ADR1399 reference

[iMo]

You have to burn-in it for 1k5 hours at least.. ADI will not do it for us, imho. But a good opportunity for the voltnuts here to offer the "calibration" of the module after the burn-in period. The switching regulator - best to look at the input/output voltage with an o'scope or similar.. And it is an evaluation kit, so we have to evaluate 

[Kleinstein]

Many of the commercial, isolated DCDC converters produce quite quite some common moder interference. So there is some capacitive coupled current between the 2 ideally isolated side. This may not effect the reference circuit itself, but it may effect some DMMs or amplifiers used for the test.
The problem with the common mode interference is, that it is hard to filter.

[dietert1]

You have to burn-in it for 1k5 hours at least.. ADI will not do it for us, imho. But a good opportunity for the voltnuts here to offer the "calibration" of the module after the burn-in period. The switching regulator - best to look at the input/output voltage with an o'scope or similar.. And it is an evaluation kit, so we have to evaluate 

Started evaluation by hooking it up to an unused 6.5 digit meter and it shows 7.04264 or 7.04265 from the start. Most people would do this with a pocket calibrator. I mean, the reference is a reference at the 6 digit level, yet of unknown voltage except 7.0xx V. When i ordered references from Mr. Geller 13 years ago, they came with calibration info. This know-how has been around for many many years.

When there is some spare time, i will probably add a linear supply and a 10 V output stage/buffer with a NOMC array divider in order to make it 10 V and integrate it into my scanner setup.

Regards, Dieter

PS: First step already finished. On a LM723 based linear supply with 15.002 V (switchmode supply inactive) the reference yields 7.04277 V into the same meter. An up shift of 17 ppm.
After undo and running it on USB-C again it comes back to previous value, so it isn't damaged. Supply is 14.4 V now and a bit noisy.

[iMo]

Many companies go Agile these times.. The calibration of the module will be done by ADI in the next Sprint, after the Sprint Review, Retrospective Meeting, Backlog Refinement Session, new Sprint Planning, provided the Product Owner agreed that with the Scrum Team and the Scrum Master and the Calibration will be put into the Product Backlog as the new Story in the 1399 Epic, with enough Story Points to be Burned-down during the Calibration Sprint.

[dietert1]

My previous supply test detected another design problem of the evaluation board: The ADR1399 won't be low noise and stable to 1 uV without a well made bootstrap gain stage. Calculate: 120 uV shift for a 0.6 V supply change results in a +/- 5 mV requirement for the supply. Also i am wondering about the resistor they used to feed current into the ADR1399.
Already replaced the two zero ohm resistors next to the reference by pieces of copper wire.

Regards, Dieter

[miro123]

My previous supply test detected another design problem of the evaluation board: The ADR1399 won't be low noise and stable to 1 uV without a well made bootstrap gain stage. Calculate: 120 uV shift for a 0.6 V supply change results in a +/- 5 mV requirement for the supply. Also i am wondering about the resistor they used to feed current into the ADR1399.

How do you calculate this numbers? Here are mine.
R4=1.33k R=0.04Ohms for TO46 R=0.011Ohm for LCC
To46 sensitivity is =0.04/1330 =1/ 33250  - 33,25m shift make 1uV
LCC  = 0.011/1330 = 120909   - 121mV makes 1uV

[dietert1]

My numbers are an observation. When i connected a 15.002 V linear power supply to the test points, reference output shifted up about 120 uV (17 ppm of 7 V). To check whether i damaged the reference, i reverted the change and it returned to the previous output voltage. Then i measured the supply from the switchmode converter to be 14.4 V. So a 0.6 V supply change caused 120 uV output voltage change.
There could be other side effects, like heat or RF emitted by the switchmode converter.
Did you notice that everywhere in the datasheet they recommend to feed 3 mA into the reference, but the 1K33 in their evaluation kit feeds about 8 V/1K33 = 6 mA.
Meanwhile i made the bootstrap/gain stage and i need some idea how to feed 10 V into the nice thermal protection without damaging things.

Regards, Dieter

[MiDi]

Made the rejection ratio calculations for bootstrapped LM399 & ADR1399 some time ago, Excel attached:

[opa627bm]

Apology if I didnt read through the entire thread.
Anyone is having the EVM drifting due to the 1Mohm isolation resistor between the AGND and PDNG?
For the ADR1001 it is using the same design and I have 2 EVMs that is showing the same (kind like EMI ) issue, and I ended up bridging this resistor.

