ADR1399 reference

[CalibrationGuy]

I thought they were referring to changing the heater current maybe?

TomG.

[iMo]

In 399 and 1399 the heater current is internally set such the temperature of the chip is kept "constant" at 95degC, afaik.
Thus you cannot mess with zener nor heater current with 399 and 1399.

For such experiments with zener/heater currents you want LTZ1000/ADR1000/ADR1001 for example..

I thought they were referring to changing the heater current maybe?

TomG.

[CalibrationGuy]

In 399 and 1399 the heater current is internally set such the temperature of the chip is kept "constant" at 95degC, afaik.
Thus you cannot mess with zener nor heater current with 399 and 1399.

For such experiments with zener/heater currents you want LTZ1000/ADR1000/ADR1001 for example..

I thought they were referring to changing the heater current maybe?

TomG.

I was just perusing the LTZ threads, thanks for pointing that out.

TomG.

[Andreas]

Hello,

on the other side the idea is not uninteresting.

Depending on location of heater, zener and temperature sensor on the chip
the temperature distribution could be affected when the cirquit around the zener gets more current
(and the heater less to keep the temperature).

with best regards

Andreas

[iMo]

Hello,

on the other side the idea is not uninteresting.

Depending on location of heater, zener and temperature sensor on the chip
the temperature distribution could be affected when the cirquit around the zener gets more current
(and the heater less to keep the temperature).

with best regards

Andreas

With a higher "excess" current you start to influence the heater's equilibrium, sure.
1mA more "zener excess current" will create 7mW of heat somewhere on the chip, with Rthja 220C/W (DS value for the TO-46 version) it increases the chip temperature by 1.54C, that will force the oven to decrease the heater current to go back to 95C. The excess current through the "zener" will increase the drop at the zener's bond wire(s) as well.
So you may try to increase the "zener current" till the oven gets off the regulation (my guess would be aprox 10-20mA) and then the "excess zener current" starts to dominate..
Another version is to switch off the heater, and heat up the zener with the "excess zener current" only. You would need aprox 25mA of current for say 70C (23C Tamb), imho.
The question is how the "zener voltage" with the excess current dependence is in reality.
I would not do it, however.. 

[dietert1]

In their evaluation board they use about 3 mA. If you use a 0.1 % resistor to generate that current, you expect at most 3 uA variation. Times the 0.04 Ohm dynamic impedance spec gives 120 nV. Good enough!
In the datasheet they explicitly show that the same dynamic impedance applies for a 3 to 13 mA change (delta I = 10 mA gives 0.4 mV).

Regards, Dieter

[Andreas]

Hello,

before changeing the mulltiplexer to DG4052 on PCB#2 I wanted to see wether the multiplexers have any influence on tempco of the references.
So I started with T.C. measurements.

Unfortunately I recognized that I did not repeat the T.C. measurements of the LS8- parts after I had populated the metal can parts.
Due to the additional heaters on the PCB the measured temperature range on the bottom of the die cast housing has changed even with same environmental temperature profile.

On the LS8 parts the temperature range was 12-45 deg C without population of the metal can parts.
Now it is 18-46 deg C with the metal can parts populated.
And it looks like the effective T.C. relative to the temperature sensor has increased.
So most probably my temperature correction for the LS8 parts was wrong after population of the metal can parts.

Further I recognized that there is still some latch up effect (2 times) after having changed from MAX4052A multiplexers to HEF4052.
I looked at the power supply values at the beginning of the latchup.
And surprise: on entry of the latchup the 15V supply of the LDO (LM2940) increased by 50-60 mV. Whereas the raw value 17V was about constant.
At the same time the voltage appeared more noisy. (Oscillations ?)

I double checked the data sheet and found that I was missing the 22uF capacitor at the output of the LM2940.
There are only 12 pcs 100nF capacitors on the 15 V supply (one for each ADR1399 heater) 2 for the Multiplexers and 2 directly at Output of LM2940 / input of LM2950A.
I tried to measure the oscillations with the scope. But obviously this changed something at the cirquit so no oscillations visible.

I removed the Jumper for the heaters since the "latchup" occured more at higher temperatures = lower heater power.
So in this condition (lower 15V load and 8 capacitors removed) I saw the oscillations 2Vpp with 45 kHz.
A 100uF capacitor which I had on hand fixed the oscillations.
So now I can repeat the T.C. measurements hopefully without any further "latchup" effect.

