Compensating the temperature coefficient of a LTZ1000 voltage reference? - Page 1
Forum section: electronics / general

author: imisaac
author_rank: Regular Contributor
timestamp: April 27, 2019, 10:23:10 am
content: The LTZ1000 voltage reference has a temperature dependent drift of about +0.35 μV/oC. This is a positive temperature coefficient.The temperature coefficient of a BJT transistor, according to the datasheet of ON Semi 2N4401 (tempco plot attached), is negative and is adjustable based on its collector current.Has anyone tried to combine the LTZ1000 voltage reference with a BJT to minimize the overall temperature coefficient further? Thanks in advance.

author: iMo
author_rank: Super Contributor
timestamp: April 27, 2019, 10:32:49 am
content: Imagine the Vbe tempco of the transistor would be -2000uV/K (an example).How would you transform that into a "control loop" such you compensate the LTZ?I mean what parts would you use? What should be the tempco of those parts?

author: Andreas
author_rank: Super Contributor
timestamp: April 27, 2019, 10:41:57 am
content: Quote from: imisaac on April 27, 2019, 10:23:10 amto a) this is dependant from device to device. So did you measure it for your specific reference?to b) why should one use a external transistor to compensate the internal temperature coefficient? this will lead to time lag and other problems.The 400 K resistor does a much better compensation because it uses the amplified internal temperature sensor.You only have to adjust the 400K resistor to your specific T.C.See the long LTZ1000 thread.with best regardsAndreas

author: imisaac
author_rank: Regular Contributor
timestamp: April 27, 2019, 02:55:34 pm
content: Thanks imo and Andreas.I think you are referring to the 400 kΩ temperature compensation resistor in the Typical Application section of the LTZ1000 datasheet? Yes, that could lead to the 0.05 ppm/oC tempco as specified .However, is it possible to use LTZ1000 in a typical negative feedback series voltage regulator circuit such that even the remaining 0.05 ppm/oC tempco is nullified?This surely depends if a transistor that is stable enough exists. I am not sure. For resistors, I think <0.05 ppm/oC are indeed available.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 27, 2019, 03:17:45 pm
content: The "400 K" resistor from the LTZ1000 standard circuit can be adjusted to get the TC down to nearly zero over a small temperature range. A second other temperature compensation would also need individual adjustment - so one could as well adjust the "400 K" resistor.The main problem with the TC compensation from the DS circuit is that the compensation is proportional to the heater current and thus more like a square root function, especially if the temperature set point is relatively low.If one really needs very low TC my idea would be more an additional level of temperature regulation.

author: imisaac
author_rank: Regular Contributor
timestamp: April 27, 2019, 04:21:56 pm
content: Thanks, Kleinstein.Yes, an additional oven for the LTZ1000 voltage reference is inevitable for the ultimate performance.However, it is unclear if trimming the 400 kΩ primary resistor is the same effectiveness as trimming the collector resistor (in the case of the transistor based tempco compensation). How much change in "400k" is needed to change the tempco of LTZ1000 by say 1 ppm?One potential advantage of the transistor-based tempco compensation, I hope, would be that the voltage reference output is relatively immune to the load variation. The extra current needed the compensate the load variation will come from the transistor, rather than from the LTZ1000 itself. This help stabilising the primary LTZ1000 circuit.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 27, 2019, 05:20:40 pm
content: Any load to the reference circuit would come from the OP (usually LT1013) anyway. Depending on the implementation there might be a transistor of JFET to buffer the OPs output anyway.For the residual TC I see different possible sources:1) the internal oven has non perfect regulation and possible thermal gradients on the chip, in the case . These errors should scale with the heater power.2) the TC of the external resistors. There TC is attenuated by a factor of some 100 but not perfect.With reasonably quality (e.g. TC < 10 ppm/K) resistors this should not be a problem.Similar the offset drift of the OP is attenuated - so this is usually not a problem.3) Thermal EMF due to changing temperature gradients. Like the 1st. point heater power is expected to be the driving force.In a limited temperature range and with not to low power, the heater current is a good approximation to the heater power. It is a little non linear (especially at low power), but any correction tends to be temperature sensitive. So the original circuit's 400 K resistor is already good in correcting effect proportional to the heater power (or current).To make use if a very stable reference one kind of has to use a stable environment anyway. So I don't see the importance of the TC that high. The more limiting factor is long term drift and other external influences like air pressure, humidity (via the resistors).

author: imisaac
author_rank: Regular Contributor
timestamp: April 27, 2019, 06:05:34 pm
content: Thanks again, Kleinstein for the insight.It seems that two of the three main temco contributions of the LTZ1000 reference circuit you mentioned comes from the internal heater related issues. Namely, the thermal gradient across the circuit and the temporal change of this thermal gradient.In addition, the aging of the voltage reference, hence the annual drift in the accuracy, could also come from the prolonged heating of the circuit.I could be wrong, but I think the humidity and the pressure effects are more pronounced for the resistive elements and to a lesser extent for the solid-state based zener reference.Therefore, it may be possible that without using the heater function (i.e. turning off the internal oven) and opt for the external temperature stabilisation across the whole board (e.g. stabilize at slightly below 23oC) may achieve a better overall stability?For example, a typical room temperature change in a climatized room is about 1oC. Combining this with a state-of-the-art LTZ1000 reference circuit tempco of 0.05 ppm/oC gives 0.05 ppm stability. However, if there is a tiny oven that can stabilize the temperature to 0.05oC, then a 1 ppm/oC voltage reference can do the job just as well.Does anyone know the tempco of LTZ1000 without the heater function?Last but not least, regarding the long-term drift of resistive elements due to pressure, humidity...etc, it feels like the only remedy is to calibrate them as frequent as needed against primary standards.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 27, 2019, 06:43:02 pm
content: The LTZ1000 TC without the heater is at some +50 ppm/K (relatively little variations), so not really good. So to get a 0.05 ppm/K TC with the oven, the oven would need to reduce temperature variations by a factor of 1000. This is already quite demanding, both for the regulator and the stability of the temperature sensor.The big advantage of the unit internal oven (heater + sensor) is that it can be fast and can thus also compensate external variations. In addition the small oven just for the reference also uses less power. If the whole circuit is stabilized it would need considerable more power, as the needed oven power kind of scales with the power of the circuit that is stabilized. So an outer oven would need something like 5 times the power.Humidity is mainly a issue with the external resistors, if not very expensive hermetically sealed. Even than, there can be swelling of the board and thus an indirect effect due to stress. Due to the attenuation the effect is not that large. For the pressure there is a small direct effect on the metal case of the LTZ1000.

