Now let´s finally look into the series production ADR1001!
If you haven´t already seen my LTZ1000A update you should read that before going on:
https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/msg5540135/#msg5540135
This ADR1001 was produced in calenderweek 7 in 2024.
The electrical connection of the package lid has not changed compared to the engineering sample. It is still connected to the inner housing base, but it is otherwise insulated.
The internal structures are very similar to the ADR1001 engineering sample.
Like in the ADR1001 we see this glass bead polymer mixture.
The diameter of the glass beads is approximately 0,1 mm. In contrast to the material used in the LTZ1000A, only very similar diameters and no significantly smaller particles can be found here. There is one layer of spheres under the die. The gap is therefore approximately 0,1mm. In the ADR1001 engineering sample the layer appears to be somewhat thicker.
Although some of the glass spheres contain air bubbles, they appear to be basically solid. They are probably the same spheres that are described as new material in PCN 23_0011 of the LTZ1000A. These would then be the glass spheres from COSPHERIC with diameters between 90µm and 106µm.
As with the LTZ1000A, there is also a small area that has been left open.
Removing the die reveals that the die attach material has just been applied to the outer areas of the die. A cavity in the middle provides better thermal insulation. As shown with the LTZ1000A, the cavity is absolutely necessary as the glass beads still conduct heat relatively well. In contrast to the LTZ1000A, however, the cavity here is relatively small.
In this context it is interesting that the engineering sample of the ADR1001 has no opening at the edges of the die. Either the material has been applied over the entire surface or there is a closed volume under the die. It is at least conceivable that a sealed volume could have a negative effect on the specification of the reference. On the one hand, substances could remain in this volume that are normally flushed out at the end of production. In addition, temperature changes result in pressure differences in relation to the rest of the package, which in turn generate mechanical stress in the die.
It is now clearly visible that there is exactly one layer of glass beads under the die.
Except for one point, the die has the same structures as the die in the engineering sample. This image is also available in higher resolution:
https://www.richis-lab.de/images/REF01/50x10XL.jpg (44MB)
We have talked about the parasitic resistor in the output of the output amplififer. The voltage drop across this resistor changes with load changes. As the voltage drop is outside the control loop, it is not compensated.
Obviously, the output resistance has also been recognized at Analog Devices. It would have been ideal to resolve the node at the output of the opamp so that the feedback resistors could be contacted externally. However, this would have meant changing the pinout. Instead, the output stage is now connected to the bondpad via a relatively large metal surface. The connection to the feedback resistors is made via a long contact between the two metal layers. This contact is located as close as possible to the bondpad.
The long contact of the feedback resistors would not have been necessary, as not much current flows through this path. In fact, it would have been better to choose a small contact and bring it even closer to the bondpad. However, this path is no longer critical. Its resistance should be 15-30mΩ. The bondwire alone adds a resistance of 140mΩ. In addition, there are contact resistances at the bondpad and at the contact pad of the housing.
https://www.richis-lab.de/REF45.htm 
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