ADR1001 - Ovenized Voltage Reference System

[magic]

Ok, after a good lunch my current understanding is as follows:
1. there is an 1.7ohm resistor wired after the divider
2. with the 10V the resistor applies as the buffer's feedback BUF_S goes to the output divider, so the resistor works as "the bond wire"
3. with 5V the resistor does not apply as the buffer's feedback goes to the output pin, after the resistor
4. they put the resistor there to mitigate instability with 1uF MLCC output capacitor
5. the buffer itself works, as I do not see a change in the feedback path when loading its output (at 10V setup)..
 

I must concede that this is what it looks like

Did you confirm that the 1.7Ω resistance disappears in 5V output mode or just speculation? (Of course, it seems almost certain at this point.)

And it looks like the resistance will also affect -5V applications.

edit
The only way it could still be your fault is if the resistance is outside the chip somehow, a dodgy solder joint or something.

[iMo]

@magic: it is my current understanding..
I have installed a new jumper switch, shortening pin 12 (BUF_F) and pin 9 (BUF_S) hard. Nothing else rewired.
With the jumper I can switch between 10V and 5V out.
I wired it such the jumper wires are bonded directly to the chip wires, to avoid resoldering other existing joints.

When I short the switch with special golden jumper I can see:

Vout 4.999718V 4.999723 (I may precise it later on as my dmm warms up, it was switched off), mind the buffer's load here is aprox 9k internal resistor against gnd.
With 47k load I do not see any change in the readings (the resolution here is +/-1uV without averaging).
With 4k7 load I see a drop of 7-8uV (7.7mOhm internal resistance?).

Edit: a night run shows 4.999723 within 5uV.. The output divider ratio gives aprox 2.000116 then (both 10V and 5V ran with the same output load current).

[magic]

Yep, this confirms that the ref and the amp are working correctly.
1.7Ω is a lot of resistance; if it exists on your board you should be able to find it with a DMM.
If it isn't there, it has to be in the chip.
(We already know that it exists somewhere between the internal feedback divider and your jumper.)

This means load regulation from this 10V/-5V output is worse than TL431

edit
Theoretically, a different explanation is possible too. If input bias current of the amp is significant and changes with output load current, then BUF_S voltage is no longer half the output voltage and things get weird. But let's do the math: with 1mA load current, 10V output decreases 1.7mV. BUF_S is constant, so current through the upper 9.3kΩ resistor decreases 0.2µA. This would be caused by 0.2µA change in input bias current, which seems very unusually large.

I don't even think it's worth trying to modify the circuit to test this theory. I'm running out of even the most crazy and far-fetched alternatives to the simple explanation that there is 1.7Ω in series with this output not compensated by feedback.

[iMo]

I've measured the resistance between my chip's board's bonding wire on the pin 12 and the "+10V" post.
34401A shows some noisy milliohms, an another meter a zero.

[iMo]

I have installed a standard output buffer with 20mA current limit (bottom left in the shot), currently with an OP27 for initial experiments, later on I have to decide whether I put there an OPA189 or 2057. The ADR's output is filtered by 1u/50V foil wima and then the opamp etc. The voltage at the opamp's output is around 4.5V-4.7V based on the load.
With 820ohm load the voltage drops by 18uV in this setup (15V). The 10V output voltage is by couple of ppms higher than before (opamp's offset, or less load via the 1.7ohm bonding, it seems).

[iMo]

Yep, the OPA189 looks better there, I do not see an offset (op27 did +5.5ppm). The loading with aprox 12mA shows a 16uV drop, thus +- same as with the op27.
PS: the question now is whether to add a series resistor with the ADR's output, like 4k7-10k, to create a low pass with the 1uF foil at the opamp's input.. And keep, say, an 100nF foil at the ADR's output for its stability.
PS: I did with 10k and 100nF foil as below, no change at the output (within the mesurement noise)..