author: imisaac
author_rank: Regular Contributor
timestamp: April 27, 2019, 08:14:30 pm
content: It appears worthy to check how much the +50 ppm/oC tempco of the raw LTZ1000 can be compensated to without using the internal oven. If +1 ppm/oC tempco can be achieved, then a <0.05 ppm overall stability may still be possible when an external oven is added (assuming the power consumption is not an issue). The point here is to see limiting factors of this approach.The pressure and humidity related drift of the voltage reference is also very interesting for the metrological point of view.....I may need to find a climate chamber to study them after the reference is built.The issue with the swelling of the PCB, however, is a bit challenging. I don't have an idea how to apply a controlled amount of stress to the pins of LTZ1000 yet. In addition, I suspect the stress effect is somewhat anisotropic as well. That complicates things further....Thanks for the tips, Kleinstein.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 28, 2019, 07:25:25 am
content: The LTZ is not that sensitive to board stress. It is more resistors that can react to board stress, especially SMD form factor. The problem with humidity effects is that they tend to be slow and with different time constants. So some resistors may react to the average humidity of the last few days, the board itself may need weeks for the full area or a day for the edge and other resistors could need months.There is a known way to reduce the intrinsic TC by adding a series resistance to the positive side of the zener. However as a downside this adds another rather critical resistor.The pressure effect is mainly important if he pressure really changing, e.g. if one sends a precision reference to NIST in boulder at a rather high altitude (my guess is some 1600 m like Denver). The normal variations with the weather should not be a problem.

author: imisaac
author_rank: Regular Contributor
timestamp: April 28, 2019, 01:51:42 pm
content: Regarding the stress related issues of SMD components, I think the remedy is to use PCB substrates with low deformation such as ceramics or glasses.Regarding the pressure related issues of the voltage reference, a pressure regulated thermostat could help.Regarding the tempco compensation of a zener, there are indeed several techniques found. But I am not sure which one would work the best yet without further evaluation.....

author: imisaac
author_rank: Regular Contributor
timestamp: April 29, 2019, 10:31:53 pm
content: In an early eevblog post, several members indeed have attempted to trim the temperature coefficient of a zener to nearly zero. However, the main part used there was LTFLU.https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/In one message (https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/msg1285987/#msg1285987), one member (Dr. Frank) mentioned that:QuoteDoes anyone know the reason why the +50 ppm/°C tempco of LTZ1000 w/o oven can not be trimmed further? For a 7 V output, this tempco corresponds to +0.35 mV/°C. Is he implying that finding a component with the negative tempco (i.e. -0.35 mV/°C) would be difficult?Thanks in advance.

author: Gyro
author_rank: Super Contributor
timestamp: April 30, 2019, 01:08:03 pm
content: QuoteThere's an app circuit for unstabilized temperature in the LTZ1000 datasheet if you want to have a play. It doesn't specify the degree of temperature compensation that you might achieve though...

author: iMo
author_rank: Super Contributor
timestamp: April 30, 2019, 01:36:03 pm
content: I think fine trimming with the 70k in above schematics may work too..

author: Kleinstein
author_rank: Super Contributor
timestamp: April 30, 2019, 04:15:41 pm
content: The 70 K resistor is only suitable for very fine trim. Reducing the current to half gives about some 20 mV lower voltage and some + 65 nV/K to the TC. So to get near zero TC one would need some 2-3 MOhms and thus so little current that noise and amplifier bias becomes a problem. In addition resistors in the > 1 M range tend to be less stable and leakage currents become a problem.The more suitable way is the resistor at the positive side of the zener. Better not use a trimmer there, but find a suitable resistor (e.g. some 20 Ohms) and than maybe do the fine trim with the "70K". However this extra 20 ohms resistor would be rather sensitive ( e.g. like 1 ppm change in output voltage from some 50-100 ppm change in this resistor) - so not much saved from not needing the divider for the set temperature. The initial TC of the LTZ is just a little to high to make it worth it.

author: imisaac
author_rank: Regular Contributor
timestamp: April 30, 2019, 09:29:43 pm
content: Thanks Gyro, imo and Kleinstein for the circuit suggestion!Looking at the left most "arm", the output voltage Vout= V200Ω+Vzener+VBE.Looking at the middle arm, Vout= V70Ω+VCE.The operational amplifier will make the two input pins equal to each other, i.e. VCE=VBE. Therefore, the following conservation relation should hold true always:V200Ω+Vzener= V70Ω.Adjusting the V200Ωever so slightly by changing its resistance value away from 200Ω will change the zener voltage in order to balance out V70Ω, and vice versa.Because the positive tempco of a zener depends on its operating voltage, the tuning of 200Ω is a means to tune the zener tempco.Tuning the 70Ω collector resistor, however, may have two consequences. Firstly, it will change the collector current and hence the negative tempco of the BJT transistor. Secondly, it will change the positive tempco of the zener according the the conservation relation. The two effects somewhat cancels each other out. This is the reason that it is only used for fine tuning of the system tempco.The 1N4148 diode at the op amp output is only for protection, and the 0.022 μF is for noise filtering.Is the above interpretation of your circuit correct?Thanks in advance.

author: iMo
author_rank: Super Contributor
timestamp: April 30, 2019, 11:28:42 pm
content: I've found a model of a Zener with a tempco, and I've created the simulation where the tweaking with the serial Zener resistor works.After some tweaking I've found the point where the 4 degC (25-29degC) makes 2.38uV Vref change.You may iterate closer to zero if interestedWith 200ohm you get 10mV, with 5ohm 1.8mV.See below.

author: imisaac
author_rank: Regular Contributor
timestamp: May 01, 2019, 07:57:26 am
content: Super! Thanks imo for the simulation.It clearly shows the temperature dependence of the output voltage under different series resistance values.

author: iMo
author_rank: Super Contributor
timestamp: May 01, 2019, 09:40:50 am
content: And the sweep with the 70k finetuning resistor.(Fixed the pictures type to .png above).

author: Kleinstein
author_rank: Super Contributor
timestamp: May 01, 2019, 09:50:33 am
content: Quote from: imisaac on April 30, 2019, 09:29:43 pmThe diode is for protection and maybe start up - it is not really needed.The 22 nF cap is for frequency compensation of the control loop - the transistor adds extra gain to the loop and thus some extra compensation is needed, so the extra gain is gone when the OP's loop is closed. For the DC calculation one can ignore it.One can see the resistor to set the zener current and the 200 ohms pot as a kind of "amplifier / divider": they see both the same current. So the extra resistor adds a fraction of VBEon top of the zener.Changing the 70 K resistor mainly changes VBE, the effect on the Zener current is small and does not change the TC very much. Usually less current for a zener gives a more negative TC - so the 2 effects don't compensate but go in the same direction. The problem is just that it takes a much lower current to get enough TC from VBE.

author: iMo
author_rank: Super Contributor
timestamp: May 01, 2019, 10:18:40 am
content: Changes of the 70k resistor.R TC=065k 34C70k 28C75k 22C

author: iMo
author_rank: Super Contributor
timestamp: May 02, 2019, 08:04:09 am
content: Made for fun in simulation.You may try in HW, sureWould require an opamp and resistors with low TCs and the opamp capable working with 0.5V at its inputs (in that wiring). Also good thermal zener-transistor coupling.What would be a long term stability of a cheapo Zener in glass?PS: a first help for the owners of an LTZ1000 with broken heater

author: Kleinstein
author_rank: Super Contributor
timestamp: May 02, 2019, 09:38:58 am
content: The cheap zeners in glass case could be stable (though likely not really good), but they are generally rather high noise, so it is difficult to measure. The noise is not only a little higher but more like a factor of 100.There are 1N82x zeners which are not cheap, but they can be reasonably long time stable and the noise may be acceptable. However it may need some selection and luck to find a good one.