But I fear the ageing measurements up to now where useless:
Wrong T.C. and one never knows how the unstable 15V supply influenced the measurements.

I am feeling like a bloody beginner.
On the other side: the oscillations are only visible under rare conditions otherwise I would have seen them on cirquit start up.

with best regards

Andreas

[Alex Nikitin]

Guys, just for clarification - the ADR1399 can not work properly below about 2.5mA reference current, with 3mA is not just a datasheet figure but the correct minimal current. For that reason if you replace LM399 with ADR1399 you must take care of the current, in many old units LM399 is run at just above 1mA reference current and without modifications to the circuit the ADR1399 as a replacement will work poorly.

Cheers

Alex

P.S. - did you really think you can get lower noise for free, without an increase in current? 

P.P.S. - I've scanned through the circuits uploaded in this thread, it looks like I am overreacting  .

[Andreas]

Hello,

if it calms down your nerves: I am using 2K4 to 15V in my ageing box.

with best regards

Andreas

[Alex Nikitin]

Hello,

if it calms down your nerves: I am using 2K4 to 15V in my ageing box.

with best regards

Andreas

It certainly does  ! I would be heartbroken if all your work on this reference could be wasted!

Cheers

Alex

[Alex Nikitin]

Now I can explain a bit, my earlier strong reaction. A few days ago I bought HP3245A unit which was modified by replacing the original LM399 with ADR1399 (on top of other service work, a new mains filter, new Dallas chips etc). Unfortunately, the minimum current issue was missed and the unit (recently calibrated using HP3458A) was not very stable. I've added a 2K7 50ppm/C resistor in parallel to R401 and had to recalibrate manually, as the reference output (and hence the unit output) increased by 2.35mV or over 300ppm! Here is the stability test results overnight in my lab with the ambient temperature variation during nighttime about 4 degrees C, before the mod and after the mod and calibration. The voltage was measured by one of my Keithley 2015 units (NPLC10 +10 averages, a rough equivalent of NPLC100), so essentially against a very well aged LM399.

Cheers

Alex

[iMo]

I reported that 1399 instability in my "noise indicator" thread - the 1399 really needs at least those 3mA and the RC "snubber" (!!!). So for standard 7->10v you need 1k cathode resistor or even smaller..

[IanJ]

My 3245A with a newly fitted ADR1399 (24hrs ago).
Recently re-capped.
3245A Vref circuit op-amps changed out for Chopper (yes I know....but I am experimenting!).
Red trace is workshop temperature (regulated).
PS. My workshop is quite noisy, LED lights, PC running, Server cabinet.......all within fairly close proximity.

No snubber fitted to ADR1399:
10NPLC, 5sec interval, 78mins total.
Max-Min across graph = 2.224uV

Snubber fitted to ADR1399:
10NPLC, 5sec interval, 83mins total.
Max-Min across graph = 2.127uV





Ian.

[iMo]

Also do not solder the snubber directly on the 1399 pins (ie from bottom side of the pcb). I did it in my 34401A and the noise (stddev) rose by 20%. After removing it it went back. Caused by the thermal flows as the capacitor and resistor worked as a heatsink. I did it with the stock 399 there as the preparation for 1399 replacement, but stopped the experiment as that would require an adjustment of the meter (adjustment of everything).. 

[Alex Nikitin]

Ian, the reference noise in not a problem in HP3245A. Here are some interesting measurements I've just made using a fully passive 0.1-10Hz filter and Keithley 2015M at 100mV range, 1 NPLC, changing voltage output from HP between 1.2 and 1,3V.

Cheers

Alex

[IanJ]

Ian, the reference noise in not a problem in HP3245A. Here are some interesting measurements I've just made using a fully passive 0.1-10Hz filter and Keithley 2015M at 100mV range, 1 NPLC, changing voltage output from HP between 1.2 and 1,3V.

Cheers

Alex

I have seen this before when I was playing with WinGPIB. I guess the PWM isn't properly controlled when changing output voltage. Am guessing in Lo Res mode the 3245 doesn't do it?

Ian.

[Alex Nikitin]

It is not PWM control, it is a range change, 1.25V is the maximum output for the lower range. Moreover the level of noise changed with the range but doesn’t change with the voltage, it is the same voltage p-p for 3V and for 10V.

Cheers

Alex

P.S. - Low Res behaves in the same way just the ranges are different, for example if you go from 1.250V to 1.251V in Low Res, the noise doubles from about 12-13uV p-p to 25uV p-p.

