large thermal gradient from zener to copper tracks on PCB, causing uneven EMF. No matter how balanced is layout, it would not be exactly perfect anyway. By having chip close to PCB (mostly it's bottom sitting right on PCB surface!) PCB get warmer and there is less temperature gradient.
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Offline IanJ

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Re: Ultra Precision Reference LTZ1000
« Reply #1390 on: March 28, 2016, 05:50:58 pm »
Hi all,

Just a wee note on the LTC2057..............I tried it on my precision digital voltage source on the final output after the DAC a few months ago but found it had a rather annoying glitch that I couldn't get rid of. Don't remember the exact nature of the glitch apart from it was just a few tens of uV.
I did try a few (same batch) to no avail, but changing to a different device the problem went away immediately.

Ian.
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Manufacturer of the PDVS2 & PDVS2mini
 

Offline Galaxyrise

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Re: Ultra Precision Reference LTZ1000
« Reply #1391 on: March 28, 2016, 06:41:09 pm »
would be anyway interesting which device calls for short legs.
I remembered the one from the LM399, but looked at the LT1007 datasheet when I checked if the LTZ1000 had a similar entry.  But the LT1007 doesn't have much self-heating to worry about, and the LM399 doesn't usually have its leads under a wind screen.
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Offline Andreas

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Re: Ultra Precision Reference LTZ1000
« Reply #1392 on: March 28, 2016, 07:33:48 pm »
Hello Ian,

sorry but the info from your side is a bit vague for me.

How did you measure the tens of uV change? (Oscilloscope or multimeter)
Which bandwith/NPLC settings did you use?
Wiring capacitance of your setup? (long coax-line?)
What was the other part number with no issues? (bipolar or CMOS?)
Is your output stage a voltage follower or a inverting cirquit?

I also had some problems with some uV drift measured on DMM for the LTC2057 in voltage follower cirquit.
I blame it on the CMOS protection diodes at the negative input.
But as Ken already described this can be solved with a output filter capacitor and a capacitive isolation cirquit.
It is generally a good idea to have some kind of isolation cirquit on precision outputs.

With best regards

Andreas

 

Offline IanJ

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Re: Ultra Precision Reference LTZ1000
« Reply #1393 on: March 28, 2016, 07:57:33 pm »
Hello Ian,

sorry but the info from your side is a bit vague for me.

How did you measure the tens of uV change? (Oscilloscope or multimeter)
Which bandwith/NPLC settings did you use?
Wiring capacitance of your setup? (long coax-line?)
What was the other part number with no issues? (bipolar or CMOS?)
Is your output stage a voltage follower or a inverting cirquit?

I also had some problems with some uV drift measured on DMM for the LTC2057 in voltage follower cirquit.
I blame it on the CMOS protection diodes at the negative input.
But as Ken already described this can be solved with a output filter capacitor and a capacitive isolation cirquit.
It is generally a good idea to have some kind of isolation cirquit on precision outputs.

With best regards

Andreas

Hi,

Yes a bit vague because it was a few months ago and I don't remember much about the exact nature of the noise. I do remember though that it wasn't at the chopper switching freq, it was a lot less.....I think at the time I thought it was picking up noise from something else on the pcb (lcd, Atmel uC etc)......and that I was a bit surprised to see it given I had tested quite a few other chopper op-amps and none of the rest exhibited the same issue......at all!

To answer your questions:-
- Not drift, but glitching. I have never had a problem with drift using any of the choppers I tested.
- Oscilloscope (Rigol 2202) and using a ground clip on the standard Rigol scope lead.
- The 2202 is a 200MHz scope, I tried with the 20MHz limit in & out (as I usually do).
- Output not connected to any DMM or any other leads used when testing noise with the scope.
- Voltage follower x2 gain (5ppm/degC resistors)
- My circuit has a similar setup by way of series resistor to isolate, and a snubber etc.
- The feedback cap is not in my circuit as standard, but only because it made no difference. My circuit is quite noise free (well, as much as the noise floor of the Rigol will allow). I remember trying the feedback cap on the LTC2057 but it made no difference.

I have tried a couple of other precision choppers.

LTC2050 - Limited to 11.5vdc supply max, nice op-amp, but output relatively easily damaged. No current limit or short circuit protected.

LTC1250 - Advertised as a bridge amplifier, but works great in all respects. The best all round solution for me, albeit the most expensive op-amp. Robust output.

I tried a bunch of others.......don't have the part numbers to hand.

Hope that helps.

Ian.
Ian Johnston
www.ianjohnston.com
Manufacturer of the PDVS2 & PDVS2mini
 

Offline Kleinstein

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Re: Ultra Precision Reference LTZ1000
« Reply #1394 on: March 28, 2016, 08:43:56 pm »
Chopper OPs can be a little more sensitive to EMI problems and poor decoupling. Also high impedance sources can be a problem and may require an extra capacitance to ground. The input bias current is different from other FET OPs: as charge injection current the Bias is about opposite sign for both inputs - so no compensation  trough equal source impedance. Also the temperature dependence is not as strong as with other FET OPs. So the 30 pA for the LTC2057 are also valid at 50°C not only at 20 C.