author: iMo
author_rank: Super Contributor
timestamp: May 02, 2019, 10:05:47 am
content: Sure, it is rather a simulation "experiment".To the above tempco - based on the definition of the box method (for example the OP177 picture above):from 16C to 34C is (Vmax-Vmin) / 18C = 8.9e-7V/C, with Vref=6.99569V.That is 0.13ppm/C, isn't it?

author: Kleinstein
author_rank: Super Contributor
timestamp: May 02, 2019, 10:32:53 am
content: This is only a simulation and the models for temperature dependence are often not that good. Usually they don't contain case effects, but more the the theoretical silicone part only.The OP177 example shows another simulation problem: the real OP177 is not single supply.

author: iMo
author_rank: Super Contributor
timestamp: May 02, 2019, 10:36:08 am
content: Moreover, the opamps models usually do not include the TCs of the drifts. Therefore my note above when somebody tries it out in HW.Below with LT1013A, 0.13ppm/C (box).

author: imisaac
author_rank: Regular Contributor
timestamp: May 02, 2019, 11:35:18 am
content: Thanks imo for the beautiful simulations. Thanks Kleinstein for the deep insights.I attempted to reproduce your circuit simulation in LTspice, but with a 6.2 V buried zener instead (attached). This was the reference circuit that the following parameter values were derived.The sensitivity coefficient of the voltage reference out to the resistor R1 (series resistor to the zener) and R3 (collector resistor) were about +0.02 ppm/ppm and -0.005 ppm/ppm respectively.The observations are:1. The two sensitivity coefficients were opposite in sign.2. The zener series resistor had the much stronger effect in the system tempco.3. The zero tempco operating temperature was about 22.3oC.If I use a resistor with a tempco of 5 ppm/oC for both R1 and R3 and operate the system at the "zero tempco" temperature, then the system temco would be 0.1 ppm/oC, mainly dominated by R1. This represents the lower bound for the topology and component specs used.The tempco of R1 seems to be very critical.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 02, 2019, 12:35:01 pm
content: The sensitivity to R1 changes is the main problem with the TC adjustment. Here it is not only the TC of R1, but also the long term drift of R1 that is a problem.Where the zero TC point is depends on the details and exact values. So with suitable resistor values one can chose the value.When an oven oven is used, there is no need to adjust so accurate. It already helps a lot if the raw TC comes down from some 50 ppm/K to maybe 5 or 10 ppm/K. This lowers the requirements on the oven (and the temperature related resistors) by a factor of 5 or 10. However the problem is that the sensitivity to R1 is higher than the sensitivity to the other resistors of the LTZ1000 circuit.

author: iMo
author_rank: Super Contributor
timestamp: May 02, 2019, 02:55:33 pm
content: Random +/-5ppm resistors and the Vref.The top pane shows the three TCs used (for a doublecheck only).For each TEMP sweep is generated a random TC coefficient (-5ppm..+5ppm) for each resistor.The LTspice's random(x) function is something I still do not understand fully..

author: imisaac
author_rank: Regular Contributor
timestamp: May 02, 2019, 06:33:08 pm
content: Yes, imo. The voltage reference sensitivity to the resistor R2 should also be checked as you have done here.I extracted the R2 sensitivity coefficient manually and plotted it together with the other two coefficients (attached plot).Surprisingly, the sensitivity coefficients of R1 and R2 have opposite sign and are roughly equal to each other in magnitude.Generally speaking, is it possible to find two resistors that have equal and opposite tempco and aging effect?If so, they would be the perfect pair for this circuit.

author: Andreas
author_rank: Super Contributor
timestamp: May 02, 2019, 08:38:44 pm
content: Quote from: Kleinstein on May 02, 2019, 09:38:58 amHello,I have different measurements.https://www.eevblog.com/forum/metrology/ultra-low-noise-reference-2dw232-2dw233-2dw23x/msg2207748/#msg2207748Even a cheap SMD Zener in plastic package has only ~2uVpp 1/f noise.Ok it mainly depends on zener voltage and current.The lowest noise is usually around 5-6V for the zener and high current.with best regardsAndreas

author: 3roomlab
author_rank: Frequent Contributor
timestamp: May 02, 2019, 08:44:00 pm
content: this is my "scratch pad" simulation of a BJT with -2mv/C (-200mv/100C ?), and a diode with +50ppm/C (for vbr around 6.4v = + approx 32mv over +100C ?)I think there should be a curve, but I do not know what the curve should look like, so I made it more linearthe BJT tempco parameter appears to be XTB (in my case, its -2.3, used on a 2N2222)the diode tempco parameter are TRS1 and TRS2 (in my case, its TRS1 approx= -0.255, TRS2 1n)TRS2 controls the quadratic curve fit.TRS1 controls the linear slope fit.the original BJT 2N2222with XTB =1default does not include a XTB parameter, it appear to fit around -1.5mv/C. if this is default, then all default BJT spice models are not "fitting" 2mv/C. maybe I am missing some other theory, else then, what I found here should be true? (I tried with a mmbt3904, it is nearer to 2mv/C than 2N2222 in fit, mmbt3904 XTB = 1.5 by default model, not 1)the above require user to edit the dio and bjt files in "cmp" directory.putting the 2 back into the same .op run from 25C to 125C, produces something that looks like -600ppm/C overall ref circuit, no heater of course. but was it suppose to be -600ppm? is this right?by modifying the TRS1 to around -0.155. I could then get the bowed curve in the 2nd picture. the curve then becomes manipulated by a pre bias resistor {rt}. I think this part is discussed in the LTZ pdf, unheated compensation? I have seen some online docus talking about this, the curve is not suppose to be bowed but a "s" curve (^3) .Im not sure if the BJT should be constant current fed or constant voltageso maybe someone else can improve the model further with a known curve fit.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 02, 2019, 10:22:04 pm
content: The zener TC depends very much on the voltage, current and to some degree on the design.For the BJT a reasonable good approximation is to assume an about linear TC and VBE to extrapolate to some 1.2 V at 0 K. So the higher VBE the less TC and if one starts at some 600 mV at 300 K would get the rule of thumb -2 mV/K.There is a little curvature in the TC curves, but I would not be so sure the BJT / Zener models are correct on this.For the zener noise, it depends on the types - the ones I measured so far (in glass) and most datasheets I found where quite horrible. The 2DW232 zener is a rather special low noise type. I don't know if there are other low noise types that really deserve that name.

