Looks like i am facing A3
Board replacement, drift is 1.5ppm a day according to CAL72 values i measured.
Wait a moment,please.
Which reference do you use (10V, 6ppm???).
Is the environmental temperature absolutely constant?
The AN 18 leaves our completely, that CAL? 72 constant depends also on both parameters.
If you monitor additionally the internal temperature TEMP?, you may get a rough idea of the T.C. of your 3458A, for 10VDC, w/o ACAL.
That dependency you have to subtract, before you judge the timely manner drift of your
AD chip.
Rough idea, because the T sensor sits on opposite PCB.
Frank