Hey folks, I finished the layout of my LTZ1000 reference module. It is built around an 3458A reference form factor. 6 layers, uses mostly through hole parts, the divider is VHD200, the other sensitive resistors are VHP202Z, I also use MKP capacitors, and plan on using the free space for a few LMT70 temperature sensors, a USB interface and microcontroller for tracing and calibration.
I appreciate any feedback on the layout, specially around grounding.
OK, I can't resist any more to make some comments.
At first, this 6 layer board is a complete over-engineering, and mostly counter-productive.
These additional layers do more harm in terms of thermal-electric voltages, than simply using a single sided board only.
Latter would reduce the number of solder- and via- joints to the absolute minimum, would let you control them much better as you would have a single iso-thermal plane only, not randomly distributed over 6 planes.
Alternatively, you could place all analogue signal joints on one layer, and the supply voltages on a 2nd layer.
Please revise your star point concept for the two reference points, especially concerning the current flow from and into the different resistors around the LTZ1000.
I can not recognize at all, if these paths are all correctly Kelvin- sensing designed. Also, you should try to get a symmetrical layout for the reference pins.
These GND and Vcc shielding planes are useless, because it's a DC-application, but not an RF board.
In contrary, possible EMI disturbances will come from outside the board only, therefore an EMI shield (tuner box) all around the PCB would be more useful.
This could also serve as a shield against air draughts inside the 3458A. As often discussed before, the constant air flow inside the 3458A contradicts the data sheet recommendation to keep all solder joints away from this.
Maybe you also want to reduce the oven temperature to reasonable values, i.e. 12.5k/1k for 60..65°C.
The only proven measure to equalize the temperature gradients of the LTZ1000 is to apply a broad copper ring AROUND the chip.... the electrical connections to the LTZ have to be made on the opposite layer, of course. This way it's done inside the Datron/Wavetek M7000 reference, which is the best LTZ1000 based 10V reference.
Frank