ADR1399 reference

[Andreas]

Hello,

no its not specified explicitly.
But you should stay away from the 90 deg C setpoint with the chip temperature.
And that with the 3 mA zener current.

I don't see why the FlexPCB should be worse in performance.
Does that sound reasonable to you?

It is worth a try. (in comparison to the TO-46 package).

By the way did you test already the T.C. of the devices in TO-46 package?
My LS8-values are higher (>=0.08 ppm/K) than the values that I have from the LM399 measurements (<= 0.07 ppm/K) near room temperature.

with best regards

Andreas

[branadic]

By the way did you test already the T.C. of the devices in TO-46 package?

No I didn't and probably won't for the individual samples, but for the overall circuit including the gain stage. In the end I'm not that much interested in what every component contributes, but in compensating the final circuit for low (almost no) t.c.

-branadic-

[Andreas]

Hello,

to come back to the "position dependant drift"
https://www.eevblog.com/forum/metrology/adr1399-reference/msg4597733/#msg4597733

after having added the level shifters between processor and multiplexers I added 2 LM399 to the PCB which I had in the drawer to exclude some drift by the multiplexers.

Unfortunately both LM399 (from LT) had a large initial drift.
Additionally #2 of the both LM399 had initially popcorn noise which I recognized on a first T.C. test

After some checks with constant temperature I recognized that the temperature compensation with respect to the PCB temperature (0.3 ppm/K on #4) showed some "over compensation".

So I finally measured the T.C. of ADR#01 - #04 again which was surprising to me.
The PCB temperature range with the same environment sweep has been reduced (and shifted to higher temperature) by the adittional "heaters" on the PCB.
The ADR in LS8 package do not correlate well to the PCB temperature.
But they obviously correlate good to the "environment" temperature (which is sensor glued to a heatspreader aluminium plate below the ADR1399 test PCB).
Attached a "overlay" between a old and new measurement of PCB related TC and environment related TC of ADR1399#03 which obviously driftet nearly zero since last T.C. measurement which was end of november.

So now I can explain the position dependant drift:
I relied on the PCB temperature sensor near the multiplexers to compensate for environment temperature.
Initially I had 2 degrees self heating of the multiplexers due to missing level shifters creating PCB temperature rising which does not correlate to the behaviour of the ADR1399 which react more to the environment temperature.
Also placing the ADR1399 PCB above another heat source gave a large temperature gradient between the both PCB temperature sensors of +7/-2 deg C difference.

Additional point:
We had some rather cold days where I had down to 17 deg C in my lab.
Normally the T.C. of my measurement devices (24 Bit ADCs) is temperature compensated so this should be no issue.
But with my other daily measurements I recognized that the standard deviation of my ADCs has nearly doubled from 0.25 ppm to nearly up to 0.5 ppm.
And partly some devices developed some kind of popcorn noise.
In the beginning I only evaluated one ADC (ADC#13). Now I am evaluating all 3 stable ADCs (#13, #15, #16).

comparison ADR1399 vs. LM399
- the 2 lm399 show a much larger initial ageing drift as the 4 ADR1399LS8 samples.
- The T.C. of both LM399 (TO-46) samples is lower than the 4 ADR1399LS8 samples.

lessons learned:
- the PCB temperature is not usable for temperature compensation of ADR1399LS8.
  instead I should use a Metal housing and measure the temperature below the PCB.
- Never rely on one single measurement device for ageing measurements.
- Build a ageing PCB with some kind of injection channel for a proven stable reference (Like my LTZ#4 and/or LTZ#9) as sanity check.

with best regards

Andreas

[r6502]

Hello All,

I will get some ADR1399 in LCC version  and I lokked at the data sheet if there are informations given, where to place the damping network, sould it be placed cloce to the pins, like you do with caps for decoupling, or can it be a litttle more far away? Should they be close by pins 1/7 or better close by pins 2/6 or where both connections are joined when usig kelvin connection. Or connect pins 1/2 and 6/7 direct at the chip and place the damping network there?

Guido

[Andreas]

Hello,

if you look at the user guide (evaluation board) schematics:
https://wiki.analog.com/_detail/resources/eval/adr1399e_schem.jpg?id=resources%3Aeval%3Aadr1399e-ebz
its more on pin 2 + 6.
But on the other side they have used the bridges instead of the transistor.

So I have simply connected both pins at the chip and put the network directly under the chip on the soldering side (together with the cap for the heater).

with best regards

Andreas

[Kleinstein]

The damping part is mainly relevant for frequencies of some 50 kHz to some 1 MHz. So the exact position should not really mater. More distance from the chip (e.g. away from the heat) and also more to the sense or force should not matter that much. A position more to the force side makes sense, especially if there is an additional transistor driver in between as in the evaluation board.

From the shown imedance curve it looks like the capacitance is more in the low side. Especially with MLCC capacitors I would plan with space for 2x1 µF to at least compensate of the biasing effect and not get much below 1 µF.

[alex-sh]

Hello all,

Did anyone compare the LS8 package vs TO?
Particularly, I am interested in the long term drift, but suppose nobody has this info at this stage?

