ADR1001 - Ovenized Voltage Reference System

[Kleinstein]

The need for the extra capacitor is likely due to the compensation of the output amplifier. It looks like this is not a normal, unity gain stable OP-amp. This is absolutely OK for this use and 1 µF at the output would not be a problem. There is still the question if this applies to the actual chip or is only a model thing.

Considering the compensation, this makes me think how they get stability for the reference circuit itself. The LTZ1000 circuit usually needs at least 2 (usually 3) capacitors for the compensation to make it stable. Usually the capacitors are quite sizable (e.g. 100 nF) and would be hard to integrate to a chip. For the actual reference it may work with a modified compensation of the internal OP-amp. Without those capacitors the internal control loop would likely be somewhat different. This may effect the higher frequency noise - though usually not a thing to worry about.

The tricky part could however be the temperature control loop. This part includes a rather long time constant corresponding to the thermal loop and this would be really tricky to implement with the small on chip capacitors. The thermal loop would anyway be different with most of the heate power from the transistor and not a resistor. So the thermal part will be different from the LTZ / ADR1000.

[iMo]

Below setting the REF_TEMP via the upper resistor in the 10k/10k divider.
The resistor values from 1k to 10k step 1k, the lowest temperature with 1k.
With 100nF at ADR_OUT it stops oscillate, btw.

[iMo]

And R_ISET resistor (100-200ohm step 10ohm) vs. ADR_OUT at 59degC.
The higher the value the lower the output voltage.
PS: aprox -160uV/ohm..

[dietert1]

Recently i discovered oscillation in a DIY reference output buffer. There was a large cap on the amplifier output, but in my case with a small resistor 51R between the amplifier output and the cap. While there was no oscillation visible on the cap (reference voltage output), i found a very small AC signal on the amplifier output, maybe 100 or 200 uVss, hardly visible on the scope. First i couldn't believe it and began measuring Gnd and Vcc in various places, to see what "no oscillation" looks like. But finally i decided there was oscillation and modified compensation to fix it.
And it's somewhat plausible as those amplfiers usually include a miller type compensation inside which gives near 90° phase shift at higher frequencies. Then the output stage with that large cap gives another 90°, so it's an oscillator. Of course if you omit that 51R resistor, you won't have a chance to see the oscillation on a scope, but it may still be there.
Don't know how a 20 uV high frequency oscillation affects the input parameters of an operational amplifier. I'd guess it may cause some uV of offset shift.

Regards, Dieter

[iMo]

Out of curiosity I've tried to model the experiment with the 120ohm ISET resistor janaf made 7y back with his LTZ1000..
The model shows -258uV output change with the +1.2ohm delta (janaf's measurement was -91uV @7V).

[iMo]

Still looking for the DS..
Especially I want know what is ADI's recommendation for those 3 resistors around the package. The 120ohms ISET (zener's current) one on their evaluation board is an 0805 smd VPG "foil" one. Why they have not placed that 120ohm resistor inside the package? Is it expected to set the zero TC with it?

[Kleinstein]

The 120 ohm resistor set the operating current. Some 0.5 to maybe 10 mA are realistic, depending on the power one is willing to spend on the reference. The low current would come with more noise and likely a slightly worse TC (at least for the non heated case). Very high a current also comes with problems, like more effect of lead resistance. Higher current will also need a higher set temperature.
So it makes sense to have the resistor external to have the choice.

[iMo]

Below some internal resistances in my sample..
PS: a planned prototype board with a dead bug..

[iMo]

Before I start to mess with my iron - here is the schematics..
Not sure whether to put the 78L15 on the board (the protoboard is 5x7cm large), also see below my idea on wiring all the GND paths (?).
All parts are TH, no smds at this stage..
The 120ohm will be a 220ohm trimmer at the very beginning of the experiment, the 2k67/10k temperature divider should set 59C (based on sim, provided it is enough as the dead bug wires will work as a heat-sink  )..
Any feedback welcomed..

PS: UPDATE: schematics - added SET TEMP Jumper, fixed the heater currents, added an input filter and 4n7 at the output..

[Kleinstein]

Using a trimmer for the 120 Ohms is not really going to work well: the wiper resistance is usually not really stable. If one wants to test a few different currents it would be more like using more resistors in parallel with jumpers for some of them.

The capacitors for the heater current look a bit generous. One extra capacitor should be enough.

For the capacitance at the very output one should check the data-sheet / instructions on how much capacitance is needed / wanted. The old plan had 1 µF.  Chances are some extra capacitance with ESR would help. I would at least plan for space for something like an additional 1 µF+10 Ohm loadging the output. A 1 µF electrolytic may already have enough ESR.

[iMo]

I've made a change with the heater caps above. The trimmer will be not placed on the pcb, my idea is to set the output voltage (at 59C) as close as possible to 10V with the "120ohm", and then replace it with PTF or foil resistor. There will be place for 1uF at the output.
I've been using a protoboard so no problem with adding or removing parts, except the ADR1001, of course 

PS: I tried to find the zero TC by varying the ISET in the simulation, with no success. It looks like the zener in the model does not have all the spice params required for TC.

PPS: the heater opamp is powered from VIN, imho, so the question is whether the C4 should not be grounded to power gnd..

[Kleinstein]

Changing the current changes more than just the absolute value of the voltage. It may work if you are lucky and get 10 V with a resistor close to 120 ohm. However the required current can also be rather high or rather low and this way compromise the TC or noise or power consumption (may than need a higher set point).

There should be no really high frequencies involved. So the exact position of the capacitor ground should not really matter.

[iMo]

Yep, messing with the 120ohm is the main exercise for this week.. I want to see how the output follows the ISET (compared to above sim). The sim above shows some 0.2mV @10V per ohm around the 120ohm (and 2.2-4.4mA Iz for 200-100ohm).

