a bias voltage

[Emily-hh]

When I try to use the digital-servo-box to lock a cavity by the PDH method , I meet a problem. I use the AIN1 as the error signal input, Meanwhile, I use the AOUT2HV as the feedback to the pzt of the cavity. But when I disabled the Sweep of the AOUT2HV with the Filter enabled, there is a bias voltage about 10V in the AOUT2HV. Why? How I solve the problem? Thank you.