author: 3roomlab
author_rank: Frequent Contributor
timestamp: May 02, 2019, 10:42:35 pm
content: found ithttps://www.geosci-instrum-method-data-syst.net/6/301/2017/gi-6-301-2017.pdfthe docu that I saw about unheated compensationvery interesting docu, but it apprears to be part of some huge report over 300-400 pages

author: iMo
author_rank: Super Contributor
timestamp: May 03, 2019, 06:22:00 am
content: With Rb=15k.PS: I do not see an effect of the PTAT resistor R5 (from that pdf) above, however..

author: 3roomlab
author_rank: Frequent Contributor
timestamp: May 03, 2019, 05:26:35 pm
content: I cannot remember if I tried to simulate this, but they have got an extra R5. and in order for the response to be correct (like a ^3-ish curve), the quadratic curve in the diode needs to be accurate/populated, which ... nobody knows whats the value is, so my estimate is just to fit a line, but in real life is suppose to be a curve. it will need a dozen ^ dozen "if then or else steppings" for sure.** did you try to tempco curve/line fit your diode/bjt? (aka xtb + trs1 ?)

author: iMo
author_rank: Super Contributor
timestamp: May 03, 2019, 09:01:11 pm
content: FYI - VBe TCs of some transistors from the lib.-1,9mV/C in average, except the ON model.

author: imisaac
author_rank: Regular Contributor
timestamp: May 03, 2019, 10:16:14 pm
content: Here is some complementary information.In Fig. 3 of the wonderful paper that 3roomlab cited, there is a base resistor rb=15 kΩ added that can correct the circuit tempco at the two temperature extrema (for better agreement between the simulation and the measurement data).If we assume the choice of the base resistor value is independent to all the other components, then we can again derive the sensitivity coefficients.It turns out that the base resistor has a negligible effect on the sensitivity coefficients of all the other resistors in the circuit.Last but not least, there is an additional resistor from the collector to Gnd ("R5" in the paper figure). I feel that it behaves similar to the "R3" collector resistor here (perhaps with a positive tempco effect instead) and therefore may not be worthy to simulate its influence in full.

author: imisaac
author_rank: Regular Contributor
timestamp: May 03, 2019, 10:28:12 pm
content: QuoteI think there are two buried transistors that come with the LTZ1000 TO-5 metal can package. Their tempcos are unknown. However, since they are buried, they may have low tempcos than usual.

author: 3roomlab
author_rank: Frequent Contributor
timestamp: May 03, 2019, 11:37:41 pm
content: in this simulation, I put in TRS2= 0.5m for the LTZ. with this curve, you can now get a "flat zone"finding/characterizing a LTZ with a TRS2=0.5m will be another rabbit holebut bouncing to a CERN paper which I spoke about in another thread on hpm7717, they are designing a TEC box which completely takes out the variable "external temperature factors". if memory serves, there was a laser hologram nut in here who did his TEC to 1milli celcius accuracy. I think this pretty much ignores tempco and brute forces the box into a temperature compliance.

author: imisaac
author_rank: Regular Contributor
timestamp: May 04, 2019, 08:45:25 am
content: Thanks 3roomlab for the revised circuits and the references.I have a few questions:1. Regarding the Δ1 mK thermal electric cooler, I think you may be referring to this TEC design here?http://hololaser.kwaoo.me/electronics/Arduino_TEC/Arduino-TEC.html2. Regarding the CERN paper, could you provide a reference?3. Is the voltage reference output in your circuit at the output point of the resistor R10?

author: Kleinstein
author_rank: Super Contributor
timestamp: May 04, 2019, 09:08:30 am
content: Quote from: imisaac on May 03, 2019, 10:16:14 pmThe rb is needed to match the simulations to the real LTZ1000. This resistance may be part of the real LTZ1000 chip - however there may also be just slightly different parameters for the real transistors / zener. Some internal base resistance is normal and this could also have quite some TC. Anyway there is no way to adjust rb.The extra R5 reduces the gain of the transistor stage. So it makes the OP (noise and offset drift) a little more important.AFIAK the transistors are not that special and the main effect on the TC is due to the absolute value of VB: a low VB gives a larger temperature effect.

author: 3roomlab
author_rank: Frequent Contributor
timestamp: May 04, 2019, 11:17:46 am
content: Quote from: imisaac on May 04, 2019, 08:45:25 am1) I think thats the one, I think it might be possible to use full analog w/o arduino2) see pdf3) you mean R12

author: iMo
author_rank: Super Contributor
timestamp: May 04, 2019, 11:36:01 am
content: @3roomlab: the Q3 is within the control loop, afaik, so it does nothing..Does that Scurve come from your TRS1/2?PS: the HPM-7177 project looks like a wish list only.. the repo is empty..

author: 3roomlab
author_rank: Frequent Contributor
timestamp: May 04, 2019, 11:57:46 am
content: Quote from: imo on May 04, 2019, 11:36:01 amTRS2 = 0.5m, try itfor hpm7177, I think tempco = zero is a big deal

author: iMo
author_rank: Super Contributor
timestamp: May 04, 2019, 01:28:12 pm
content: Added TRS2=0.5m but I see no change..

author: iMo
author_rank: Super Contributor
timestamp: May 04, 2019, 01:49:52 pm
content: Ok, it looks like you must define both TRS1 and TRS2.Tried below.PS: BTW, 0.04ppm/C (box) the best result below

author: imisaac
author_rank: Regular Contributor
timestamp: May 04, 2019, 07:45:03 pm
content: Thanks, imo. The tempco is indeed impressively suppressed.A couple more questions arise:1. Are TRS1 and TRS2 user adjustable parameters in the real circuit (via a resistor in series..etc)? Or they are only used in the simulation for a better measurement data and simulation results agreement?2. I suppose that the output buffer transistor would help reducing the load influence on the voltage reference output. However, shouldn't the tempco of the buffer resistor ("R5" in your circuit) contribute to the overall tempco as well? In that case, I think its contribution would be even stronger than the R1 and R2.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 04, 2019, 08:20:29 pm
content: The Resistors and the transistor for the output buffer after the OP have essentially no influence. They are inside the control loop and thus there change. The series resistance might be needed for the simulation to start.I don't think one can change the TRS parameters.

author: imisaac
author_rank: Regular Contributor
timestamp: May 04, 2019, 10:23:17 pm
content: Thanks Kleinstein for the prompt information.How are the resistor values selected for the "control loop"? It seems that the output voltage only maintains at 6.9 V if R5<<R6.

author: iMo
author_rank: Super Contributor
timestamp: May 05, 2019, 06:22:15 am
content: The only parts which contribute to the Vref's tempco shape are the R1, R2 and R3 resistors with "tc", the zener diode, transistor and the opamp (when not "ideal").The components in the opamp's loop do not contribute (the output transistor and the 10k and 25ohm) as the opamp does compensate the changes. Basically "anything" you put between the opamp's output and Vref node is compensated off.The output transistor buffers the 5mA zener current, as I think 5mA is too much for a precision opamp to stay cool.The TRS1 and TRS2 are the intrinsic params of the zener diode and cannot be changed from outside, afaik. The TRS values used in my simulation are the examples from 3roomlab.

