Hello,
can be many root causes.
One is popcorn noise (either your instrument or the reference).
The other is EMI influence from outside (USB-cables, refrigerator, mobile phone ...)
I have the impression that the ADR1000 is more sensitive to EMI as the LTZ1000.
But I have at the moment no comparable cirquit where I can prove the impression.
I had (during 1/f noise measurement) 2-4 uV negative spikes all 15.4 seconds
which were related to a unused FTDI RS232 cable which was laying (unintentionally)
on the same desk where I did the 1/f noise measurement.
with best regards
Andreas