Hi Qun Zhou ,

does this drift remain even after warming up for 1 hour, as specified on page 2-2 of the instruction manual?

Hello Qun Zhou ,

I guess you have Zero Check switched on.

A possibility to circumscribe the cause of the drift is to use the preamp out signal available at the rear panel (see page 11-12 of the manual): With External Feedback disabled, you could measure the preamp out signal and see if it is drifting. If not, the cause has to be located behind the preamp.

If the preamp is drifting, you could enable the External Feedback, and use a simple resistor (well shielded) in the external feedback loop. If the drifting disappears something might be wrong with the internal feedback.