author: imisaac
author_rank: Regular Contributor
timestamp: May 05, 2019, 01:52:24 pm
content: Super, imo.I added one extra op amp at the buffer stage to scale the output up to ~10 V, a convenient voltage for a DAC.

author: imisaac
author_rank: Regular Contributor
timestamp: May 11, 2019, 09:46:08 am
content: Does anyone have experience in which operational amplifier is the best for the stable voltage reference circuit?Three possibilities are listed below to begin with....OpAmp|VOSTC(nV/oC)|IOS(fA)|VOS(μV)|Noise(nV/√Hz)ADA4530-1|130|1|40|14LT1013|300|150|150|22LT1006|200|120|20|23Thanks in advance.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 12, 2019, 12:17:45 pm
content: The L1013 is well good enough for the job. So no real need for experiments. As the transistor inside the LTZ1000 already provide a gain of about 200, the demands one the OP are not that high. So neither offset, drift or noise are that important. So in theory a LM358 might be good enough - but possibly problems with phase reversal. But why experiment on a $2 OP if a $50 reference is at risk ?The LT1006 is essentially the single version to the LT1013.Just be careful with the odd pin-out of the SO8 version of the LT1013.If there would be no LT1013 available for some reason my next best guess would be TLE2022.Lots of discussion on the OPs is in the long LTZ1000 thread.

author: imisaac
author_rank: Regular Contributor
timestamp: May 13, 2019, 08:10:26 am
content: Thanks, Kleinstein.If there is an offset voltage at the input sides of the OP, they will show up at the output even if the gain is small, I think. The OP at the 10 V boosting stage is the one that will affect the output more strongly. In particular, its tempco (plus the tempco of the resistor divider) will directly determine the tempco of the 10V voltage reference. After all, the resistor trimming tricks for zero tempco discussed so far is done prior to the boosting stage.It could be true that the requirements (noise, tempco, aging, offset...) for the OP on the zener side is not so critical. However, the same conclusion may not be true for the boosting OP, right?ADA4530-1 appears very interesting since it outperforms LT1013/LT1006 in the tempco by ~3 times.....

author: Kleinstein
author_rank: Super Contributor
timestamp: May 13, 2019, 04:37:59 pm
content: The OP in the LTZ1000 circuit itself is less critical as the extra gain reduces the effect of errors (offset, drift and noise) by about a factor of 200, at least for the low frequency (some 1 KHz) part. Similar the resistors in the circuit have a reduced effect by something like a factor of 100 or more. So 1 ppm of change in the output would correspond to some 100 ppm (the exact numbe depends on the resistor) change in resistor vales.A possible boost stage is much more critical. The offset is directly added to the 7 V reference level and the resistors are responsible for some 1/3. So 3 ppm resistor ratio change to give an 1 ppm output effect.The ADA4530 is no the right choice here - this is a super low bias OP, but with high noise. LT1013 is also not that good here.The more practical candidates would be LTC2057, ADA4077, OP177 - so more OPs made for a lower impedance.

author: imisaac
author_rank: Regular Contributor
timestamp: May 14, 2019, 08:10:07 pm
content: Thanks Kleinstein. I re-tabulated the (boosting stage) op-amp selection table based on your suggested criteria. In particular, all input bias parameters are removed from the table. All noise related parameters are inserted.Here is an update.OpAmp|VOSTC(nV/oC)|0.1~10 Hz Vpp(nV)|Inoise(fA/√Hz)|Vnoise(nV/√Hz)ADA4625-1|200|150|4.5|3.3ADA4077-1|250|250|200|7LTC2057|15|200|170|11OP177|100|?|?|10If the tempco is of the most importance, then LTC2057 seems to be the best (as the boosting op amp).If the input voltage noise is of the most importance, then ADA4625-1 seems to be the best (as the boosting op amp).Is the above understanding of your suggestion correct?

author: imisaac
author_rank: Regular Contributor
timestamp: May 15, 2019, 10:33:41 pm
content: Just a brief clarification.To ensure what the "boosting stage" meant in the previous message is clear, I drew a hypothetical boosting stage consisting of two op amps. Of course, the tempco of this boosting stage should be comparable to the tempco of the LTZ1000 voltage reference (the circuit that produces 6.9 V).It seems that a low bias op amp may produce a more accurate 10 V output in comparison to low noise op amps because of the small offsets.....Is it still true that the noise consideration outweigh the input bias when choosing both op amps for this circuit?

author: Kleinstein
author_rank: Super Contributor
timestamp: May 16, 2019, 08:06:08 am
content: The circuit with the extra boosting stage is not good: it adds the errors of the 2nd OP. So normally the divider should go from the output. So the current boosting buffer would be inside the loop. In this case there is no real need to use an OP, even just a transistor is enough, as the divider is already plenty of load / biasing current.The divider is using rather low resistor values. This would produce quite some heat and related problems. I would more consider resistor values about 10 times higher.Unless noise is really high, the noise is not that critical - it should just be considerably lower than the reference noise. If at all it is the low frequency noise that can really be a problem.Bias of the OP is also no that important - the bias is usually relatively constant and only the variable part would really be a problem. The really tricky parts are more like offset drift with time and possible offset hysteresis - these parameters are not often specified.Especially when using an AZ OP like the LTC2057 it would be a good idea to have some filtering (RC like 5 K and 1 µF) before the amplifier, to keep chopper spikes away from the actual reference circuit. The high frequency components can cause hard to predict effects, a little like EMI effects. For a BJT based OP the filter could also provide source impedance matching the divider.

author: iMo
author_rank: Super Contributor
timestamp: May 16, 2019, 08:35:44 am
content: For example

author: imisaac
author_rank: Regular Contributor
timestamp: May 17, 2019, 09:08:45 am
content: Thanks, Kleinstein and imo for the wonderful insights as always been.1. The first half of the "boosting stage" is needed to scale the 6.9 V LTZ reference circuit voltage up to 10 V. In the example circuits that imo designed, U1 serves this purpose as well. Therefore, this voltage boost step appears to be inevitable if one needs a voltage that is different from the default 6.9 V.2. When a transistor is used to supply the current to the load, the voltage noise of the +15V power supply directly connected to the collector will be converted to a voltage noise at the emitter and hence producing a voltage output noise. The capacitor C1 (in the imo's circuit) may be able to alleviate the problem by shorting the high frequency noise to the ground. However, is this noise suppression scheme as good as having a high PSRR when an op amp is used?

author: Kleinstein
author_rank: Super Contributor
timestamp: May 17, 2019, 10:05:32 am
content: The transistor as an emitter follower would be inside the control loop. So the OP would compensate for any change coming from the collector side. Anyway there is not much coupling through the early effect, as VBEfor a given current does not depend much on VCE. So not sure an OP would be better. The main advantage of using an OP as current boost stage would be that most OPs have a current limited output - so they could survive a short.