[dietert1]

The filter you inserted will also suppress chopper noise in reverse direction. As far as i remember input current spec of OPA189 is +/- 70 pA times 10 KOhm gives 0.7 uV offset. I used the same for an ADR1399, except i had 2x 5 KOhm to make a Sallen-Key low pass. The 1 uF 50 V WIMA may not be the best solution (leakage depends on temperature). Better pick an MKP cap for higher voltage with less leakage current.

Regards, Dieter

[iMo]

For example Wima MKP 4 series? Two 1u would not fit on my current board, sure
There is a low pass filter already before the ADR's buffer (5->LP->10V), there is also a Wima 3u3/50V in that first LP I had on stock.
Not sure the second one LP with OPA189 would help much, except the chopper noise.

[Kleinstein]

For reference filtering an MKS capacitor should be OK. It will take some extra time for settling, but the reference would anyway likely want at least a minute or so before getting sub ppm stability anyway.

A PP capacitor would be lower leakage and faster settling, but also larger. There may be slightly smaller form factors (e.g. japanese brands) than Wima MKP4 series.

It depends on the use of the reference if the fitler function helps much. The more higher frequency noise that is filtered out may or may not be relevant.

[iMo]

This is the "final" run with OPA189 buffer, the RC filters as above, 15V power, against a 34401A 100PLC over night (20C ambient). This time with median7 and TEMA filter I test just now. You may see the LM399 typical drift I saw also this fall as I did a check of the dmm with the LTZ1000 based calibrator. You may also see the change in the temperature noise - that is because the sensor was loose in the box, I put it somewhere on the board and it moved closer to the "chimney" (the cut off area around my ADR "trampoline") as I touched the box. The air turbulence there is large thus the sensor provides more noisier data. The ADR1001 itself has to be put inside an insulation as it is done in the ADI's evaluation kit.
The next precision measurements are on the experts here who are equipped with the best rigs, and I will let my PreProd Sample burn-in in peace.
Hopefully the ADR1001 comes soon, as the work with it is easy and cheap...

[Noopy]

Hopefully the ADR1001 comes soon, as the work with it is easy and cheap...

Latest news on my side: The release of the ADR1001 has been pushed out to July of 2023 

[Noopy]

Where do you all get these pre-production samples from???

[Villain]

Where do you all get these pre-production samples from???

Sample request on AD wegpage.

[Noopy]

Where do you all get these pre-production samples from???

Sample request on AD wegpage.

I don't find it on the Analog website and I even talked to the FAE... 

[Villain]

Where do you all get these pre-production samples from???

Sample request on AD wegpage.

I don't find it on the Analog website and I even talked to the FAE... 

It usually helps to work in the industry. And talk to people that want to sell you stuff, not the one's engineering it.

edit: Being in an engineering job i curse about sales people all the time, when you need something they are your best friend though

[Noopy]

Where do you all get these pre-production samples from???

Sample request on AD wegpage.

I don't find it on the Analog website and I even talked to the FAE... 

It usually helps to work in the industry. And talk to people that want to sell you stuff, not the one's engineering it.

Check!
Check! 
Nevertheless... 
Will try again... 

[Villain]

In theory one could have a OPA189 at BUF6V6, one at VDOUT, one at BUF_F to be able to monitor 5V, direct zener voltage and 10V respectively or anything speaking against that?

[Kleinstein]

It should be OK to have more than 1 output buffer at the same time. Here it would help, if the input to the buffer also has some filtering (e.g. 1 K and 100 nF range) to keep switching spikes away from the reference chip.

[iMo]

The buffer with the OPA189 I've been using is unstable with output loading capacitancies like 100n-220nF ceramics. With a tantalum 1.5uF (or 1.5uF ser 4R7) the output is stable. In the tina-ti simulation it looks stable for any Cout, but frankly, I would be much more happier to do it in the ltspice - is the OPA189 model somewhere available?

[Villain]

The buffer with the OPA189 I've been using is unstable with output loading capacitancies like 100n-220nF ceramics. With a tantalum 1.5uF (or 1.5uF ser 4R7) the output is stable. In the tina-ti simulation it looks stable for any Cout, but frankly, I would be much more happier to do it in the ltspice - is the OPA189 model somewhere available?