[branadic]

An update of my ADR1399-PWM-DAC.
Recap: I'm observing the bootstrapped reference, that uses PWM in the 7 V --> 10 V gain stage, with a S7081 and a Prema 6048 at the same time.

I repeated t.c. measurement, result attached.
Long-term drift observation unfortunately were corrupted by a data crash, resulting in a total loss of data 
Since then 1000 h of new drift data were recorded, which appear rather stable.

-branadic-

[Andreas]

Hello,

update on my ADR1399 ageing reference board after having added 100uF to the output of the LDO voltage regulator on day 246.
(HEF4052 multiplexer has been used since day 190 / previously it was a MAX4052A)

Now I have a much lower standard devation especially on the metal can packages (from 0.25-0.3 ppm to 0.1 - 0.15 ppm)
But also the LS8-packages went down from 0.15-0.3 ppm to 0.1 - 0.17 ppm

And again the output voltage shifted. So I fear that I will have to repeat the ageing drift measurements with a new set of references.

With best regards

Andreas

[EC8010]

My ADR1399 (TO46 variant) evaluation board arrived a few days ago. I replaced the 1k33 resistor with a temperature compensated LM334 set to 3mA. The periodicity is due to the central heating coming on in the (cold) lab, probably a change of 5C or so. Deviations are smoother and smaller than with resistor. Negative polarity is because I inexplicably plugged to the board the wrong way round and didn't notice. 

Power was 12V 7Ah lead-acid battery, meter was HPAK34470A.

Next step will use op-amp bootstrapped Zener circuit instead.

[Andreas]

Hmm,

so you have around 1 ppm/K temperature drift.
I think that your DMM contributes most of the T.C.

Did you try to thermally isolate the ADR1399?
Did you do the TC-Compensation of the LM334 from datasheet or did you adjust the 2 resistors to minimum T.C.
(according to my experiences with LM334 for a LM399 project the resistor values from datasheet are not perfectly).

with best regards

Andreas

[EC8010]

Thanks for your thoughts. That's what I'm thinking; it's too close to the limits of my present measurement ability. I keep wondering about an 8 digit DMM. Or better, an array of at least four temperature controlled voltage references that can be compared using existing kit.

The 1399 is in the cardboard box with foam the evaluation board came in, but not properly sealed; I have done much better thermal control on other experiments. This is early days. I'm also wondering about that plastic hat the device is in. For instance, is the plastic triboelectric where it wobbles and scrapes on the leads? Is the plastic hat there simply because it's cheap and easy to add by the manufacturer, but not genuinely optimised? Would foam be better? The wobblyness and rattling concerns me, too. At this level, microphony is often an issue.

The LM334 is pretty much as per data sheet. I have tried tweaking the ratio of the resistors but it didn't help, suggesting that its remaining tempco was actually down to the (metal film) resistors themselves. I could perhaps do better using low-tempco wirewound resistors and trimming, but I think that's a bit of a dead end. I only put the compensated LM334 in because I had one ready-made and set for 3mA. I think the bootstrapped op-amp solution for the Zener should be better.

[Alex Nikitin]

It might be simpler (and better) to use the LM317 + 330 Ohm resistor (and another 2K7 to ground as an additional load for the LM317) as a current source, the current variations should be below 10uA in a reasonable temperature range  (10C-50C) which would give a voltage change only around 1ppm (over the range) even for LM399, with less than 0.1ppm for the ADR1399.

Cheers

Alex

[EC8010]

That's a thought. And I have an LM317-based variable CCS that could easily be connected. If I see the same result as with the LM334, fingers will be pointed at the 34470A.

[Mickle T.]

I slightly complicated the circuit diagram from the previous page, adding a few more components to the ADR1399 and the ability to reproduce AC and DC voltage with ranges of 1 and 10 V, six decades of resistance from 10 Ohm to 1 MOhm and current with five ranges from 10 μA to 100 mA. DAC - as usual AD5791A, but selected for INL<1.5 ppm.
The most difficult and time-consuming thing, as it turned out, was making the case. Although SCHROFF CompacPRO is convenient for installation and assembly, I did not like it at all due to its large and non-standard dimensions, and rather large weight (the entire assembled device weighs about 5 kg!). The extruded aluminum case is quite simple to machine, but unfortunately it does not accommodate domestic sealed PWW resistors and I have to install BMF or small-sized non-sealed PWW