author: imisaac
author_rank: Regular Contributor
timestamp: May 17, 2019, 10:33:26 am
content: If I understand you correctly, you are saying that the 10 V voltage output on the emitter side is inside the control loop. Therefore, any deviation from the 6.9 V will be compensated by the op amp that is comparing the 6.9 V and the voltage divided 10 V signal.This compensation is also fast enough to act as a low pass filter to get rid of the high frequency noises coming from the power supply.In this case, the bandwidth of this op amp (the one inside the control loop and scaling 6.9 V to 10 V) becomes an important factor?

author: Kleinstein
author_rank: Super Contributor
timestamp: May 17, 2019, 01:51:00 pm
content: The higher frequency part is usually not that critical, as it can be filtered. The difficult part is the low frequency part. There should not be much higher frequency noise in the supply, so there is not much need for a fast OP in a DC reference.The really critical part are the resistors to do the 7 to 10 V amplification. These are much more (e.g. 30 times) critical than those at the LTZ1000. Today the amplifier part is more on the easy side.

author: imisaac
author_rank: Regular Contributor
timestamp: May 20, 2019, 10:46:58 am
content: Thanks, Kleinstein. You are absolutely right that the voltage divider for the 7 V to 10 V amplification is definitely critical.For that divider network, I am thinking about using the DSMZ Z-foil voltage divider (R1/R2 ratio of 2) from Vishay. The tempco of the ratio (TCR tracking) for this resistor network is < 0.1 ppm/oC (tempco spec attached).The output will not be exactly 10 V, however, with this off-the-shelf ratio.Does anyone have an experience with this resistor network or perhaps have a better resistor option in terms of stability (tempco and aging)?Thanks in advance.

author: TiN
author_rank: Super Contributor
timestamp: May 20, 2019, 11:10:07 am
content: I think all this should be in original LTZ1000 thread. Feeling like a dejavu already, as many of such topic discovered in that thread already.I've trimmed multiple LTZ refs with "400K" resistor totempco <0.05 ppm/K. Issue to get better TC often not with LTZ circuit itself but in thermal management and layout of the parts. Also detecting such low tempco especially on limited Trange is difficult and not cheap. I have to use modified multiple 3458As for that to be reliable (meters have zero TC for 20-30c range, down in own noise). No amount of simulation will help to find "receipe" that works in real PCB. I can only suggest to build few boards and start experimenting with real hardware.imisaac, also don't believe marketing posters about 0.0 ppm/K resistors, there is no such thing unless you buy hundred resistors and cherry pick one golden sample out of them. I've testedhundreds of various resistors, from $2 USD a piece to $240 USD a pieceand still didn't find magical 0.0 ppm/KTo be fair - not that you need that for LTZ reference anyway, even 2ppm/K will be fine and performance of the reference is limited by design/LTZ chip itself.

author: imisaac
author_rank: Regular Contributor
timestamp: May 20, 2019, 02:41:48 pm
content: Thanks, TiN for the references! The -0.03 ppm/oC tempco of your 10 V reference is super impressive!1. In your circuit (screenshot attached), you used a ADA4522 chopper amplifier. Its Vos tempco is 4 nV/oC, seeming like a good choice. However, you used two of them and then added them up at the output. Was the idea to average down the chopper noise?2. You used VHD200 as your (7V->10V) voltage divider network. It has got the same TCR tracking (0.1 ppm/oC) as the DSMZ divider network. Did you choose VHD200 because it was hermetically sealed and hence had a smaller ageing?3. A minor comment about the well established LTZ1000 thread. That thread has reached 105 pages, with perhaps more than 2000 posts. Many of the discussion topics are beyond my current electronics level, I think. It is a bit difficult for me to dig the specific answer out of it. In addition, the tempco compensation solution that imo/Gyro/Kleinstein/Andreas/3roomlab suggested in this thread appears a bit different from most existing LTZ1000 reference circuits in the sense that the internal heater on the LTZ1000 is not utilized to reduce the tempco, but with a special resistor trimming approach. It is interesting to see how this topology performs in comparison to the established LTZ1000 reference circuit.4. Do you have an estimate on the annual drift of your 10V FX reference?Thanks.

author: TiN
author_rank: Super Contributor
timestamp: May 20, 2019, 04:36:06 pm
content: VHD network used only in prototype. Final units including the one I linked use unobtanium laser-trimmed hermetic Fluke film resistor (used in Fluke 8846 DMM).Output stage opamps used for both force and return drive bipolar ability. This way you can abuse output with caps, shorts, reasonable reverse backdrive and there is no major risk to kill expensive LTZ.As of annual stability, I will know answer to that question next month, currently in progress to obtain SI Volt from Josephson reference via transfer by Fluke 732.Existing LTZ thread have lots of data, so time spent to read it thru can answer some of your questions here already. Either way LTZ designs aren't what many can call "beginner" level

author: Andreas
author_rank: Super Contributor
timestamp: May 20, 2019, 05:54:46 pm
content: Quote from: imisaac on May 20, 2019, 10:46:58 amBeginning from here 2 T.C. measurements: (sorry no ageing data)https://www.eevblog.com/forum/metrology/t-c-measurements-on-precision-resistors/msg521240/#msg521240like all plastic/epoxy packages (absorbing humidity and swelling with time constants of some days) the T.C. is very dependant on humidity history.0.1 ppm (typical ?) tracking could not be reproduced on my 2 samples.with best regardsAndreas

author: Gyro
author_rank: Super Contributor
timestamp: May 20, 2019, 06:36:16 pm
content: Quote from: imisaac on May 20, 2019, 02:41:48 pmAgreed, the other thread is mainly heated LTZ.This thread definitely needs to be moved from the Beginners section to Metrology though.

author: iMo
author_rank: Super Contributor
timestamp: May 21, 2019, 03:07:04 pm
content: Quote from: Kleinstein on May 16, 2019, 08:06:08 amFYI - While messing with my junkbox I discovered a pack with OP07AY in CerDIP14 packages, 1979-85 by PMI and "M". After an intensive search I found the datasheet with package.Enclosed the shot from the datasheet (PMI 1982) as the cdip14 pin layout it is not easy to find.Hopefully the chips could be used with 399/LT1021/REF01 or LTZ.

author: Gyro
author_rank: Super Contributor
timestamp: May 21, 2019, 07:10:48 pm
content: A very nice hermetic collection, especially being A spec. I've never seen a 14 pin OP07 in the flesh.

author: imisaac
author_rank: Regular Contributor
timestamp: May 21, 2019, 10:10:55 pm
content: QuoteThanks, TiN. I have another question related to the "output stage opamps" in your circuit. If the two BJT transistors are replaced by two buffer opamps (see figure attached), would you expect it to worsen the voltage reference stability/noise performance? I am also not sure myself if isolating the ground with two opamps is any better than just using the ground pin directly.Secondly, there are two stages of low pass filter at the output of the 7 V reference. Wouldn't you add more 1/f noise, due to capacitors of the filter, to the 7 V reference then without using it? It may be true that the high frequency noise is reduced, but 1/f noise may be the more important one here in this part of the circuit. If I move the low pass filter to the output of the ADA4522 (in the "High" portion), it may work better?Last but not least, how do I move this thread to the "Metrology" section?Done already.Thanks in advance.