[dietert1]

By "bridging the resistor" you mean a short? This shouldn't be necessary. You can keep isolation by using a small capacitor to bridge the resistor, like the 4.7 nF found in many switchers. Then you keep the reference voltage isolated.
My impression was the switchmode converter isn't stable enough, so i am using an alternative linear supply. My ADR1399 EVM also got the bootstrap/gain stage. Not the one calculated by midi, but the "real thing". It has a chopper amplifier and a NOMC divider and outputs 10 V to some ppm. Put the whole circuit into a metal case. Running in my scanner setup, hourly averages exhibit noise of about 2 uVpp plus strong initial drift of -2 uV/day. As this is a new build, both noise and drift should calm down soon.

Regards, Dieter

[opa627bm]

By "bridging the resistor" you mean a short? This shouldn't be necessary. You can keep isolation by using a small capacitor to bridge the resistor, like the 4.7 nF found in many switchers. Then you keep the reference voltage isolated.
My impression was the switchmode converter isn't stable enough, so i am using an alternative linear supply. My ADR1399 EVM also got the bootstrap/gain stage. Not the one calculated by midi, but the "real thing". It has a chopper amplifier and a NOMC divider and outputs 10 V to some ppm. Put the whole circuit into a metal case. Running in my scanner setup, hourly averages exhibit noise of about 2 uVpp plus strong initial drift of -2 uV/day. As this is a new build, both noise and drift should calm down soon.

Regards, Dieter

Yes, I can try the capacitor too. The ADI engineer told me the PSRR from the part should be good enough for it ...

[Andreas]

Hello,

some further measurements regarding PSRR of the heater voltage.
compare also the measurement of macaba:
https://www.eevblog.com/forum/metrology/adr1399-reference/msg4114465/#msg4114465
In my case I use a positive heater voltage from 9-30V (supplied by a external analog power supply).

Below 15V the PSRR gets quickly worse from ~0.5ppm/V up to 1.2 ppm/V.
2 samples of LM399 (in TO-46) seem to be slightly better especially at low heater voltages.

unfortunately I still have not found the "position dependant drift"
https://www.eevblog.com/forum/metrology/adr1399-reference/msg4468918/#msg4468918

maybe there are several different reasons:
Partly a explanation may be that on the LM399 ageing box the temperature distribution at the output connector is more equal.
Measured 2-3 degrees difference near left and right pin of the connector due to the heaters of the ADR1399 on one side of the pcb.
But also direct soldering of the measurement lines near the "far" multiplexer did not solve the differences between hot and cold place.

I additionally found out that the temperature of the multiplexers varies by up to 3 deg C depending on the level of the Adress/chip select lines.
So obviously the level shifters in the MAX4052A need considerably high power when not being at the Gnd/VDD level. So I might need external level shifters to keep heating away from the multiplexers.

with best regards

Andreas

[Kleinstein]

The quite variable power level for the max4052 is interesting. This could indeed be a problem for a precision measurement. There can be 2 parts: the level shifters and inputs that get a relatively weak signal. 74HCT... also show surprisingly high supply current with 3 V at an input, though a perfectly OK input level.

[Andreas]

Hello,

yes current consumption is unexpected high.
With 15V supply voltage and 5V from processor on the adress and CS lines I get up to 85 mW power consumption for one MAX4052A (with the same date code).
So I am waiting for the level shifter boards. (BSS138 based).

with best regards

Andreas

[dietert1]

The ADR1399 evaluation board i mentioned above was put into a hermetic enclosure. It got an OPA189 buffer that is a 7 to 10 V gain stage with a NOMC divider and a Sallen-Key low pass filter with two Wima MKP4 10 uF/250V. After observing a TC of about +0.2 ppm/K i made the enclosure a simple thermostat with an analog PI controller. Image shows heater resistor, transistor and NTC. Controller and power supplies are still external.
After the most recent temperature controller mod about 32 hours ago (arrow) reference noise is down to about 0.8 uVpp in comparison to the average of our two LTFLU references. ADR1399 datasheet allows about 2 uV per 10 V. Drift estimate after one month of operation would be 180 nV/day = 6 ppm/year (probably overestimated).
There are still events decreasing voltage, so it needs more "education".

Regards, Dieter

[branadic]

Why don't you add a bit of copper in the 7V --> 10V gain stage to compensate for the remaining t.c. overall?