[Andreas]

Hello,

I still have no TO-46 samples.

The only thing that I can say:
I have difficulties with the LS8 package to monitor any useful ageing data.
The tempco is rather high on my samples.
And the LS8 package is rather picky about the location of the temperature sensor.
I hope that I will have a better setup for ageing monitoring at end of this week.

with best regards

Andreas

[alex-sh]

The tempco is rather high on my samples.

Andreas
Do you think this is just your samples or this is a general rule of thumb for LS8?

[Andreas]

Hello,

We have still not enough measurements (4 on my side) to make any statistics.

with best regards

Andreas

[branadic]

I have measured my ADR1399-PWM-DAC 10 V voltage reference over temperature (18 ... 28 °C) against an internal temperature sensor with the result of -0.223 µV/V/K.

-branadic-

[JodelJonny]

Hello there
I am new here. After sucessfully repairing a HP3456A I solwly but surely waddle alogng the path towards voltnuttery. And after years of silent observing I now have decided to try to build a 10V reference based on the ADR1399. The LTZ1000/ADR1000 is just unreasonably expensive and overkill for now.

I designed a simple PCB based on the circuit from the datasheet.
I think that should work out. Do you argee? Are there any errors or things I should improve before ordering the parts?
The resistor divider to amplify the aprox. 7V to close to 10V is planned to be build from Vishay TNPU e3 0805 smd resistors with 0,1% and 5ppm. There are a multiple footprints to be able to tune the output close to 10V and they are all encloed with copper planes on both sides of the pcb to keep the temperature differences between them small.

I plan to supply it with stable 15V DC from a simple linear transformer power supply. Or would it be better to include a 7815 on the pcb?
The reverse polarity protection is handled by a DMP3099L-7 P-channle mosfet. I think it can do that job without any additional components, or did I overlook something?


Thanks for the feedback! :-) 

[iMo]

I would not place the 7815 on the board (it changes its temperature based on the mains voltage and adds a heat source).
The 7815 should be there but after the protection mosfet.
I would use the 1399_zener_bootstrapping from the opamps output. That would require to add a transistor in order to de-load the opamp's output.

[dietert1]

Maybe produce a reference modul that fits into the HP 3456A? The ADR1399 can be used as an upgrade for the existing LM399. The 10 V to 7 V divider and the buffer transistor that are external to the HP reference module can be optional parts, maybe hidden on the bottom of the board.

Regards, Dieter

[JodelJonny]

@imo Good point about the bootstrapping. Didnt really understand that before but now I think I do. I added Q2 and reduced the value of R1 to 1k to maintain 3mA zener current.
What transistor should I use? I dont have much expereince with all this. I guess the specs are not that important.

@dieter1 The 3456A ist right now the only thing in my posession of wich I can at least assume that it is reasonably stable and likley even still kinda accurate.
Long story short. I wont touch it until I am sure to be able to monitior the cosequences of my actions. But this might be an interesting projet for the future.

[dietert1]

I did not propose to touch the HP 3456A, but to contribute something useful. It's a plug-in module with connectors, easy to exchange. I would never unsolder a LM399 from a HP reference module, but make a replacement module.

Sooner or later compatible reference modules with ADR1399K will be requested. They also fit the HP3457A. We have in total 7 of those meters.

Regards, Dieter

[iMo]

@imo Good point about the bootstrapping. Didnt really understand that before but now I think I do. I added Q2 and reduced the value of R1 to 1k to maintain 3mA zener current.
What transistor should I use? I dont have much expereince with all this. I guess the specs are not that important..

Any modern transistor will do. It also depends whether you want an output current limitation for safety reasons, that would require 2 transistors, and also sometimes people shift the opamp's output voltage to a lower value by a zener diode, like below. But is depends what you want..
PS: the R7 sets the current limit roughly..

[Kleinstein]

There is usually no need for the extra diode D1 in series with the zener.
In many cases one would also need additional capacitive feedback directly from the OP-amp for stability.

For the OP-amp there is no need for the relative expensive LT1012. A relatively cheap OPA202 may be a better choice.

[JodelJonny]

Thanks for the great advice!

So in the circuit shown by imo the opamp is sinking current or pulling Q1 down towards the Out_level against R4, correct?
May I ask why one would like to shift the opamp's output voltage lower? And what is the idea behind D1?

I copied the LT1012 from the ADR's datasheet but I'll happily save the money if OPA202 is good enough.
Overcurrent protection would be nice to have. Little accidents do happen.

What I want is a simple and cost effective voltage standart to give me a stable reference to compare my 3456A to and use is as a transfer standart so I could bring home a known voltage if I come across an oficially calibarted precision multimerter.  That combined with learning about analog desing, pcb layout, metrology and so on. This part is already working.

@dietert1 Sure but right now I have no way of evaluating the perormance of such a desing. Thats one of the reasons I am building a Voltage refernece
 

[Kleinstein]

For a 10 V reference there is no real need to use the zener diode to shift the level. One sometimes does this to get an output voltage closer to the supplies (e.g. 14 V out from 15 V supply).