author: CalMachine
author_rank: Frequent Contributor
timestamp: May 21, 2019, 10:34:41 pm
content: Quote from: imisaac on May 21, 2019, 10:10:55 pmThat LPF is put there to protect the LTZ from current spikes of the chopper

author: TiN
author_rank: Super Contributor
timestamp: May 22, 2019, 04:23:41 am
content: I don't see much benefit on using buffer opamps, instead of simple cheap transistor at the output. My goal was to enable +/-20mA at the output without degrading voltage output accuracy, to use as stable voltage source for bridge circuits for example. If that is not needed, transistors can be removed altogether as well.RC network is to provide some isolation between LTZ ref output and chopper output stage. Noise caused by it is tiny compared to own LTZ output noise, so it can be ignored for typical application, where reference output is integrated/averaged over long time.

author: imisaac
author_rank: Regular Contributor
timestamp: May 22, 2019, 06:21:51 am
content: Thanks, CalMachine and TiN for the explanation.1. Would an additional low pass filter at the output side of the chopper amplifier be helpful to prevent the chopper noise going to the load (e.g. a capacitor across your R2)?2. Was there a need to filter out the noise from the +/-15 V power supply before connecting them to the transistors? In other words, did the noise from the main power supplies show up at the load voltage?Thanks in advance.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 22, 2019, 08:59:09 am
content: The transistors are quite good in suppressing supply noise, so no trouble there. The +-15 V (if -15 V is needed at all - could be just -2 V) should be reasonable clean from a linear regulator, but I see no need to spend too much there.I would be more worried about the supply current to the chopper OPs. So some isolation resistors/ferrites between the +-15 V and the ADA4522 can be a good idea, not to inject too much noise to the supplies.

author: Dr. Frank
author_rank: Super Contributor
timestamp: May 22, 2019, 09:51:51 am
content: Quote from: imisaac on April 29, 2019, 10:31:53 pmIf you read my other post:https://www.eevblog.com/forum/metrology/eu-calclub/msg2420097/#msg2420097, then you would understand, that this generic T.C. of the LTZ1000 is due to physics, or let's say due to the bad match between the diodes and the zeners T.C.s.The LTFLU, the SZA263 and the other predecessors, are all optimized in this aspect, by simply having a lower reference voltage.We already have discussed and tested the reduction of the T.C. (generic and composite one) of the LTZ in many, many posts, and there is simply no good way to reduce the generic T.C. (e.g. by adding a 22 Ohm resistor in series with the zener), and there is a really simple way to have near zero composite T.C. of the whole circuit, description e.g. here:https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/msg1336581/#msg1336581Frankly speaking, you're.Why do you want to re-invent the wheel, searching for a complicated, over-engineered solution, instead of simply adapting the already described schematics and procedures?Frank

author: imisaac
author_rank: Regular Contributor
timestamp: May 22, 2019, 10:34:55 am
content: Thanks, Kleinstein as usual. By "isolation resistor", do you mean to put a highly ohmic resistor in series to the +/-15 V power supply outputs? If so, doesn't that limit the amount of current available for the ADA4522.

author: imisaac
author_rank: Regular Contributor
timestamp: May 22, 2019, 11:03:19 am
content: Thanks, Dr. Frank for your detailed references.1. I am trying to understand the physics limitation that you referred to:QuoteThe LTZ1000 is made of a transistor and a zener diode. I agree that the tempco of the two may not perfectly cancel each other and therefore leading to the 50 ppm/oC without using the on die heater. However, it is not so clear that this tempco can not be trimmed further down by a special circuit topology. At least in the simulations that have been carried out in this thread, it is indeed possible that this tempco can be manipulated further. It looks like this approach has not been attempted so far in this forum (please correct me if I am wrong).Therefore, where is the solid state physics limitation coming from again? Thanks in advance.2. It has been very informational and helpful to be able to discuss with the experts like you and many others in this thread. I think part of this fruitful learning is due to the fact that discussion has been tailored for a not so advanced person. It is unclear if this learning process can ben deemed as "reinventing the wheel"......It would be interesting to see the measurement results from this unattempted circuit scheme and compare with existing LTZ1000 based reference circuits.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 22, 2019, 12:30:38 pm
content: The way the LTZ1000 is build, there just is no good way to compensate the unheated TC. It is possble to use the 22 Ohms resistor, but this resistor would than have more influence than the savings from a better TC before the heater: one resistor with 1 ppm/K (the 22 Ohms) tends to be more expensive than 2 resistors with 3 ppm/K (the 1 K / 14 K divider). Things could change if the LTZ would start with a lower TC, but the way the LTZ are made is like it is. No need to repeat this discussion.I would very much like an LTZ1001 starting with some 6.9 V and low unheated TC, so that the collector current (e.g. R2) could be used to adjust the unheated TC to zero. However this would be a different chip. I doubt one could get such out of tolerance chips.The "isolation resistor" in the supply is a common technique used if RF circuits - get the supply to circuit parts through a resistor or inductor, so that local decoupling of the supply is really effective. So for the ADA4522 with some 2 mA supply current (ideally not much load current added) one could use some 100-500 Ohms with only some 0.2 -1 V lost for the supply. Modern OPs don't need a super stable supply, it's more improtant to keep out RF trouble.

author: imisaac
author_rank: Regular Contributor
timestamp: May 22, 2019, 07:23:15 pm
content: Thanks, Kleinstein.Is there a need for a bypass capacitor right at the power supply pin of the ADA4522 as well? If so, then the capacitor and the isolation resistor form a low pass filter, right?To facilitate discussion, an effective circuit containing various suggestions collected so far in this thread is attached.

author: MiDi
author_rank: Frequent Contributor
timestamp: May 22, 2019, 10:11:13 pm
content: U7: Inputs flippedI guess the bjt buffers Q2, Q3 want to be inside nfb of op amps.Not shure about C4: phase margin, leakageTo protect op amps, inputs should be current limited.Edit:R13, R14 make only sense with bypass capacitor.

author: chuckb
author_rank: Frequent Contributor
timestamp: May 23, 2019, 01:37:38 am
content: Pin 7 of U5 (LTZ1000) needs to be grounded for Q1 to work correctly.

author: TiN
author_rank: Super Contributor
timestamp: May 23, 2019, 04:54:04 am
content: In my reference design power is also separated, LTZ reference is powered by +11V from LT3042 LDO, while output amplifiers are powered directly from +/-12V input voltage. As result I do not have R13/R14 resistors, only decoupling caps at the opamp power pins.

author: imisaac
author_rank: Regular Contributor
timestamp: May 23, 2019, 06:43:43 am
content: Thanks, all!There were many mistakes picked out. Here is the revised version. In particular, the outputs are now in the control loops of the chopper amplifiers and the emitter of the LTZ1000 Q1 transistor is correctly grounded.Question:Is it ok if 100 nF decoupling capacitors to GND are added to all the bases, collectors, and emitters of the buffer NPN and PNP transistors at the output? The hope is to reduce the chopper noise at high frequency appearing at the load.