-branadic-

[dietert1]

I didn't have the time and right now i don't know whether
- the NOMC divider suffered a minor TC while fine tuning for 10 V or
- whether it is the ADR1399 or
- whether it is the Pomonas' fault (thermal EMF).
It's an experimental setup. Next time i'd probably use soldered connections for the reference voltage output or move the hot reference further away from the Pomonas.
The temperature controller and the precision parts of the power supply should go into the box. One of the tests to do yet is running the ADR1399 with its heater off. The thermostat is good for +/- 20 mK.
My conclusion is that the ADR1399 with its increased zener current and its current price is
very interesting. Once the TO-46 version arrives at the distributors one should try an array.

Regards, Dieter

[Andreas]

Hello,

I want to give some update to the "position dependant drift".

The main factor is obviously thermal drift (ppm/K) of the references.
If I do not use the environment temperature but the PCB temperature the value increases up to 0.3 ppm/K.
(see overview).
 
I tried to compensate the readings with the PCB temperature.
But the improvement is much lower than I would expect it.

There is a remaining "low frequency noise / instability" of the values of about 2 ppm.
This is more than that what I usually get with my measurement system.

So the questions for me are:
- Is this related to my setup or do others also have larger instabilities with ADR1399 than with LTZ1000 or LM399?
- Is it only the LS8 package or is it also characteristic for the TO package?

At least it seems that the initial ageing drift is rather low.

with best regards

Andreas

[Andreas]

Hello,

I also did some tilting tests in 6 orientations of the PCB.

0 = top = component side of PCB on top
5 = bot = solder side of PCB on top
1 = north = Pin8 ADR1399 on top
2 = east  = Pin6 ADR1399 on top
3 = south = Pin4 ADR1399 on top
4 = west  = Pin2 ADR1399 on top

I had guessed that between north and south (PIN 4 and 8 ) there would be the largest differences.
https://www.eevblog.com/forum/metrology/adr1399-reference/msg4457818/#msg4457818

But there it seems that I was not completely correct.

Pin 4 on Top = south = 3 gives mostly (not always) the highest output voltage.
Pin 6 on Top = east   = 2 is likely to give the lowest output voltage.

The tilting drift amplitude with some thermal isolation is comparable to that what I also usually have on LM399 devices.
(see overview in previous post)

with best regards

Andreas

[branadic]

Some of you already know that I've revised the PWM-DAC board, now called xx99-PWM-DAC, that allows to populate the ADR1399 and its snubber network too. One of the boards was assembled and tested. I populated the same components such as 20 MHz crystal that I've used on the LM399-PWM-DAC and used the same firmware (8.16), but measurements weren't satisfying and I observed huge spikes.

I finally ended up sacrifying my LM399-PWM-DAC board which was used for the EU-CalClub test run and frankensteined the ADR1399 to it. I also changed the 2W NTC connection to a 4W one and took, after some adjustment of the gain, first measurements. For some yet unknown reason no such spikes are visible here, but with the filtering as it is by now the noise at the output is larger than it should be, but still an improvement with respect to the former populated LM399.

-branadic-

[Andreas]

Hello branadic,

is it the TO-46 package?
how much thermal isolation has been used?

with best regards

Andreas

[branadic]

Yes, TO-46 package as I don't have trust in that ceramic SMD packages without proper low strain assembly solution such as FlexPCB.
There is a bit of foam block around it, similar to what was used for LM399-PWM-DAC.

-branadic-

[Andreas]

as I don't have trust in that ceramic SMD packages without proper low strain assembly solution such as FlexPCB.

Hmm,

I measured 55 mA Heater current @15V and room temperature for 4 populated ADR1399 in LS8 package on my PCB with some thermal isolation:
one cotton pad around the edge of the PCB + some thin (2 mm) foam foil.
So only 14 mA per LS8-package (+3mA Zener current)

I fear with a FlexPCB the environment temperature limit to no thermal stability is reached soon.

with best regards

Andreas

[branadic]

I don't see why the FlexPCB should be worse in performance. What I have in mind is a small breakout board LS8 to through hole connections on one side only, so that the FlexPCB is floating underneath the LS8 footprint to prevent mechanical stress to the package. Does that sound reasonable to you?

-branadic-

[iMo]

I mentioned this issue in the adr1001 thread too - a need for a flex board as well..
@Andreas - what is the env temp limit to no thermal stability? Is that defined somehow in the DS?