The diode D1 may be wanted, so that the base current to the transisor is only from the resistor and not from the OP-amp. Usually the op-amp outputs are current limited and thus no real problem without D1, but the diode would not hurt either.
The OPA202 is nearly rail to rail out, and one may still need some startup mechanism, if the zener current is taken from the 10 V. The zener diode part would provide the start-up.

Ideally one would want to have both an output for the 10 V level and the raw 7 V from the reference, so that the gain drift does not matter and can be checked separately. It could be enough to just have a series resistor (e.g. 1 K range) as a crude protection for the raw reference.

[Andreas]

I copied the LT1012 from the ADR's datasheet but I'll happily save the money if OPA202 is good enough.
Overcurrent protection would be nice to have. Little accidents do happen.

What I want is a simple and cost effective voltage standart to give me a stable reference to compare my 3456A to and use is as a transfer standart so I could bring home a known voltage if I come across an oficially calibarted precision multimerter.  That combined with learning about analog desing, pcb layout, metrology and so on. This part is already working.

Hello,

I think: for a single device the price of the OP-Amp should not play any role.

I think you should take more care about star grounding. (A power plane easyly can carry some 10 uV from heater current).

Thermal management is also to consider. (Thermally isolating and packaging into a metal housing to avoid large temperature gradients).
Depending on how stable your lab temperatures are you should take also care to select the right reference and divider resistors.
A LM399 might give better results than a ADR1399.
At least some temperature sensor would be needed if you want to transfer the "volt".

I would at least put some 100 nF capacitors directly at the Heater pins and the zener output to avoid EMI drifts. (for the ADR the 5R 100nF should be placed very close to the zener).
And the buffer amp output also should be also shorted with a 100 nF (of course with a isolation cirquit against capacitive loads).
see example here:
https://www.eevblog.com/forum/metrology/emi-measurements-of-a-volt-nut/msg2684100/#msg2684100

I would build at least 3 devices to observe ageing drifts or drifts related to temperature shocks during transport of one device.
Otherwise you will never know wether the volt that you measured on calibration is the same that you imported into your lab.
I had a 4 ppm drift on a LM399 before and after calibration. I finally recognized that the LM399 output is dependant on orientation.
So I now mark the housing of the LM399 reference where the "Top side" has to be.

with best regards

Andreas

[JodelJonny]

Thanks again!

I tried to understand and implement your suggestions. To be honest, the amount of information in all those threads is quite overwhelming at times, especially since I have no formal education in electronics.

I plan to mount the pcb in a metal housing with temperature sensing but I am not sure were I want to measure the temperature.

@Andreas I have now added two 100nF caps (C6 &C7) directly at the pins of the ADR1339. One of which is now in parallel with the 5.1Ohm 1uF (R3 & C3) RC circuit from the datasheet which I also moved on the backside and very close to the pins. Is that correct or are those circuits redundant?

I am not sure about what you meant with the 100nF to short the buffer amp output and the isolation circuit against capacitive loads. I added C5 in the schematic.

I switched to a ground plane on the back side and added some slots to break temperature gradients and thermally isolate the ADR a bit.

I hope it is now better and would appreciate further feedback :-)

[Kleinstein]

The temperature sensor should be more at the resistors to set the gain - not directly at the ADR1399, where the temperature is regulated in parts..
For the temperature sensor I would more prefer an anlog one, like PT1000 , 10 K NTC or maybe just a diode.

The ground plane is not a valid replacement for a star ground. I would prefer less ground around the LM1399 to reduce the heat loss even more.

The configuration for the gain setting resistors looks a bit odd. The required gain is a bit less then  3/2. So the fist approximation could be 3 identical high grade resistors (e.g. 10 K) and than some additoinal fine trim pallel to the one on the high side.  To get fine trim without very high values the fine trim would ideally be 1 resistor (~ 65 K ?) in series with some 3-4 in parallel (e.g. 5 K + .... ).

[iMo]

Here is an interesting setup with 8 resistors (an array), I've tried to sim it and a resistor in series with the low side 10k one, around 450ohm, could be used for the fine trimming (do simulate the stuff with your values first).

https://www.eevblog.com/forum/metrology/statistical-arrays/msg4703825/#msg4703825

Also, as I wrote above, the 15V voltage regulator should be placed "after" the mosfet, otherwise you get -2mV/C at the 15V supply.

The 22k and 15ohm in the buffer were examples only in my schematics, you have to adjust that values based on your requirements.

To the 100nF feedback capacitor at the opamp - do we really need it? Where the instability comes from? From the output load capacitance? Could we somehow calculate the value really required? The output buffer is an emitter follower with no gain, imho.. Moreover I did some simulation in past and I saw decreasing PSRR of the opmap with increasing the value of that capacitor. But perhaps that is just an issue with the opamp model used..

[Kleinstein]

Some capacitance for C5 is likely needed due to the capacitive loading. How much depends.  With the output capacitance one shoud aim for C5 * R5  > 2*C6*R12 . So less than 100 nF should be OK,  more like the 1 nF range.

The MOSFET used as a kind of ideal diode may give less than -2mV/K, but still not ideal. I would not mind a LDO on the PCB that much, at least as an option to populate.