author: TiN
author_rank: Super Contributor
timestamp: May 23, 2019, 06:57:33 am
content: I recommend adding low ohm resistor on low side output opamp between Low post and inverting input to have room with EMF battle.Also what is the purpose of R8?

author: Andreas
author_rank: Super Contributor
timestamp: May 23, 2019, 07:49:11 pm
content: Hello,Quote from: TiN on May 23, 2019, 06:57:33 amIt is absolutely necessary to limit the input current to maximum 10mA. Otherwise the clamping diodes between +/- input will be destroyed when the negative output is forced to +10/15V.I would also compensate the output against capacitive loading and balance the input resistors.But there are at least two further errors in the cirquit ....Who will find them?with best regardsAndreas

author: TiN
author_rank: Super Contributor
timestamp: May 24, 2019, 12:10:19 am
content: Missing cap around U6, and not a kelvin connection/wrong reference for low side output opamp?Ah, Q3 as well

author: branadic
author_rank: Super Contributor
timestamp: May 24, 2019, 08:43:48 pm
content: QuoteFrank, did you compensate the zener circuit only or the overall circuit including the t.c. of the 10V boost stage?-branadic-

author: imisaac
author_rank: Regular Contributor
timestamp: May 24, 2019, 08:54:53 pm
content: Thanks, TiN. I need some help in the choice of several resistors in your circuit.1. A 1Ω resistor is now added in the negative feedback path of the "Low" ADA4452 (revised figure attached). Other resistors are also copied from your original boosting stage design. By the way, how was the 1Ω value determined? Was it calculated from the biasing parameters of the ADA4522, e.g. Vos, Ios...etc? Or was it related to the noise of the opamp?2. The base current of the BJT transistors are very small because β is presumably large. Why are R17 and R18 needed and how are their values derived?3. The input bias current, multiplied by the R16, will give a small voltage offset at the non-inverting input of the ADA4522. This small offset will show up at P2 (output Low pin). How is this desirable?4. I am not sure the functionality of R8 myself. I thought it was for protection (when combined with the diode D1) or some sort. Maybe imo or some other experts can explain better....sorry.5.QuoteWhere exactly in U6 and Q3 to add a capacitor?

author: imisaac
author_rank: Regular Contributor
timestamp: May 24, 2019, 09:02:02 pm
content: Thanks, Andreas.QuoteCould you provide more hints?

author: Andreas
author_rank: Super Contributor
timestamp: May 24, 2019, 09:20:15 pm
content: Quote from: TiN on May 24, 2019, 12:10:19 amI only meant Q3 which is in reverse mode.and of course C8 which gives instability of the output.(should be between Pin 1 + 2 of the OP-Amp and could be reduced to some nF)Quote from: imisaac on May 24, 2019, 08:54:53 pmR15+R16 in sum have to limit the current across the input pins to less than 10-20 mA. (in case of short cirquit of the -output to 10V or 15V).I also fear that you need some load from the negative output to the positive, otherwise you will have instable output.(you could also connect R2 to the negative output instead of gnd).Do you really want to use VDD + VFF as extra supply? (if yes why is the high current output connected to the zener supply)with best regardsAndreas

author: imisaac
author_rank: Regular Contributor
timestamp: May 25, 2019, 10:20:32 pm
content: Thanks, Andreas.1. Q3 polarity is now corrected.2. A capacitor is added between pin 1 and pin 2 of the "high" arm ADA4522. Is the purpose of this capacitor shorting the chopper noise back to the op amp instead of the load? If so, why not short the chopper noise directly to the ground?3. Why is the output unstable in the open circuit condition (and hence needing a dummy load)?4. If I understand you correctly, you suggest to connect R2 to the P2 such that any drift in P2 will not change the voltage difference across P1 and P2, right?5. I do not understand your question about the power supply. Which "zener supply" are you referring to?The revised circuit is attached for further examination.

author: TiN
author_rank: Super Contributor
timestamp: May 25, 2019, 10:47:17 pm
content: Sorryimisaac, it sounds like you would be much better off actually building circuit on breadboard and playing around it with DMM, as it feels like we design your ref for you. While it's all good and fun for all involved, we have no idea if our understanding of the project matches what you want out of all this. This already went quite a bit out of topic, which was about LTZ circuit itself. Boosting 7V to X V is separate (much more challenging than LTZ ref itself!) topic.This is just one example:QuoteIf you don't know what R8 is doing (neither I am), why you add it into circuit?You don't need LTZ reference to test out the output stage, you can take LM399 which just need power and one resistor (PTF56 will be fine) and a DMM. Then by experimenting you can answer all of your questions (and surely will also get much more questions that might need attention instead). You will see from actual results if the outcome fits your needs or not, from which point you can go refining the experimental setup. Many of the high-end metrology circuits are counter-intuitive (e.g. adding more "parasitic" parts that does not do anything or make more noise to balance out/mitigate worse issues like thermal EMFs), and trying to take all of these hacks you see on board A or board B without knowing the impact will only confuse you more.Heck, the heated debates about two little capacitors in LTZ circuit here on forums already caused few people moderating themselves out and unwanted personal attacks.Again, I'm not trying to discourage you from posting or discussing things on forum. My intention is the opposite, have alternative approach and play with the circuit on the bench, as no theory or simulation can account for all real-world complications. Otherwise we collectively just tearing your design apart, everybody pulling resistors and signals direction they see best fit (out of best intentions, not trying to harm). I designed and implemented my output stage because it fit my project needs well enough. Is it most stable possible output stage? I'm sure it is not, but it works for me.

author: imisaac
author_rank: Regular Contributor
timestamp: May 26, 2019, 07:59:22 am
content: Thanks, TiN for the advice.Regarding the boosting stage, it does seem that the boosting stage is trickier than the LTZ reference and all the questions that was raised so far related to the circuit was to understand the circuit before building so that the component quality can be decided. That is, I agree that the function of R8 should be understood before being placed on the real circuit.I think you are suggesting to have a new thread for this boosting stage because it is off topic from this original thread topic, right? Yes, we shall do that. The reason that the boosting stage was discussed here was that the tempco of the LTZ1000 tempco setting resistors can actually correct for the tempco of the boosting stage circuit as well.....so not completely off topic.In addition, I agree that building a real circuit is better than simulation when it comes to circuit evaluation. However, some simulation is still invaluable at the beginning stage to have a rough picture of the circuit behaviour.QuoteYou are right that this circuit is being designed collectively by all of us in this forum. The original goal was to explore how the tempco of LTZ1000 can be compensated by a transistor so that a better stability may be reached. Therefore, many theoretical questions about the circuit limitations were raised. I do not understand why you think this circuit is being designed just for me. That is in contradiction to the open nature of the forum.....maybe that is not what you meant. Anyway, the most important point of doing any discussion is thatQuotequote by TiNIn summary, I agree that we should move the discussion of the boosting stage to a different thread after some initial test results are available.