[iMo]

It works!!
The first smoke test shows 9.980414V 10.004074 output with still cold 34401A.
The package is below 50C (finger test, 21C amb) with the TSET divider as above.
1uF/63V alu capacitor at the output.
The zener current with R_iset=120.5ohm is 2.9mA.

EDIT: fixed the output voltage.

[Kleinstein]

If the wires are solid copper (not manganin) it looks like they conduct quite some heat away from the chip.  Chances are one should run the chip with a relatively low set temperature.

An interesting point to observe may be the heater current. The 10 ohm resistor in the heater supply could be used for this, not just supply filtering.
Chances are one would see the effect of air fluctuations.

[iMo]

Yep, the heater takes 50.5 11.4mA (amb 22C) with the setup in free air.
The heatsink wires are made of oxygen free copper  .
An insulation around the package may help a little bit.

The voltage at the zener opamp's output is 6.58043V. It could be the TSET divider loads the output too much. Waiting on the DS..

+1V change at the input of the 78L15 makes no change at the output voltage (or it is less than 10uV).

PS: a pity they wired the buffer output directly to the output divider. There is still one pin NC (2), with the hot side of the divider on that pin 2 one could fine tune the output voltage then..

PPS: when I put my finger on the top of the package the current goes up to 57.2mA.

I replaced the 120.5ohm Iz set resistor with a 98.6ohm one, the output voltage went up by aprox 5.5mV, that is aprox -250uV/ohm (at 10V out). Close to the above simulation.. No chance to get closer to the 10V by altering Iz.

EDIT: fixed the heater current.

[branadic]

Yep, the heater takes 50.5mA/14.54V (amb 20C) with the setup in free air.

Wow, that is quite some juice. Looking forward to see more results (t.c., noise, drift, ...) and of coarse datasheet plus availability of it.

-branadic-

[iMo]

That heater current is on the free air at aprox 20-21C ambient, temp set to 59C (based on the model), bonded with around [EDIT: I finally have checked out the dia of the wire] 0.25mm dia Cu wires (the total cross section 0.98mm2). The package power loss P=U*I=0.2W..
Good for initial playing with without making a pcb, it should be done differently, sure.
I ran a measurement last night, but not worth of publishing it at this stage, the ISET resistor was 100ohm unknown TC from my junk box, it took around 5 hours to stabilize (the board put into an insulation box, 35 tbd C in the box created by the beast package, ambient aprox 21C), the last 8 hours it stayed within 1ppm @10V. It needs to be burned-in for at least 3000h, imho.

PS: I was told by Jaromir that this is the first documented home made board with ADR1001, so this is the first naive measurement, not worth of publishing.. 

EDIT: fixed power loss.
Note: below measurement with 50mA heater current.. (wrong setting).

[branadic]

Measurements such as t.c. and noise don't require burn-in before.

On the other hand I would characterize t.c. with the heater turned off to spot the zero t.c. temperature for a given current first. If the zero t.c. temperature fits your requirements of the final application (max. ambient temperature) with enough headroom for the oven control to operate, I would set the oven temperature exactly to that sweet spot. If not, adjust the current, measure t.c. again, find the new zero t.c. temperature and adjust your oven to that new temperature. Similar to how it's done in W/F7000. That is a valid general approach for any oven controlled, temperature compensated zener, except ADR1399 and LM399 as you don't have access to the zener itself.

It also makes sense to observe the first 3000 h or whatever the datasheet claims, too. Especially the difference between the zener and the 10 V output to see how stable the amplification is over time.
This way you - and if you publish your findings here, we - get the most knowledge out of your experiment with this component.

-branadic-

[sahko123]

So far this is quite similar to what other drift curves have come out look like. So its fairly passable. Chances are your unit didnt have any burn in at all at the factory hence the rapid initial fall off.

[iMo]

Hopefully nobody from ADI here watching my experiments.. 
I have got fully different situation after today's lunch.. 

I installed a pin header (a jumper switch) for switching the external TEMP set divider on/off.
I will update the schematics above soon.
I fired up myADR1001, watching the new data while messing with the jumper.
What I see now is following - the output voltage shows 10.0002V or 10.004V based on whether the TEMP divider header is open/closed.
The heater current changes from 20mA to 11.4mA based on the jumper, on free air..  OMG..

I've spent an hour thinking what went wrong before when I saw 50mA heater current - the only conclusion here is one of the PTF resistors had no contact (unbelievable) and the heater's setting point was pulled/pushed by the second resistor hard to one side, forcing the heater to max current.


I will investigate a little bit more - and I will fix all above info.. Starting again.. I love these experiments, indeed!
 

PS: updated above the schematics, and fixed the heater current info. Starting again with some basic measurements..

PPS:

LTspice simulation:
TEMP JUMPER
CLOSED: 59C with heater current 13.2mA
OPEN:   70C with heater current 17.7mA

Board:

[sahko123]

can you explain more about the temp divider your using? like a quick schematic photo to expain what your actually doing.

[iMo]

Schematics with description in my Reply #58..

[sahko123]

Yeah afaik you shouldnt have the temp set coming from a divider instead have it tied (THROUGH A RESISTOR) to ground or ref6v6

shorting to GND will give you a Tset of 125c

[iMo]

In my case the upper 2k67 was opened, thus the lower 10k set the high current. That is strange as the soldering was ok, imho, as you may see on the photo above. I messed around the resistors as I was installing the jumper and it started to work the right way. I have to doublecheck whether the resistor has not get cracked or what.. Anyhow, the reference has got good burn-in.. The outer package temperature was below 50C so it did not smoke in reality..