ADR1399 reference

[Andreas]

Hello,

for me the resolution is sufficient to see that on the bottom side of the PCB the temperature difference is ~4 deg C from center (KOVAR) to edge (Copper) for the long leads and > 10 deg C for the short leads on the unslotted side of the PCB.

with best regards

Andreas

[MK]

Hi Andreas,

My thinking is that it is necessary to keep the pads at the same temperature, a gradient in the copper is not as important.

[oz2cpu]

THANKS for a seperate ADR1399 thread :-)
see the picture mine just showed up

[iMo]

Hello,

for me the resolution is sufficient to see that on the bottom side of the PCB the temperature difference is ~4 deg C from center (KOVAR) to edge (Copper) for the long leads and > 10 deg C for the short leads..

Fun with 1399 

[iMo]

@1399 owners: could you somehow compare the 1399 vs. 399 popcorning?

[branadic]

I couldn't observe any popcorn on my ADR1399 sample by now.

-branadic-

[iMo]

LCC version in LTSpice..

[Andreas]

Hello,

my first LS8-samples from DigiKey showed up yesterday.
From datecode the LS8 version seems to be much older than the TO-46 devices here in this thread. 

Unfortunately I still do not have a "ageing board" for those.

with best regards

Andreas

[iMo]

Dead bug style?

[Andreas]

Hello,

dead bug is not planned as I want to see the influence of the PCB (humidity) on the output voltage.

With best regards

Andreas

[branadic]

Hi Andreas,

I would appreciate if you would split that group into halve, one group mounted to a rigid PCB, the other one with the reference on a flexboard that is attached to the rigid PCB. This way you could proof if a semirigid solution is better, since it is a standard industrial solution, reproducable and without magic.
If you need the flex board made by someone else for this investigation, I could provide you with some.

-branadic-

[Conrad Hoffman]

Just got an LCC8 package today. Hope is to dead bug it and mount it inside an old Eppley (or Weston) standard cell enclosure as a cool looking . Not sure what circuit or output voltage I'll use yet but probably a traditional opamp with the output feeding back around to the zener, though I'm not sure the 1399 needs that. Maybe just a buffer?

[iMo]

For example in my replay #31 above there is the simplest version with the buffer transistor only.
The ADI's eval kit 1399 has got the transistor too..
PS: the R4 there in my sim is not part of the schematics..

https://wiki.analog.com/resources/eval/adr1399e-ebz

PPS:
Below a Sim of ADI's Eval schematics - the TC at 1mA out.
The max +REFS current is 2.19mA in this sim.
The max +REFF current is 5.47mA in this sim.

Interestingly the ADI Eval schematics has got the RC filter at the +REFF pin (my sim above has it at +REFS pin), not sure ADI's schematics RC wiring is the correct version (ADI's typo?)..

[Conrad Hoffman]

Would have been nice if they had revealed their board layout and done a buffered application. Hey, more to experiment with!

[iMo]

The I_load char with the by transistor buffered 1399.
-1ppm at aprox 0.4mA..

[Kleinstein]

Driving the reference with just 1 external transistor is an interesting option.
The Eval board has the 1µF + 5 ohms RC element at the ref force side, not at the sense side. This does make a difference for the stabilty against oscillation.
The capacitor at the force side should be closer to the case with the 4 pin TO46 version and likely the better placement.

It is somewhat the question if one really needs the extra external transistor. The main advantage of the external transistor is unloading the reference chip from possibly variable currents.  For most cases the load to the reference is essentially constant and this essentially no gain from the extra transistor. The extra transistor as emitter follower may make sense when there is a variable load to the reference. So the extra transistor does not replace the bootstrapping ciruit to provide a very stable ref. drive current, but is more like an optional extra buffer for the 7 V reference, that may follow the reference.
The separate pins allow for a lower ref. impedance and thus less need for the bootstrapping part. To a large part this already applies to the simple circuit without the transistor. The extra transistor allows for a larger resistor at the force output (so the 1.3 K are rather low) and thus a little less effect of supply variation. So it helps to some degree (e.g. a factor of 5), but would still need a stable supply.

It is a bit unclear how the AC stabiltity of the reference is effected - the DC load current from the sense input is essentially constant, but it is a bit unclear how this part behaves AC wise.

