The structure in the center of the die strongly resembles the combination of Z-diode and transistor known from the LTZ1000 and the ADR1000. What seems absolutely logical at first glance raises questions when you recall the structure of this device.
As shown in the ADR1000, the base of the transistor contacts the substrate. The emitter potential is thus more negative than the substrate. In the LTZ1000 and in the ADR1000 this is not a problem, because there are no additional circuit parts on the die apart from a transistor for temperature measurement and a heater resistor.
In the ADR1001, however, a very extensive circuit is integrated on the same die. In addition, the emitter of the Z-diode/transistor combination is connected to further circuit parts. Either the reference structure in the ADR1001 is designed differently than in the ADR1000 or a process was used in which the active structures are completely isolated from the substrate.
The multiple collector connection known from the LTZ1000 and the ADR1000 is not found here. The only contact leading to the collector layer is found at the top right. Surprisingly, the collector is connected to the base of the transistor structure.
Around the combination of Z-diode and transistor there are four more transistors. Two of the transistors (T1/T2) are used for temperature control. The other two transistors (Q2/Q3) seem to have a functional part in the voltage reference and represent the branch that one would actually look for within the special structure.
I wonder where that 1.7ohm resistor at the buffer's output comes from.. Could we see it on the die?
..It's how ya doin'. Ideally, the feedback network should have a separate bonding wire going to the package pin or even a separate pin to include PCB parasitics (or buffers) in the feedback loop. They could have freed up one pin by removing REF6P6_F and connecting it internally to the cathode; not sure what's the point of making this connection externally.
Those beads looks like fish eggs.
I think what happens here is that the central structure is just an ordinary buried zener. There is no transistor there and no connection to the substrate. The contact in the center is a sense connection to the anode and a separate force connection ("ISET") surrounds the cathode like in ADR1000.
Q2 and Q3 perform the function of Q1 from ADR1000. The schematic you suggested doesn't work, because with one zener and two PN junctions in series their thermal coefficients wouldn't cancel out.
@Noopy: TCHIP pin - that pin is not indicated in the known pin description - there is the "NC".
In the LTSpice model there is a signal called TCHIP, which outputs the chip temperature in Celsius (at least it is my current understanding).. What that pin 2 actually does then?
Weird..
PS: it goes to the T transistor near the zener, but the signal follows also somewhere around the zener (and there is a resistor hanging on it as well)..
In my view pin 2 is the pin to check the temperature with the transistor T. Put a Pull-Up or a current source there and you get the Vbe of the transistor.
In the schematic of the Eval-Board there is just n.c., perhaps because it´s just for internal use...
Take a closer look at the transistor, it´s just connected to TCHIP (and to GND). The two metal layers are hard to read.
17.3.23 - the first "metro-LAB measurement" Vref = 9.999.895Volt, powered off for 3 hours
17.3.23 - the first "metro-LAB measurement" Vref = 9.999.895Volt, powered off for 3 hourswith the same uncertainity as the other measurements?
I now assume in the ADR1001 it looks like this. The emitter is missing and the contact in the middle changed to a Anode Sense.
If you look closely and compare it with the ADR1000 structure you can see that there is a ring missing. That is the emitter area.
I am not sure of the process allows for good PNPs, that maybe needed for a single supply OP-amp.