I recently started working with the Axopatch 200B in current-clamp mode (patch). As a simple test, I checked to make sure that the open circuit (nothing connected to the headstage) reading was 0 mV, but the reading was much larger than 0 mV (greater than 300 mV in some cases). I restarted the equipment and ran into the same problem. Based on my understanding, in current clamp mode, the Axopatch 200B uses a feedback circuit to automatically adjust the pipette voltage to keep the pipette current at 0 pA. If nothing is connected to the headstage, then current cannot be flowing, so I would expect to see a reading of 0 mV since the system does not need to adjust the pipette voltage to keep the current at 0 pA.
Can you think of a reason why I would see a large voltage (>300 mV) V_m reading while everything is disconnected from the Axopatch 200B headstage?
Thank you in advance for your help.