The simpler case is to just connect the force and sense pins externally to the chip and use the extra pins just for compensation of the exffect of the bond wires. So the circuit is than essentially the same as for the 4 pin version.

[iMo]

No idea how the ADR1399 model in LTSpice has been designed, but I've tried a simple test with pulsing 1mA load and the peak-peak transients with RC on the force side are 4x higher than when RC is on sense side (with the transistor buffer).. So the ADI's eval kit schematics with RC on the force pin (with the transistor buffer) is not correct, imho..

[Kleinstein]

With 2.2 mA the transistor as emitter follower has an output impedance of some 12 Ohms. So the somewhat worse reaction to fast peaks is not such a surprise. The extra transitor changes the circuit and can change the suitable compensation quite a bit. 1 µF at the base looks a bit high - chances are a smaller cap there and than some RC (could be different from 1 µF and 5 ohms) at the ouput would be OK.
The spice model could be helping, but it really depends on how good the model actually is. A first test on the model would be how well it reporduces the output impedance in the LM399 like mode without the extra transistor. Even than it is not clear how much of the internal compensation is from which pin.

[branadic]

Just got an LCC8 package today. Hope is to dead bug it and mount it inside an old Eppley (or Weston) standard cell enclosure as a cool looking . Not sure what circuit or output voltage I'll use yet but probably a traditional opamp with the output feeding back around to the zener, though I'm not sure the 1399 needs that. Maybe just a buffer?

You could consider building a circuit similar to the LM399-PWM-DAC, which works pretty decent and this reference was used for the EU CalClub test run within Germany.

-branadic-

[Conrad Hoffman]

After playing with the LTSpice model for a bit, I'm sure I don't understand how the F & S lines work and where/why current flows where it does. On a board where everything is close together, it seems safest to just hard wire them together.

[KT88]

With 2.2 mA the transistor as emitter follower has an output impedance of some 12 Ohms.

That's not what is happening...
The emitter follower is the external part of a closed loop amplifier - it is just only a uni-polar output stage. No need to sink current...
Running the sim in LTSpice gives an output impedance of only 1mOhm.
Some significant advantages come with using an emitter follower:
-The current through the shunt reference remains basically constant - the classic circuit shares the bias current with the load.
-The bias resistor won't need to provide the load current - it can be larger.
-A wide range of load currents are challenging for a shunt circuit - in paticular if larger load currents than the max bias current are required.
-The power dissipation within the reference IC remains constant with changing load currents.

Cheers

Andreas

[MiDi]

You could consider building a circuit similar to the LM399-PWM-DAC, which works pretty decent and this reference was used for the EU CalClub test run within Germany.

Documentation for PWM-DAC is in thread LM399 based 10 V reference:
Schematics & Gerber
Firmware

[Conrad Hoffman]

Had some time for a crude measurement using manganin wire to isolate the package thermally. 0.011" diameter and about 0.2 ohms per inch. Just wired the heater and ran it on 15 VDC. Current stabilized at about 10 mA in free air, so I think this might be a useful trick. Probably better with some sort of insulation. I used to do the same thing with cooled CCD chips to lower the heat loss through all the wires. I got an IR reading off the edge of the chip of about 150F (65C) which is probably too low. Don't know the emissivity- usually I put a dab of flat black paint on things. Pretty sure the heater is working correctly. If you blow on the chip the current doubles. Still thinking about how to handle the reference wiring.

[iMo]

You could consider building a circuit similar to the LM399-PWM-DAC, which works pretty decent and this reference was used for the EU CalClub test run within Germany.

Documentation for PWM-DAC is in thread LM399 based 10 V reference:
Schematics & Gerber
Firmware

Is that the latest schematics and firmware? I think there were some changes after the evaluation here by Andreas and Kleinstein..

[iMo]

The manganin leads shift the voltage a little bit (the refsense is rather fake here), but otherwise it should work.
In the ADI's schematics (with the transistor) the higher the bias resistor R1 the worse regulation under variable load.
@Conrad: here are the 3 classic examples - the worse load char is with the ADI's transistor, those with an opamp are the best (the graphs below are "ppm vs. load current" from 1uA to 5mA/10mA/20mA).