For the LTZ1000 Reference I see not real need for a chopper OP, as reference chip has gain inside. The buffer Chip right after the reference is way more critical.

For the thermal design, the thermal resistance of a board without strong thermal cutouts will be somewhat comparable to the thermal resistance of the chip and pins - so the temperature of the solder joints will not follow the chip temperature, but only about half the way - maybe a little more with cutouts and less with a solid board with much copper. What really helps is that only the difference between the solder joints is what counts, and the coupling between the joints is usually way better than to the outside. Making the board thermal resistance really large compared to the pins is not that easy - one would need rather thin copper lines (e.g. 0.1-0.2 mm over 50 mm length). So I would still see an slight advantage for the longer pins - though I don't think the thermal EMF is really a problem in both cases.
 

Offline zlymex

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Re: Ultra Precision Reference LTZ1000
« Reply #1395 on: March 29, 2016, 02:56:49 am »
Another consideration of the opamp is the tempco of the bias current, better to be small, to allow the divider pair to have larger values other than 1k:13k, for instance 2k:26k, 3k:39k or any value that not too large and having the same ratio. This is very helpful when buying off-shell parts where the value selection is limited.

Looking at the datasheet of LT1013, the temco of the bias current is about 30pA(per deg C), less than 0.1ppm of the current of the divider pair(around 500uA). We all know that the divider pair has about 1:100 ratio on final voltage, so this 30pA variation will affect the final voltage by 0.001ppm(=0.1ppm/100).
Therefore, if you don't mind "increasing" the tempco by 0.001ppm/K, you can use an 2k:26k pair.

While the voltage noise of a LT1013 looks a bit high(a little below half of the typical LTZ1k), the current noise of the bias is small enough to be ignore safely.
Apart from choppers, there are other options to replace LT1013, such as OPA2188 and OP727.
 

Offline Kleinstein

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Re: Ultra Precision Reference LTZ1000
« Reply #1396 on: March 29, 2016, 11:17:35 am »
Due to the internal amplification, the OP used directly at the LTZ1000 is not that critical. So the LT1013 is fine and
there is little reason to use an auto zero OP, as the low drift, low LF noise and high DC gain are not needed.
More important parameters are power consumption and stability in the loop with extra gain  - some AZ OPs could have a problem here and may need adaption of the circuit.

The higher frequency noise is usually set by the OP, due to the filter function of the circuit, so here the noise of the OP might be important.

There is one more effect of longer leads - the long leads could have a resistance up to the 0.1 Ohms range and this way have an significant influence at pin4 with the typical 120 Ohms resistor. As the resistance of kovar is not that stable with temperature it can also contribute to the TC.  However resistance of the wires also sets a limit on how good thermal isolation on the board can be made.
 

Offline xyrtek

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Re: Ultra Precision Reference LTZ1000
« Reply #1397 on: March 29, 2016, 06:45:00 pm »
Anyone got working LTZ1000 board PCBs to sell?

For anyone interested in the pcb design please check this.
https://xdevs.com/article/kx-ref/

My apologies for cluttering the thread.
« Last Edit: March 30, 2016, 03:43:19 pm by xyrtek »
 

Offline Andreas

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Re: Ultra Precision Reference LTZ1000
« Reply #1398 on: March 29, 2016, 08:13:58 pm »

There is one more effect of longer leads - the long leads could have a resistance up to the 0.1 Ohms range and this way have an significant influence at pin4 with the typical 120 Ohms resistor. As the resistance of kovar is not that stable with temperature it can also contribute to the TC.  However resistance of the wires also sets a limit on how good thermal isolation on the board can be made.

When calculating with the data:
http://www.calfinewire.com/datasheets/100105-kovar.html

I get about 3.1 Ohms/m for the 0.45 mm diameter of the wire.
with 12.7mm (shortable) wire length this gives about 39mOhms for Pin 1 where I have influence.
Over 20 degs temperature difference the 0.35%/K give a factor 1.07 or 2.7 mOhm difference.
so against the 120 ohms around 23 ppm resistor change.
With the factor 0.14/100ppm this gives a error 0.032 ppm over a 20 deg C range.
Or 0.002ppm/K.

I guess I cannot measure that.
So the thermal isolation has much more effect.

With best regards

Andreas


 

Offline Kleinstein

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Re: Ultra Precision Reference LTZ1000
« Reply #1399 on: March 29, 2016, 08:37:46 pm »
The wire resistance has an effect mainly with pins 3 and 4, as here the highest current flows.
With pin 3 and 4 mA zener current, a 2.7 mOhms change in resistance for a 20 K increase in temperature gives 10-12 µV of extra voltage. However the Kovar wire will also partially profit from temperature stabilization, so a smaller effect of maybe half the size is more realistic. So about an additional +0.04ppm/K for the TC with longer leads. That is quite a lot for chip that should reach 0.05 ppm/K.

The other effect would be from Pin 4 - from Andreas' calculation this effect is much smaller. So even with opposite direction it can not fully compensate.

So the length of the wires is not only important for the thermal part.

Besides the extra TC the lead resistance can also explain much of the change in absolute value.
 


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