How is behaviour without any output capacitance? KX ref for example does not use any output capacitance if i remember correctly. Is it needed?

[iMo]

The buffer with the OPA189 I've been using is unstable with output loading capacitancies like 100n-220nF ceramics. With a tantalum 1.5uF (or 1.5uF ser 4R7) the output is stable. In the tina-ti simulation it looks stable for any Cout, but frankly, I would be much more happier to do it in the ltspice - is the OPA189 model somewhere available?

How is behaviour without any output capacitance? KX ref for example does not use any output capacitance if i remember correctly. Is it needed?

I've soldered the OPA189's load capacitors (4n7||220n||1u5) already in the pcb and cleaned up so I cannot tell yet. I wanted to remove the 1u5 tantalum I had there before and replace it with 220nF and I got weird voltages (bridging the 10V output posts with the 1u5 tantalum cured that instability, thus I soldered it in afterwards). I wanted to simulate it but with no result as I do not have OPA189 model in my ltspice..

PS: the KX does not use an output buffer with the power/current limit stage, afaik, moreover TiN is using the 2057 in his designs, not the OPA189..

[Kleinstein]

The buffer with OP and extra transistor could use a split feedback to improve the stability with capacitive load. So  an extra resistor (e.g. some 1 K) in the DC FB path and an capacitor (e.g. 10 nF) for a direct FB from the OP-amp.
The higher GBW makes the OPA189 more sensitive to capacitive loading.

[iMo]

FYI - my power supply for myADR1001 board - built around MC1723 from Motorola, date code 8445
It works nice!

[MiDi]

is the OPA189 model somewhere available?

Spice model for OPAx189 is awailable: https://www.ti.com/product/OPA189#design-tools-simulation -> OPAx189 TINA-TI Spice Model (Rev. E)

To include the model in LTSpice seamlessly:

Copy OPAx189.LIB into:

Code: [Select]

%UserProfile%\Documents\LTspiceXVII\lib\sub\Contrib\TIIf it is put in other directory, change path in OPAx189.asy accordingly.

In:

Code: [Select]

%UserProfile%\Documents\LTspiceXVII\lib\sym\Contrib\TI
create new file:

Code: [Select]

OPAx189.asy
and insert:

Code: [Select]

Version 4
SymbolType CELL
LINE Normal -32 32 32 64
LINE Normal -32 96 32 64
LINE Normal -32 32 -32 96
LINE Normal -28 48 -20 48
LINE Normal -28 80 -20 80
LINE Normal -24 84 -24 76
LINE Normal 0 32 0 48
LINE Normal 0 96 0 80
LINE Normal 4 44 12 44
LINE Normal 8 40 8 48
LINE Normal 4 84 12 84
WINDOW 0 16 32 Left 2
WINDOW 3 16 96 Left 2
SYMATTR Value OPAx189
SYMATTR Prefix X
SYMATTR SpiceModel %UserProfile%\Documents\LTspiceXVII\lib\sub\Contrib\TI\OPAx189.LIB
SYMATTR Value2 OPAx189
SYMATTR Description 55 V, EMI Enhanced, Zero Drift, Ultralow Noise, Rail-to-Rail Output Op Amp
PIN -32 80 NONE 0
PINATTR PinName In+
PINATTR SpiceOrder 1
PIN -32 48 NONE 0
PINATTR PinName In-
PINATTR SpiceOrder 2
PIN 0 32 NONE 0
PINATTR PinName V+
PINATTR SpiceOrder 3
PIN 0 96 NONE 0
PINATTR PinName V-
PINATTR SpiceOrder 4
PIN 32 64 NONE 0
PINATTR PinName OUT
PINATTR SpiceOrder 5

Enjoy new third party part in LTSpice!

[iMo]

LTspice misses something - it loads the OPAx189 part into the schematics, but I get
Fatal Error: Unknown subcircuit called in:
   xu1 n007 out n001 0 n005 %userprofile%\documents\ltspicexvii\lib\sub\contrib\ti\opax189.lib opax189